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We explored picophytoplankton in the surface (0m) and bottom (2.3–8.7m) layers of a shallow (, Marine Environmental Research, 184, art. no. 105852; 2022

After the construction of the Three Gorges Dam (Changjiang River), the northern East China Sea has been exposed to major environmental changes in the summer due to climate change and freshwater control. However, little is known regarding phytoplankton in this area. Here, we investigated differences in the summer phytoplankton-community structure as a consequence of marine-environment changes from 2016 to 2020. In the 2000s, the key dominant species in the summer phytoplankton community in the northern East China Sea were diatoms and dinoflagellates. In this study, however, nanoflagellates of ≤20 µm were identified as the dominant species throughout the survey period, with abundances ranging from 43.1 to 69.7%. This change in the phytoplankton-community structure may be ascribed to low nutrient concentrations in the area, especially phosphate, which was below the detection limit, seriously hampering phytoplankton growth. The relative contribution of picophytoplankton to the total chlorophyll a biomass was highest in the surface mixed layer with low nutrient concentrations. Spatially, higher percentages were observed along the east-side stations than the west-side stations, where nutrient concentrations were relatively high. Conclusively, decreased nutrients led to phytoplankton miniaturization. Accordingly, as the dominance of picophytoplankton increases, energy transfer is expected to decrease at the upper trophic level.

Simple Summary In order to accurately characterize the composition of the picophytoplankton community by size in the marine environment, we used a size-fractionated filtration plus a high-throughput sequencing molecular method. The results showed that this method can detect the composition of picophytoplankton in different particle size ranges in detail, and it is accurate in assessing the vertical distribution patterns and size of phytoplankton. This method can enrich the study of phytoplankton communities with diameters of less than 2 μm. Abstract In this study, flow cytometry (FCM) and size-fractionated filtration, together with high-throughput molecular sequencing methods (SM), were used to investigate picophytoplankton. A particle separation filter and a higher-throughput sequencing method were used to evaluate the composition of a euphotic zone of picophytoplankton—especially picoeukaryotic phytoplankton—in the Western Pacific, and the results of flow cytometry, which is a classic way to detect picophytoplankton, were used as a standard to evaluate the reliability of the results of the SMs. Within a water column of 200 m, six water depths (5, 25, 50, 113 (DCM), 150, and 200 m) were established. In order to further study the particle size spectra of the picophytoplankton, size-fractionated filtration was used to separate water samples from each water depth into three particle size ranges: 0.2–0.6, 0.6–1.2, and 1.2–2 μm. A total of 36 (6 × 3 × 2) samples were obtained through PCR amplification of the 18S rRNA V4 hypervariable region and 16S rRNA, which were biased toward phytoplankton plastids, and then high-throughput sequencing was performed. The estimation of the picophytoplankton diameter relied on forward scattering (FSC) through FCM. The estimation of the vertical distribution and diameter of the picophytoplankton using the SM was consistent with the results with FCM; thus, we believe that the estimation of picophytoplankton composition with the SM has value as a reference, although the size-fractionated filtration seemed to cause some deviations. In addition to Prochlorococcus and Synechococcus, the SM was used to evaluate the composition of picoeukaryotic phytoplankton, which mainly included Prymnesiophycea (Haptophyta) (38.15%), Cryptophyceae (Cryptophyta) (22.36%), Dictyochophyceae (Chrysophyta) (12.22%), and Mamiellophyceae (Chlorophyta) (3.31%). In addition, the SM also detected Dinophyceae (Dinoflagellata) (11.69%) sequences and a small number of Bacillariophyceae (Diatom) (1.64%) sequences, which are generally considered to have large particle sizes. The results of the SM also showed that the picoeukaryotic phytoplankton were not evenly distributed in the euphotic layer, and the vertical distributions of the different picoeukaryotic phytoplankton were different. An analysis of correlations with environmental factors showed that temperature was the main environmental factor controlling the vertical distribution of picophytoplankton.

Compounds in sunscreen such as ultraviolet (UV) filters protect human skin from damage caused by UV radiation exposure. However, sunscreen components reach marine ecosystems after their release from human skin during activities such as swimming and washing, and are potentially toxic to marine organisms. TiO2 and ZnO nanoparticles (NPs) are commonly used as inorganic UV filters. In this study, we explored the effects of TiO2 and ZnO NPs on natural phytoplankton assemblages in coastal seawater. Growth rates of natural phytoplankton assemblages were significantly decreased by 10mgL−1 TiO2 and 1 and 10 mg L−1 ZnO NP treatments. NP addition also modified the size structure of phytoplankton assemblages, and small phytoplankton (mainly cyanobacteria) are vulnerable to NPs. Because herbivore food preferences depend strongly on algal cell size, NP contamination could also affect higher trophic levels. Notably, small phytoplankton are an important component in microbial loop, and this energy transfer pathway may be more vulnerable to NP contamination., Marine Environmental Research, 183, art. no. 105826; 2022

Biogenic silica (bSi) standing stocks and carbon (C) biomass of small plankton are rarely studied together in previous analyses, especially in oligotrophic gyres. Within the oligotrophic western tropical North Pacific, based on size-fractionated bSi and biovolume-derived C analyses in three size fractions (i.e., 0.2–2; 2–20; >20 μm), we observed that picophytoplankton (2 μm) to total bSi standing stocks and living C biomass were 9 and 16%, respectively, suggesting that the role of diatoms in marine Si and C cycles may have been overestimated in previous analyses. Due to the overwhelming predominance of picocyanobacteria in the oligotrophic western tropical North Pacific, their contributions to total bSi stocks and C biomass were quantitatively important and accounted for more of the bSi and C associated with living cells than did diatoms. In addition, water temperature and light intensity were likely the key determinants of the variations in size-fractionated bSi standing stocks and living C biomass, but not nutrient availability. Collectively, these findings encourage a reconsideration of the previously underestimated role of small plankton in understanding the Si and C cycles in the ocean, and may provide insights into the interpretations of disproportionate budgets of Si and C in oligotrophic oceans.

Picophytoplankton were investigated during spring 2015 and 2016 extending from near‐shore coastal waters to oligotrophic open waters in the eastern Indian Ocean (EIO). They were typically composed of Prochlorococcus (Pro), Synechococcus (Syn), and picoeukaryotes (PEuks). Pro dominated most regions of the entire EIO and were approximately 1–2 orders of magnitude more abundant than Syn and PEuks. Under the influence of physicochemical conditions induced by annual variations of circulations and water masses, no coherent abundance and horizontal distributions of picophytoplankton were observed between spring 2015 and 2016. Although previous studies reported the limited effects of nutrients and heavy metals around coastal waters or upwelling zones could constrain Pro growth, Pro abundance showed strong positive correlation with nutrients, indicating the increase in nutrient availability particularly in the oligotrophic EIO could appreciably elevate their abundance. The exceptional appearance of picophytoplankton with high abundance along the equator appeared to be associated with the advection processes supported by the Wyrtki jets. For vertical patterns of picophytoplankton, a simple conceptual model was built based upon physicochemical parameters. However, Pro and PEuks simultaneously formed a subsurface maximum, while Syn generally restricted to the upper waters, significantly correlating with the combined effects of temperature, light, and nutrient availability. The average chlorophyll a concentrations (Chl a) of picophytoplankton accounted for above 49.6% and 44.9% of the total Chl a during both years, respectively, suggesting that picophytoplankton contributed a significant proportion of the phytoplankton community in the whole EIO.

In oceanic communities, picophytoplankton often dominates phytoplankton biomass and productivity. Diel variations in picophytoplankton abundance and growth have been well documented. In the current study, we used flow cytometry to assess the short-term variations (3 h) of the abundance of the most dominant picophytoplankton, Synechococcus spp. and picoeukaryotes, in the coastal regions of northeastern Taiwan. To explore the change in growth and mortality rate in the daytime and over 24 h incubation, we performed a two-point modified dilution experiment for measuring growth, viral lysis, and nanoflagellate grazing rate. In this study, the growth rates of picoeukaryotes were 0.21 and 0.06 h−1, and those of Synechococcus spp. were 0.15 and 0.06 h−1 for daytime and 24 h incubation, respectively, and the values were higher at significant levels in the daytime than those for 24 h incubation. These growth rate values of picoeukaryote and Synechococcus spp. after incubation for 24 h were approximately underestimated at 71% and 55%, respectively. This finding suggests that estimates based on 24 h sampling may not accurately reflect the true growth rate of these populations on ecologically relevant timescales.

Understanding picophytoplankton variations that play important roles in the material circulation and energy flow are critical to assessing overall status of waterbody, especially for clean reservoirs which remain a relatively stable community structure and high species diversity due to lower nitrogen and phosphorus nutrients. However, their response to key environmental factors and tightly acting microbial remains poorly understood. Traditional quantification methods are limited, such as chlorophyll-a, turbidity and microscope. There are still many defects with present molecular analysis. In this study, a flow cytometric analysis and high-throughput sequencing combination methodology was developed and tested on clean water from a reservoir, by a monthly dynamic for a vegetative period April-September in 2019 to improve the accuracy of dynamic monitoring for the picophytoplankton system. More species of Pico-Cyanobacteria and Pico-Eukaryotes were discovered. The increased percentage of pigment compounds from 8.2% to 76.3% proves the effective reduce of heterotrophic disturbing and enrichment of target populations. Picophytoplankton that was previously neglected due to their low relative abundance has once again entered the scope of our eyes. Phytoplankton were divided into three categories. The first one was the highly abundant and frequently present taxa, the second one was the low-abundance but highly-transient population, and the third one was the low abundance and stable group. Synechococcus, Emiliania, Tetraselmis and Thalassiosira were dominant picophytoplankton and displayed obvious temporal and spatial distribution characteristics. Pico-PE rich Cyanobacteria and Nano-Eukaryotes with high transience abnormally increased in summer. Temperature, ammonia-N, nitrate-N, turbidity and total nitrogen were most influencing factors, while some picophytoplankton with special physiological structure showed distinct competitive advantages in the microbial community. As for the off-flavor compounds, the concentration of 2-methylisoborneol and geosmin were high even 66.7% and 20.8% of the samples exceeded their olfactory threshold. Chrysochromuina, Planktothrix and Microcystis might be the potential producers.

Dynamics of picophytoplankton and photosynthesis will be inevitably impacted by changing marine environment, such as ocean acidification and nutrient supply, but related studies are very scarce. Here we cultured the picophytoplankton-dominated surface water of the oligotrophic Eastern Indian Ocean (EIO; R/V Shiyan-3, 20 March to 18 May 2019) at two levels of pCO2 (400 and 1000 ppm) and phosphate (0.05 and 1.50 µM) to investigate the interactive effects of elevated pCO2 and phosphate (P) on the dynamics of picophytoplankton and photosynthetic properties. High pCO2 and P levels interactively increased the abundances of Synechococcus, Prochlorococcus and picoeukaryotes by 33%, 18%, and 21%, respectively, of which high P level had a major promoting effect. Conversely, rising pCO2 alone decreased their abundances by 9%, 32%, and 46%, respectively. For the photophysiological responses in relation to the combination of high pCO2 and P levels, there was an increase in the maximum (Fv/Fm) and effective (Fq'/Fm') photochemical efficiency, the electron transfer rates (ETRRCII) and the charge separation rates (JVPSII, an indicator of primary production), but a decrease in the non-photochemical quenching (NPQNSV). Elevated pCO2 alone facilitated the NPQNSV process significantly, ultimately leading to reduced light use efficiency (e.g., Fv/Fm, Fq'/Fm' and ETRRCII) and primary production (JVPSII). There was a strong coupling of picophytoplankton and JVPSII, suggesting the EIO primary productivity was potentially controlled by picophytoplankton. Overall, our results indicate that the negative effects caused by ocean acidification may be masked or outweighted by the role that P availability plays in regulating growth and metabolism in this oligotrophic ecosystem.

Pico-sized Synechococcus, Prochlorococcus, and eukaryotes are the dominant photosynthetic organisms in the vast warm and oligotrophic regions of the ocean. In this paper, we aim to characterize the realized niches of the picophytoplanktonic community inhabiting the Red Sea, the warmest oligotrophic sea, which is considered to be a model for the future ocean. We quantify population abundances and environmental variables over several oceanographic surveys, and use stepwise regression, principal-component analysis (PCA), and compositional-data analysis to identify the realized niches of the three picophytoplanktonic groups. Water temperature varied from 21.4 to 32.4°C within the upper 200-m water column, with the warmest waters being found in the South, where nutrients increased. Synechococcus dominated the biomass, contributing 47.6% to the total picophytoplankton biomass, followed by picoeukaryotes (26.4%) and Prochlorococcus (25.9%), whose proportions contrast significantly with those reported in the subtropical ocean, where Prochlorococcus prevails. There were positive and significant relationships between temperature and the three populations, although these were weak for Prochlorococcus (R2 = 0.08) and stronger and steeper for Synechococcus (R2 = 0.57). The three populations centered their maximum abundances (Lorentzian fits) at similar low nutrient values. Synechococcus were centered close to the surface at ≈77% of surface photosynthetically active radiation (PAR) and ≈30.6°C. The picoeukaryotes were centered at lower light (≈6.4% surface PAR) and warm waters (≈30°C). Prochlorococcus was segregated from the surface waters and centered deep at low light (≈3.2% surface PAR). Light and temperature were the most influential factors determining the community composition, with Synechococcus dominating ∼74% of the picophytoplankton biovolume in the warmest (>30°C) waters. In the warm and mesotrophic southern Red Sea, the moderate abundances of picoeukaryotes and Synechococcus suggest increasing competition with nano and microphytoplankton. Our observations agree with predictions of increasing vertical segregation of picophytoplankton communities with future warming and reveal Synechococcus’s significant capacity to adapt to warming.

Picophytoplankton (pico) in the eastern Indian Ocean (EIO) were investigated during the inter-monsoon periods. They were found to typically comprise Prochlorococcus (Pro), Synechococcus (Syn), and Picoeukaryotes (PEuks). In the survey area, the pico showed two different vertical distribution patterns in different regions, whereby the Syn abundance decreased with depth, whereas those of Pro and PEuks increased and then decreased with depth, with the maximum depths ranging from 50 to 100 m. The cell abundance and community structure of the pico were similar at the equator (EQ) and the eastern boundary of the Indian Ocean near Sumatra (EB), but the pico cell abundance was significantly lower in the Bay of Bengal (BOB). Pro dominated most regions of the entire EIO and were approximately one-to-two orders of magnitude more abundant than Syn and PEuks. The distributions of Syn and PEuks showed little difference across various regions. Influenced by the physicochemistry of circulation and water masses, there were many different environmental factors in the different regions. The abundance of pico domination by Pro showed a strong positive correlation with the nutrients and salinity in the survey area, indicating increasing nutrient availability, particularly in the oligotrophic EIO. Generalized additive models (GAMs) analysis showed the differences in their responses to environmental variability. Pro and PEuks both increased strongly with warming up to below 26 °C, and Pro and PEuks were more responsive to chemical (nutrient) variability. Syn showed a broader tolerance of low-salinity conditions. In a certain range, an increase in nitrite and nitric acid can improve the cell abundance of Pro. As a significant contributor to primary productivity in oligotrophic waters, this study provides essential information for studying pico communities in the EIO and its adjacent marine ecosystems.

An in situ mesocosm experiment was performed to investigate the combined effects of ocean acidification and warming on the coastal phytoplankton standing stock and species composition of a eutrophic coastal area in the temperate-subtropical region. Experimental treatments of natural seawater included three CO2 and two temperature conditions (present control: ~400 &mu, atm CO2 and ambient temperature, acidification conditions: ~900 &mu, atm CO2 and ambient temperature, and greenhouse conditions: ~900 &mu, atm CO2 and ambient temperature +3 &deg, C). We found that increased CO2 concentration benefited the growth of small autotrophic phytoplankton groups: picophytoplankton (PP), autotrophic nanoflagellates (ANF), and small chain-forming diatoms (DT). However, in the greenhouse conditions, ANF and DT abundances were lower compared with those in the acidification conditions. The proliferation of small autotrophic phytoplankton in future oceanic conditions (acidification and greenhouse) also increased the abundance of heterotrophic dinoflagellates (HDF). These responses suggest that a combination of acidification and warming will not only increase the small autotrophic phytoplankton standing stock but, also, lead to a shift in the diatom and dinoflagellate species composition, with potential biogeochemical element cycling feedback and an increased frequency and intensity of harmful algal blooms.

Background Picophytoplankton are abundant and can contribute greatly to primary production in eutrophic lakes. Mychonastes species are among the common eukaryotic picophytoplankton in eutrophic lakes. We used third-generation sequencing technology to sequence the whole genome of Mychonastes homosphaera isolated from Lake Chaohu, a eutrophic freshwater lake in China. Result The 24.23 Mbp nuclear genome of M.homosphaera, harboring 6649 protein-coding genes, is more compact than the genomes of the closely related Sphaeropleales species. This genome streamlining may be caused by a reduction in gene family number, intergenic size and introns. The genome sequence of M.homosphaera reveals the strategies adopted by this organism for environmental adaptation in the eutrophic lake. Analysis of cultures and the protein complement highlight the metabolic flexibility of M.homosphaera, the genome of which encodes genes involved in light harvesting, carbohydrate metabolism, and nitrogen and microelement metabolism, many of which form functional gene clusters. Reconstruction of the bioenergetic metabolic pathways of M.homosphaera, such as the lipid, starch and isoprenoid pathways, reveals characteristics that make this species suitable for biofuel production. Conclusion The analysis of the whole genome of M. homosphaera provides insights into the genome streamlining, the high lipid yield, the environmental adaptation and phytoplankton evolution.

The spatial variations of photosynthetic picoplankton abundance and biomass and the picoplankton's contribution to chlorophyll a concentration along the transect from Khatanga Bay to the continental slope in the western part of the Laptev Sea were studied in September 2017. Picoeukaryotes dominated in the picophytoplankton communities. Picophytoplankton in Khatanga Bay showed more variability than those over the Laptev shelf and continental slope: abundance and biomass were the highest in the southern part of the bay and markedly decreased with increasing salinity in its northern part. Picocyanobacteria were found over the shelf and slope at temperatures of +2.4 to -1.6°С and salinity from 22 to 34. Picophytoplankton contribution to total chlorophyll a on the shelf was higher than in Khatanga Bay. The study of picophytoplankton of Khatanga Bay and in the western Laptev Sea can serve as a baseline for future assessment of the Laptev Sea ecosystem response to interannual and climate changes., Picophytoplankton; Picocyanobacteria; Chlorophyll a; Pico chlorophyll a; Laptev sea; Khatanga bay

International audience; In parallel to phytoplankton community dynamics, transparent exopolymeric particles (TEP) and exopolymeric substances (EPS) were investigated along the salinity gradient of a temperate estuary (Seine estuary, Normandy, France) over the course of a year. The phytoplankton community was mainly dominated by marine diatom species (especially Skeletonema sp., Nitzschia sp., and Paralia sulcata) associated with a spring bloom of pico-eukaryotes and the development of Cryptophyceae in summer. The decreases in species richness and salinity were correlated along the estuary and a significant exponential relationship between species richness and primary production was identified. Concentrations of TEP and EPS (soluble and bound carbohydrates) are highly dynamic in this estuary and can reach respectively 69 mgC L−1, and 33 mgC L−1. TEP distribution was mainly related to physical factors (hydrodynamics, maximum turbidity zone formation and sediment resuspension) probably produced by stressed or dying phytoplankton, while EPS appeared to be excreted during the phytoplankton spring bloom. Soluble and bound EPS appear to be related to Skeletonema sp. and Cryptophyceae occurrences. This paper presents the dynamic pattern of these carbon pools, which play an important role in the trophic network and influence the flocculation processes involved in the fate of both organic and inorganic matter.

Seasonal variations in the picophytoplankton community structure (Synechococcus spp. and picoeukaryotes) were studied by flow cytometry in the coastal ecosystem of the subtropical western Pacific from October 2019 to September 2020. Synechococcus spp. was dominant in abundance during the study period, with its density ranging from 0.05 to 5.6 × 104 cells mL−1; its maximum occurred in July 2020. Picoeukaryotes were less abundant, with their density ranging from 0.2 to 13.6 × 103 cells mL−1. Their highest abundance was recorded in January 2020. The growth rates of Synechococcus spp. and picoeukaryotes ranged from −0.39 to 1.42 d−1 and 0.38 to 2.46 d−1, respectively, throughout the study period. Overall, the growth rate of the picoeukaryotes was significantly higher than that of Synechococcus spp. It is interesting to note that the grazing mortality of Synechococcus spp. and picoeukaryotes during the warmer period (April to September) was relatively low. Based on this study, we suggest that mixotrophic nanoflagellates lowered their feeding activity that obtained nutrients from prey and instead used additional nutrients during the incubation experiments. Our study demonstrated that a shift in the picophytoplankton community composition and grazing activity of predacious nanoflagellates in cold and warm periods can impact on the seasonal dynamics of the microbial food web.

A remarkable shift in the species composition and size distribution of the phytoplankton community have been observed in coastal waters along the Antarctic Peninsula over the last three decades. Smaller photoautotrophs such as cryptophytes are becoming more abundant and important for the regional ecosystems. In this study, flow cytometry was used to quantify the smallest phytoplankton in the central Bransfield Strait and explore their distribution across the strait in relation to physical and chemical properties of the two major water masses: the warmer and less saline Transitional Zonal Water with Bellingshausen Sea influence (TBW), and the cold and salty Transitional Zonal Water with Weddell Sea influence (TWW). Pico- and nano-phytoplankton clusters were distinguished and enumerated in the cytograms: photosynthetic picoeukaryotes, cryptophytes (about 9 µm in size), and smaller (3 µm) nanophytoplankton. It was shown that nanophytoplankton developed higher abundances and biomasses in the warmer and less saline TBW. This biotope was characterized by a more diverse community with a pronounced dominance of Cryptophyta in terms of biomass. The results support the hypothesis that increasing melt-water input can potentially support spatial and temporal extent of cryptophytes. The replacement of large diatoms with small cryptophytes leads to a significant shift in trophic processes in favor of the consumers such as salps, which able to graze on smaller prey.

We used mathematical models of photosynthesis to quantify the effects of ultraviolet (UV) radiation on the photosynthetic potential of Prochlorococcus and Synechococcus marine cyanobacteria living at 0 degrees and 40 degrees N/S latitude. We show that UV is an environmental stressor for these organisms near the ocean surface, accounting for roughly two-thirds of the potential photo synthetic inhibition. Prochlorococcus showed a higher inhibition and integrated photo synthetic potential throughout the water column than Synechococcus, since the former is more vulnerable to UV damage at the surface and more successful at greater depths compared to the latter. The maximum photosynthetic activity was reached beneath the photoactive zone, largely due to the harmful effects of UVA. UV inhibition varies with latitude, due to variability in repair capacity for Synechococcus, and the existence of more diverse mechanisms of acclimation to irradiance and temperature for Prochlorococcus. The lowest photoinhibition is estimated to occur at 0 degrees latitude, since the interactive effects of high temperature and irradiance have a positive effect on photo acclimation to UV damage.

The subarctic Atlantic phytoplankton spring bloom is one of the largest biological features of the ocean; however, processes initiating the bloom are still not well understood. We hypothesize that the microbial grazing food chain plays an important role in creating a pre-bloom condition with top-down control of small-sized phytoplankton, thus paving the way for a diatomdominated spring bloom. To assess the trophic role of protist grazers during the winter to spring transition, 3 experiments were performed using size-fractionated surface water from the Iceland Basin (March−April 2012). These experiments demonstrated heterotrophic nanoflagellates (HNF) grazing of picophytoplankton to be a key pathway, even though these are rarely considered as important phytoplankton grazers in high-latitude systems. The growth rate of HNF was significantly correlated to the biomass of picophytoplankton and was substantially higher than the growth of the larger microzooplankton (MZP), i.e. ciliates and dinoflagellates. During the first experiment, small phytoplankton dominated and overall protist grazing (HNF + MZP) was low. In the later experiments, MZP grazing on HNF became evident; however, MZP were not able to control the community of larger phytoplankton (>10 μm), which became more abundant. Our experiments thus support the hypothesis that pre-bloom conditions promote a build-up of large phytoplankton, i.e. diatoms. We found that the high growth rates of HNF together with the relaxed MZP grazing pressure allow HNF to respond rapidly to the early primary production by picophytoplankton and maintain a strong top-down control on these. We suggest that this succession may be an important mechanism that allows large diatoms, rather than picophytoplankton, to become the dominant primary producers during the subarctic Atlantic spring bloom. publishedVersion

Operational taxonomic units 94%–95% similar to the known Pedinophyceae were found as a result of high‐through sequencing of 18S rDNA V4 amplicons of environmental DNA from the summer picophytoplankton samples from the White Sea. Partial sequence of a ribosomal operon (the 5,298 bp includes partial 18S and 28S rDNA, complete 5.8S rDNA, ITS1, and ITS2 sequences) and a partial 2,112 bp chloroplast 23S rDNA sequence White Sea Pedinophyceae was amplified from metagenomic DNA by specific primers and sequenced. A new phylotype was designated as uncultured Pedinophyceae WS. On Chlorophyta phylogenetic trees the discovered phylotype occupies a basal position in the Marsupiomonadales clade. The synapomorphic base substitutions in rRNA hairpins confirm the relationship of Pedinophyceae WS to Marsupiomonadales and its difference from known genera of the order. The obtained results extend knowledge of picophytoplankton diversity in subarctic waters., Operational taxonomic units 94%–95% similar to the known Pedinophyceae were found as a result of high‐through sequencing of 18S rDNA V4 amplicons of environmental DNA from the summer picoplankton samples from the White Sea. Partial sequence of a ribosomal operon (the 5,298 bp includes partial 18S and 28S rDNA, complete 5.8S rDNA, ITS1, and ITS2 sequences) and a partial 2,112 bp chloroplast 23S rDNA sequence White Sea Pedinophyceae was amplified from metagenomic DNA by specific primers and sequenced. A new phylotype was designated as uncultured Pedinophyceae WS. On Chlorophyta phylogenetic trees the discovered phylotype occupies a basal position in the Marsupiomonadales clade. The synapomorphic base substitutions in rRNA hairpins confirm the relationship of Pedinophyceae WS to Marsupiomonadales and its difference from known genera of the order. The obtained results extend knowledge of picophytoplankton diversity in subarctic waters.

The current study aimed to assess changes in phytoplankton composition and productivity along an oligotrophic gradient in relation to changes in sea surface temperature (SST). Phytoplankton pigments, nutrients, and physical water column properties were studied along a longitudinal transect in the Mediterranean Sea (MED) in May–June 2013, covering its western (wMED) and eastern (eMED) basins. Pigments were used to determine phytoplankton taxonomic composition and taxon specific carbon fixation, whereas beam attenuation was used to calculate particulate organic carbon (POC). Nitrate, phosphate, and silicate concentrations integrated over 0–125 m declined from wMED to eMED (on average 13, 9, and 2 fold lower in eMED compared with wMED, respectively) and correlated inversely with SST (16.2–23.0 °C) for the whole transect in the Mediterranean Sea. N:P ratios in the euphotic zone averaged 5.6 and 3.1 for wMED and eMED respectively, suggesting that phytoplankton was nitrate limited. Average phytoplankton productivity and biomass were 4 and 2.5 fold higher in the wMED than in the eMED, respectively. Relative abundances of diatoms, prasinophytes, dinoflagellates, and cryptophytes showed inverse correlations with SST and positive correlations with nutrient concentrations. In contrast, pelagophytes, chlorophytes, euglenophytes, Synechoccocus and Prochlorococcus showed positive correlations with SST and negative correlations with nutrient concentrations. Particulate organic carbon (POC) of the upper 200 m showed up to 3 fold higher concentrations in the wMED than in the eMED, and correlated positively with chlorophyll concentrations, productivity, and nutrient concentrations, and inversely with SST. Inverse correlations were observed for phytoplankton biomass and SST below 19 °C. SST above 19 °C as observed in the eMED did not correlate with phytoplankton biomass and productivity, showing that in this temperature range phytoplankton productivity is uncoupled from nutrient supply from deep water. This suggests different responses of Mediterranean phytoplankton to changes in SST, depending on the temperature range.

In temperate, subpolar and polar marine systems, the classical perception is that diatoms initiate the spring bloom and thereby mark the beginning of the productive season. Contrary to this view, we document an active microbial food web dominated by pico- and nanoplankton prior to the diatom bloom, a period with excess nutrients and deep convection of the water column. During repeated visits to stations in the deep Iceland and Norwegian basins and the shallow Shetland Shelf (26 March to 29 April 2012), we investigated the succession and dynamics of photosynthetic and heterotrophic microorganisms. We observed that the early phytoplankton production was followed by a decrease in the carbon:nitrogen ratio of the dissolved organic matter in the deep mixed stations, an increase in heterotrophic prokaryote (bacteria) abundance and activity (indicated by the high nucleic acid:low nucleic acid bacteria ratio), and an increase in abundance and size of heterotrophic protists. The major chl a contribution in the early winter-spring transition was found in the fraction a > 50 µm) were stimulated by deep mixing later in the period, while picophytoplankton were unaffected by mixing; both physical and biological reasons for this development are discussed herein.

Journal of Plankton Research plankt.oxfordjournals.org J. Plankton Res. (2015) 37(2): 1 – 17. doi:10.1093/plankt/fbv011 Grazer and viral impacts on microbial growth and mortality in the southern California Current Ecosystem ALEXIS L. PASULKA 1 *, TY J. SAMO 2 AND MICHAEL R. LANDRY 1 9500 GILMAN DRIVE , LA JOLLA , CA 92037, USA AND 2 DEPARTMENT OF OCEANOGRAPHY , UNIVERSITY OF 1000 POPE RD , HONOLULU , HI 96822, USA SCRIPPS INSTITUTION OF OCEANOGRAPHY , UCSD , HAWAI ’ I AT MANOA , * CORRESPONDING AUTHOR : apasulka@caltech.edu Received November 25, 2014; accepted February 11, 2015 Corresponding editor: John Dolan Protistan grazers and viruses are major agents of mortality in marine microbial communities with substantially differ- ent implications for food-web dynamics, carbon cycling and diversity maintenance. While grazers and viruses are typ- ically studied independently, their impacts on microbial communities may be complicated by direct and indirect interactions of their mortality effects. Using a modification of the seawater dilution approach, we quantified growth and mortality rates for total phytoplankton and picophytoplankton populations (Prochlorococcus, Synechococcus, picoeu- karyotes) at four contrasting sites in the California Current Ecosystem. Grazing mortality was significant in 10 of 15 cases, while viral effects were significant for 2 cases. Nonetheless, mortality estimates for the entire phytoplankton community based on chlorophyll a were 38 + 13% higher when viral effects were included, relative to grazing alone. Mortality estimates for picophytoplankton varied in space and among groups. We also explored a potential methodo- logical constraint of this method and hypothesize that heterotrophic bacteria may be affected by the dilution of their growth-sustaining substrates. For all picophytoplankton, estimates of grazing and viral mortality were inversely related within and across experiments. Indirect interactions among grazers and viruses may be important to consider if there are tradeoffs in the grazing and virus resistance strategies of prey/host cells. KEYWORDS: microzooplankton grazing; viral lysis; picophytoplankton; mortality I N T RO D U C T I O N Mortality processes within microbial communities play key roles in regulating biomass, species composition and elemental cycling (Pomeroy et al., 2007; Suttle, 2007). Protistan grazers, the primary consumers of phytoplank- ton and bacteria (Sherr and Sherr, 1994, 2000), account, available online at www.plankt.oxfordjournals.org # Th A h 2015 P bli h d b O f d U i i P All i h d F i i l il j l i i

Распределение пикопланктона изучали в четырех водохранилищах Волги в конце лета. Среди организмов этой размерной фракции преобладали гетеротрофные прокариоты, которые составляли в среднем 98.5% общей численности и 83.5% биомассы пикопланктона. Их численность и биомасса бактерий изменялись в пределах (4.2-14.7)×106 кл./мл (в среднем (8.7±2.7)×106 кл./мл) и 45-386 (в среднем 153±89) мг С/м3 соответственно, тогда как численность и биомасса пикофитопланктона в пределах (0.6944)×103 кл./мл (в среднем (127±186)×103 кл./мл) и 0.78-204 (в среднем 46.9±53.7) мг С/м3 соответственно. Наибольшим количественным развитием пикофитопланктона характеризовалось эвтрофное Горьковское водохранилище. В его составе преобладали колониальные цианобактерии, вклад которых в формирование численности и биомассы составлял в среднем для всех водохранилищ 74.0% и 70.0% соответственно. Полученные данные свидетельствуют о важной роли пикопланктона в трофических сетях водохранилищ Волги., The quantitative distribution of autotrophic and heterotrophic picoplankton was studied in the four Volga reservoirs during the late summer. Heterotrophic prokaryotes predominated in the picoplankton. They averaged 98.5% of the total abundance and 83.5% of the biomass of picoplankton. The number and biomass of heterotrophic picoplankton varied within the range (4.2-14.7)×106 (average (8.7±2.7)×106) cells/ml and 45-386 (average 153±89) mg C/m3 respectively, while the abundance and biomass of picophytoplankton varied within the range (0.6-944)×103 (average (127±186)×103) cells/ml and 0.78-204 (average 46.9 ± 53.7) mg C/m3 respectively. Picophytoplankton achieved the highest level of the quantitative development in the eutrophic Gorky Reservoir. Colonial cyanobacteria dominated in the picophytoplankton. They contributed in average 74.0% of the abundance and 70.0% of the biomass of the picophytoplankton. The data obtained testify to the important role of the picoplankton in the food webs of the Volga reservoirs.

The differences among phytoplankton carbon (Cphy) predictions from six ocean color algorithms are investigated by comparison with in situ estimates of phytoplankton carbon. The common satellite data used as input for the algorithms is the Ocean Color Climate Change Initiative merged product. The matching in situ data are derived from flow cytometric cell counts and per-cell carbon estimates for different types of pico-phytoplankton. This combination of satellite and in situ data provides a relatively large matching dataset (N > 500), which is independent from most of the algorithms tested and spans almost two orders of magnitude in Cphy. Results show that not a single algorithm outperforms any of the other when using all matching data. Concentrating on the oligotrophic regions (Chlorophyll-a concentration, B, less than 0.15 mg Chl m−3), where flow cytometric analysis captures most of the phytoplankton biomass, reveals significant differences in algorithm performance. The bias ranges from −35 to +150% and unbiased root mean squared difference from 5 to 10 mg C m−3 among algorithms, with chlorophyll-based algorithms performing better than the rest. The backscattering-based algorithms produce different results at the clearest waters and these differences are discussed in terms of the different algorithms used for optical particle backscattering coefficient (bbp) retrieval.

The cellular size and biomass of picophytoplankton were studied by flow cytometer during spring monsoon (March–May of 2015) in equatorial eastern Indian Ocean. We established an empirical relationship between forward scatter and cellular size to address the size and biomass of picophytoplankton. Results indicated that mean cell diameter of Prochlorococcus (0.60 μm) was the smallest, and then followed by Synechococcus (0.98 μm) and picoeukaryotic phytoplankton (1.05 μm). Thereafter, the biomass converted by abundance reached 0.64 μg·C·L−1 for Prochlorococcus, 0.34 μg·C·L−1 for Synechococcus, and 0.20 μg·C·L−1 for picoeukaryotic phytoplankton. Additionally, the distinct biomass contribution of picophytoplankton appeared to be affected by abundance, but not changes in cellular size. Vertically, the cellular sizes of picophytoplankton were remarkably small in upper waters, which was predominantly controlled by the nutrient availability. In contrast, they were larger in deeper waters, which was primarily attributed to the combined effects of low temperature and reduced light availability. Spatially, under the influence of high nutrient concentration induced by the different circulations and coastal upwelling, slightly high carbon biomass of picophytoplankton was observed around the coastal zones of Sri Lanka island and Sumatra, as well as the southern Bay of Bengal.

Picophytoplankton are acknowledged to contribute significantly to primary production in the ocean while now the method to measure primary production of picophytoplankton at large scales is not yet well established. Although the traditional 14C method and new technologies based on the use of stable isotopes (e.g. 13C) can be employed to accurately measure in situ primary production of picophytoplankton, the time-consuming and labor-intensive shortage of these methods constrain their application in a survey on large spatiotemporal scales. To overcome this shortage, a modified carbon-based ocean productivity model (CbPM) is proposed for estimating the primary production of picophytoplankton whose principle is based on the group-specific abundance, cellular carbon conversion factor and temperature derived growth rate of picophytoplankton. Comparative analysis showed that the estimated primary production of picophytoplankton using CbPM method is significantly and positively related (r2= 0.53, P

In the present work, we review the current knowledge on genesis, limnology and biodiversity of salt lakes distributed around the inner contour of Eastern Carpathian arc (Transylvanian Basin, Central Romania). Transylvanian salt lakes formed on ancient halite (NaCl) deposits following natural processes or quarrying activities. Most of these lakes are located in eastern (Sovata area), southern (Ocna Sibiului), and western (Turda-Cojocna) parts of the Transylvanian Basin, have small surfaces (0.1-4 ha), variable depths (2-100 m), are hypersaline (>10%, w/v, total salts, mainly NaCl) and permanently stratified. As consequence of steady salinity/density gradient, heat entrapment below surface layer (i.e., heliothermy) develops in several Transylvanian lakes. The physical and chemical water stratification is mirrored in the partition of plankton diversity. Lakes with less saline (2-10% salinity) water layers appear to harbor halotolerant representatives of phyto- (e.g., marine native Picochlorum spp. and Synechococcus spp.), zoo- (e.g., Moina salina), and bacterioplankton (e.g., Actinobacteria, Verrucomicobia), whereas halophilic plankton communities (e.g., green algae Dunaliella sp., brine shrimp Artemia sp., and members of Halobacteria class) dominate in the oxic surface of hypersaline (>10% salinity) lakes. Molecular approaches (e.g., PCR-DGGE, 16S rRNA gene-based clone libraries, and DNA metabarcoding) showed that the O2-depleted bottom brines of deep meromictic Transylvanian lakes are inhabited by known extremely halophilic anaerobes (e.g. sulfate-reducing Delta-Proteobacteria, fermenting Clostridia, methanogenic and polymer-degrading archaea) in addition to representatives of uncultured/unclassified prokaryotic lineages. Overall, the plankton communities thriving in saline Transylvanian lakes seem to drive full biogeochemical cycling of main elements. However, the trophic interactions (i.e., food web structure and energy flow) as well as impact of human activities and predicted climate changes are yet to be assessed in these unique ecosystems with little or no match to analogous salt lakes worldwide.

Phytoplanktonic eukaryotes form huge panmictic populations and have evolved original hypervariability mechanisms to resist viral attack., Tiny photosynthetic microorganisms that form the picoplankton (between 0.3 and 3 μm in diameter) are at the base of the food web in many marine ecosystems, and their adaptability to environmental change hinges on standing genetic variation. Although the genomic and phenotypic diversity of the bacterial component of the oceans has been intensively studied, little is known about the genomic and phenotypic diversity within each of the diverse eukaryotic species present. We report the level of genomic diversity in a natural population of Ostreococcus tauri (Chlorophyta, Mamiellophyceae), the smallest photosynthetic eukaryote. Contrary to the expectations of clonal evolution or cryptic species, the spectrum of genomic polymorphism observed suggests a large panmictic population (an effective population size of 1.2 × 107) with pervasive evidence of sexual reproduction. De novo assemblies of low-coverage chromosomes reveal two large candidate mating-type loci with suppressed recombination, whose origin may pre-date the speciation events in the class Mamiellophyceae. This high genetic diversity is associated with large phenotypic differences between strains. Strikingly, resistance of isolates to large double-stranded DNA viruses, which abound in their natural environment, is positively correlated with the size of a single hypervariable chromosome, which contains 44 to 156 kb of strain-specific sequences. Our findings highlight the role of viruses in shaping genome diversity in marine picoeukaryotes.

Highlights: • Picophytoplankton was able to grow under a wide range of environmental conditions varying seasonally. • Picophytoplankton variability was more marked in the bay than in the lagoon. • Picophytoplankton decreased in the lagoon under enhanced anthropogenic stress. • Synechococcus, the most abundant group of the community, was frequently present in both ecosystems during the monitoring. We investigated at the single cell level during 16months (June 2012 to September 2013) the temporal distribution of picophytoplankton (picoeukaryotes, Synechococcus and Prochlorococcus) communities in two contrasted ecosystems: the Bay of Bizerte characterised by an oligotrophic regime typical of the Mediterranean Sea and the Bizerte Lagoon that exhibits a mesotrophic/eutrophic state. We aimed at depicting seasonal variations and quantifying the relationships between the environmental factors and the structure and abundance of picophytoplankton communities. Results showed that picophytoplankton groups were able to grow under a wide range of environmental conditions varying seasonally, although their abundances and contributions to the total chlorophyll biomass significantly varied and showed importance in the Bay of Bizerte. Synechococcus was the most abundant group reaching 225*103 cells·cm-3 in the Bay and 278*103 cells·cm-3 in the lagoon. This group was present all over the year in both ecosystems. Structural equation model results pointed out a different configuration regarding the picophytoplankton environmental drivers. The complexity of the configuration, i.e. number of significant links within the system, decreased under enhanced eutrophication conditions. The less exposure to anthropogenic stress, i.e. in the Bay of Bizerte, highlight a larger role of nutrient and hydrological conditions on the seasonal variations of picophytoplankton, whereas a negative effect of eutrophication on picophytoplankton communities was unveiled in the Bizerte Lagoon. We stress that such influence may be exacerbated under expected scenarios of Mediterranean warming conditions and nutrient release in coastal ecosystems.

Microbial dynamics has been rarely investigated in Lake Geneva, which is paradoxically the largest lake in Western Europe. From a 5 month survey, we report dynamic patterns of free living viruses, bacteria and small phytoplankton abundances in response to a variety of environmental parameters. For the first time, we fractionated the primary production to separate the contribution of different size-related biological compartments and measured both bacterial and viral production in addition to experiments conducted to appreciate virus-induced bacterial mortality. We observed marked seasonal and vertical variations in picocyanobacteria, bacteria and virus abundances and production. The contribution of picoplankton and nanoplankton production to the total primary production was high in November and spring-summer transition period respectively. The impact of viral lysis on both bacteria and picocyanobacteria was significantly higher than grazing activities. Virus-induced picocyanobacterial mortality reached up to 66% compared to virus induced bacterial mortality that reached a maximum of 34% in July. Statistical analyses revealed a complex interplay between biological abundances and/or activity with environmental factors in Lake Geneva. Our results provide new evidence on the critical role played by viruses in freshwater microbial dynamics and more globally on the functioning of the microbial food webs. This study highlights the importance of further considering this biological compartment for a better understanding of the plankton ecology of Lake Geneva, especially for modeling purposes and in a context of reoligotrophication and warming of this ecosystem.

Relationships between sea surface temperature (SST, > 10 m) and vertical density stratification, nutrient concentrations, and phytoplankton biomass, composition, and chlorophyll a (Chl a) specific absorption were assessed in spring and summer from latitudes 29 to 63° N in the northeast Atlantic Ocean. The goal of this study was to identify relationships between phytoplankton and abiotic factors in an existing SST and stratification gradient. Furthermore, a bio-optical model was used to estimate productivity for five phytoplankton groups. Nutrient concentration (integrated from 0 to 125 m) was inversely correlated with SST in spring and summer. SST was also inversely correlated with near-surface (0–50 m) Chl a and productivity for stratified stations. Near-surface Chl a and productivity showed exponential relationships with SST. Chl a specific absorption and excess light experiments indicated photoacclimation to lower irradiance in spring as compared to summer. In addition, Chl a specific absorption suggested that phytoplankton size decreased in summer. The contribution of cyanobacteria to water column productivity of stratified stations correlated positively with SST and inversely with nutrient concentration. This suggests that a rise in SST (over a 13–23 °C range) stimulates productivity by cyanobacteria at the expense of haptophytes, which showed an inverse relationship to SST. At higher latitudes, where rising SST may prolong the stratified season, haptophyte productivity may expand at the expense of diatom productivity. Depth-integrated Chl a (0–410 m) was greatest in the spring at higher latitudes, where stratification in the upper 200 m was weakest. This suggests that stronger stratification does not necessarily result in higher phytoplankton biomass standing stock in this region.

Four surveys (Jun’06 and Nov’06; Feb’07 and May’07) were carried out on the northeastern shelf of the Gulf of Cádiz (southwest Iberian Peninsula) to relate the spatio-temporal distribution of the carbon cycle parameters (dissolved inorganic carbon and dissolved organic carbon) to picophytoplankton biomass and community composition. In addition, the net ecosystem production and the picophytoplankton contribution to the air-sea CO2 exchange process were investigated. The results showed that chlorophyll-a, carbon cycle parameters and picophytoplankton composition showed large seasonality, and the Guadalquivir Estuary plays an important role in the contribution of nutrient and suspended particular material over the year. Regarding picophytoplankton composition, the flow cytometry analysis demonstrated that Prochlorococcus and Synechococcus were the main populations in the studied area and their temporal and spatial distributions were complementary: the Prochlorococcus population showed its maximum concentration in May’07 and Jun’06 and in the surface oceanic water, whereas the Synechococcus population was at its maximum during Feb’07 and Nov’06, and off the Guadalquivir Estuary and Bay of Cádiz. In addition, a relationship between the studied parameters and the fugacity of CO2 was also observed, suggesting that primary production is an important factor in the regulation of this parameter in the studied area. The calculated carbon budget showed that the area acts as a carbon sink on an annual basis. Se llevaron a cabo cuatro campañas oceanográficas (Jun’06 y Nov’06; Feb’07 y May’07) en la plataforma continental noreste del golfo de Cádiz (suroeste de la península Ibérica) para relacionar la distribución espacio-temporal de los parámetros del ciclo del carbono (carbono inorgánico disuelto y carbono orgánico disuelto) con la biomasa picofitoplanctónica y la composición de la comunidad. Además, se investigó la producción neta del ecosistema y la contribución del picofitoplancton al proceso de intercambio atmósfera-agua de CO2. Los resultados mostraron que la clorofila-a, los parámetros del ciclo del carbono y la composición del picofitoplancton presentaron una gran estacionalidad, jugando el estuario del Guadalquivir un papel importante en la contribución de nutrientes y material particulado en suspensión a lo largo del año. Los análisis de citometría demostraron que Prochlorococcus y Synechococcus fueron las principales poblaciones en el área de estudio y que su distribución espacial y temporal fue complementaria: Prochlorococcus presentó la máxima concentración en primavera y verano en aguas superficiales oceánicas, mientras que Synechococcus en invierno y otoño en aguas someras. Además, se observó una relación entre los parámetros estudiados y fugacidad de CO2, lo que sugiere que la producción primaria es un factor importante en la regulación de este parámetro en el área de estudio. El balance de carbono calculado indicó que el área actúa como sumidero de carbono a escala anual.

Although picophytoplankton (PP) (0.2–2 µm) are ubiquitous in lakes and oceans, their importance in rivers has rarely been studied. We examined PP assemblages during the ice-free period in five rivers of a temperate region varying in trophic state (9–107 µg/L total phosphorus) and water discharge (1–87 m3/s). In these rivers, PP abundance reached concentrations as high as those observed in lakes and oceans (~104–105 cells/mL). The highest density of PP (4.9 × 105 cells/mL) was observed in the most eutrophic river when the water temperature (28°C) and total phosphorus (293 µg/L) were highest. For the most part, PP abundance was dominated by non-phycoerythrin-containing cyanobacteria; phycocyanin-rich cells accounted for ~75% of PP abundance in all the rivers. In multiple regression analyses, water temperature and nitrate concentrations explained about half of the variation in PP abundance across the rivers. Discharge had no effect on PP abundance or biomass, whereas it had a significant negative effect on total algal biomass among the rivers. The PP contribution to total chlorophyll-a averaged 27% (ranging 16–46%) and did not decline with increasing nutrients as found in lakes and oceans. The PP biomass from microscopic enumerations reached a maximum of 9% of total phytoplankton biomass, comparable with that observed in lakes. The results of this study demonstrate the importance of including picophytoplankton when analysing phytoplankton communities in rivers.

Phytoplankton in floodplain lakes are frequently characterised as “small” algae but are rarely described with respect to size category (pico-, nano-, or microphytoplankton). Moreover, it is not clear which size category responds first to flood or which category total phytoplankton biomass is most related to. These questions are important in understanding the development of floodplain ecosystems following flooding and were evaluated on the dataset collected from a thermally stratified floodplain lake, Lake Sakadaš (Kopački Rit, Croatia), during spring flood conditions. Relationships of phytoplankton with hydrology and environmental conditions and contribution of each size fraction to total phytoplankton biomass were evaluated using path analysis. At the surface water layer, picophytoplankton developed first, followed by nanophytoplankton and microphytoplankton, suggesting temporal niche separation. Picophytoplankton increased in abundance just before every maxima of chlorophyll concentration, a pattern absent at the bottom water layer where picophytoplankton were a dominant group. The results suggest that analyses of the smallest phytoplankton size category are needed to better understand the controlling mechanisms of whole phytoplankton dynamics in floodplain lakes, with specific emphasis on smallest pico-sized component as “first-responders” to floodplain inundation.

International audience; In this study, we investigated the food sources of eastern oysters Crassostrea virginica cultivated in Atlantic Canada. Stable isotopes (13C and 15N) and fatty acid biomarkers were used to identify these sources under in situ conditions for suspended (∼0.5 m below surface) and bottom (∼2 m) culture stocks. It was found that particulate organic matter represented the main food source, with major contributions from live phytoplankton. Higher lipid contents were detected in the digestive glands of suspended oysters compared to bottom oysters (p 2 μm, 1.05 ± 0.15 μg l−1, mean ± SEM). To determine whether the small size PPP was captured and assimilated by C. virginica, feeding trials were conducted in the laboratory using three PPP/NPP diets (20%, 50%, and 80% PPP), consisting of isotopically-labelled (δ13C) PPP cells (Nannochloropsis oculata) and non-labelled NPP cells (Tisochrysis lutea). An isotopically-labelled fatty acids analysis indicated PPP assimilation in various tissues (digestive gland, gills, mantle, and abductor muscle), including from oysters fed the reduced (20%) PPP diet. Isotopic enrichment (13C) in the FA 22:2 (non-methylene-interrupted or NMI) showed that precursors of NMIs utilized PPP carbon in its biosynthesis process. In conclusion, C. virginica assimilated primarily particulate organic matter (POM), including PPP, which dominated the phytoplankton community in near surface waters. C. virginica can exploit PPP carbon during fatty acid production and further biosynthesis.

© 2017, Pleiades Publishing, Ltd. The molecular diversity of poorly studied algae of Bolidophyceae class was first estimated by Illumina sequencing of V4 region of 18S rRNA gene in ice, under-ice water and summer water of the subarctic White Sea. We used two clustering thresholds–93 and 97%–and revealed 31 phylotypes of Bolidophyceae. Triparma pacifica and Т. strigata were identified to species level. The association of individual phylotypes to certain biotopes (ice or plankton) and stages of seasonal succession (under ice or summer plankton) has been established. Some phylotypes are found in different biotopes and over a wide temperature range. Due to changing their genetic composition, Bolidophyceae are a constant component of the photoautotrophic plankton and ice communities.

Picophytoplankton account for most of the marine (sub-)tropical phytoplankton biomass and primary productivity. The contribution to biomass among plankton functional types (PFTs) could shift with climate warming, in part as a result of different physiological responses to temperature. To model these responses, Eppley's empirical relationships have been well established. However, they have not yet been statistically validated for individual PFTs. Here, we examine the physiological response of nine strains of picophytoplankton to temperature; three strains of picoprokaryotes and six strains of picoeukaryotes. We conduct laboratory experiments at 13 temperatures between –0.5 and 33°C and measure the maximum growth rates and the chlorophyll a to carbon ratios. We then statistically validate two hypotheses formulated by Eppley in 1972: The response of maximum growth rates to temperature (1) of individual strains can be represented by an optimum function, and (2) of the whole phytoplankton group can be represented by an exponential function Eppley (1972). We also quantify the temperature-related parameters. We find that the temperature span at which growth is positive is more constrained for picoprokaryotes (13.7–27°C), than for picoeukaryotes (2.8–32.4°C). However, the modeled temperature tolerance range (ΔT) follows an unimodal function of cell size for the strains examined here. Thus, the temperature tolerance range may act in conjunction with the maximum growth rate to explain the picophytoplankton community size structure in correlation with ocean temperature. The maximum growth rates obtained by a 99th quantile regression for the group of picophytoplankton or picoprokaryotes are generally lower than the rates estimated by Eppley. However, we find temperature-dependencies (Q10) of 2.3 and of 4.9 for the two groups, respectively. Both of these values are higher than the Q10 of 1.88 estimated by Eppley and could have substantial influence on the biomass distribution in models, in particular if picoprokaryotes were considered an independent PFT. We also quantify the increase of the chlorophyll a to carbon ratios with increasing temperature due to acclimation. These parameters provide essential and validated physiological information to explore the response of marine ecosystems to a warming climate using ocean biogeochemistry models.

The species composition and size-structure of the phytoplanktoncommunity in the Boka Kotorska Bay (SE Adriatic Sea) were analysed withrespect to abundance and carbon biomass, together with the physico-chemicalparameters, with the aim of evaluating the predefined oligo-mesotrophicstatus of this transitional water ecosystem. Three stations locatedin the inner part of the Bay were sampled with seasonal frequencyin 2008/2009. Picophytoplankton cells were counted using flowcytometry; nanophytoplankton and microphytoplankton were identifiedand counted by light microscopy. The relative importance of thepicoplankton in the Bay, in terms of both abundance and biomass,during all the investigated seasons emphasized their significancein the phytoplankton community. Picocyanobacteria (Synechococcus)constituted a significant part of the summer assemblages withregard to both abundance (up to 3.38 × 108 cellsL-1)and carbon biomass (up to 73% of total phytoplankton carbon).The contribution of the nanophytoplankton was found to be generallylow (Skeletonema marinoi, dominated the microphytoplanktonfraction. S. marinoi was the most abundant in spring/winter(up to 2.86 imes 106 cells L-1) above the halocline(making a 96% contribution to the microphytoplankton). The potentiallytoxin-producing diatom Pseudo-nitzschia pseudodelicatissimawas recorded at abundances greater than 105 cells L-1, togetherwith Thalassionema frauenfeldii, as well as the dinoflagellatesProrocentrummicans and the potentially harmful P. minimum. The higher valuesof phytoplankton biomass and the dominance of phytoplankton speciesor groups with preferences for nutrient-enriched conditions appear tobe consistent with the oligo-mesotrophic status of this specific ecosystem.

The species composition and size-structure of the phytoplankton community in the Boka Kotorska Bay (SE Adriatic Sea) were analysed with respect to abundance and carbon biomass, together with the physico-chemical parameters, with the aim of evaluating the predefined oligo-mesotrophic status of this transitional water ecosystem. Three stations located in the inner part of the Bay were sampled with seasonal frequency in 2008/2009. Picophytoplankton cells were counted using flow cytometry; nanophytoplankton and microphytoplankton were identified and counted by light microscopy. The relative importance of the picoplankton in the Bay, in terms of both abundance and biomass, during all the investigated seasons emphasized their significance in the phytoplankton community. Picocyanobacteria (Synechococcus) constituted a significant part of the summer assemblages with regard to both abundance (up to 3.38×108 cells L−1) and carbon biomass (up to 73% of total phytoplankton carbon). The contribution of the nanophytoplankton was found to be generally low (

Changes in abundance and biomass of Synechococcus were studied during five cruises carried out between November 2004 and September 2005 in the Iskenderun Bay, northeastern Mediterranean. In addition, spatial and temporal variations in physico-chemical factors and Chl a were measured. Abundance ranged from 0.2×104 to 23.09104 cells ml-1, whereas the biomass ranged from 0.19 1 to 23.01 µgC l-1 in the bay. Synechococcus was found most numerous during September 2005 in the shallower part of the bay while the population was observed least during November 2004 and January 2005. The contribution of

14 pages,11 figures The major source of reduced sulfur in the remote marine atmosphere is the biogenic compound dimethylsulfide (DMS), which is ubiquitous in the world’s oceans and released through food web interactions. Relevant fluxes and concentrations of DMS, its phytoplankton-produced precursor, dimethylsulfoniopropionate (DMSP) and related parameters were measured during an intensive Lagrangian field study in two mesoscale eddies in the Sargasso Sea during July–August 2004, a period characterized by high mixed-layer DMS and low chlorophyll—the so-called ‘DMS summer paradox’. We used a 1-D vertically variable DMS production model forced with output from a 1-D vertical mixing model to evaluate the extent to which the simulated vertical structure in DMS and DMSP was consistent with changes expected from field-determined rate measurements of individual processes, such as photolysis, microbial DMS and dissolved DMSP turnover, and air–sea gas exchange. Model numerical experiments and related parametric sensitivity analyses suggested that the vertical structure of the DMS profile in the upper 60 m was determined mainly by the interplay of the two depth variable processes—vertical mixing and photolysis—and less by biological consumption of DMS. A key finding from the model calibration was the need to increase the DMS(P) algal exudation rate constant, which includes the effects of cell rupture due to grazing and cell lysis, to significantly higher values than previously used in other regions. This was consistent with the small algal cell size and therefore high surface area-to-volume ratio of the dominant DMSP-producing group—the picoeukaryotes. We gratefully acknowledge the financial assistance provided through NSF Biocomplexity funding (OPP-0083078) and an Australian Research Council Discovery Grant. We are grateful to the comments by D.J. Kieber. We recognize the participation and help of K. Bailey, J. Bisgrove, B. Blomquist, I. Forn, H. Harada, B. Huebert, D. Jones, L. Maroney, A. Neely, S. Riseman, C. Smith, J. Stefels, K. Tinklepaugh, M. Vila-Costa, G. Westby, H. Zemmelink and the R/V Seward Johnson crew. DiTullio et al., 2001; Simo ́ and Dachs, 2002; Simon and Azam, 1989; Zemmelink et al., 2005

Исследовали структуру и трофические взаимодействия в планктонном микробном сообществе Шекснинского водохранилища (Верхняя Волга). Для этого в августе 2007 г. определяли численность и биомассу основных компонентов микробной трофической сети: гетеротрофных бактерий, фототрофного пико-и нанопланктона, гетеротрофных нанофлагеллят, инфузорий и вирусов, а также продукцию фито-и бактериопланктона, и выедание бактерий флагеллятами и их лизис вирусами. Биомасса микробного сообщества на разных участках водохранилища изменялась в пределах 170-282 (в среднем 221 мг С/м3) и составляла 26.2-64.3% (в среднем 45.5%) общей биомассы планктона. Гетеротрофные бактерии были главным компонентом микробного сообщества (занимали в среднем 63.9% его биомассы) и вторым по значимости компонентом планктонного сообщества (в среднем 28.6%). Основной вклад в формирование общей биомассы планктона вносил фитопланктон (в среднем 39.2%) однако на некоторых участках водохранилища биомасса бактериопланктона превышала таковую фитопланктона. Отношение интегральных значений продукции гетеротрофных бактерий и первичной продукции планктона было высоким и в среднем для водохранилища составило 0.9. Это свидетельствует о важной роли в планктонных трофических сетях водохранилища гетеротрофных бактерий, метаболизирующих аллохтонные органические вещества. В водной толще было зарегистрировано 34 вида гетеротрофных нанофлагеллят из 15 крупных таксонов и 15 видов инфузорий из 4 классов. В среднем флагелляты выедали 24.7%, а вирусы лизировали 11.7% суточной продукции бактериопланктона., Structure and trophic interactions in the planktonic microbial community of the Sheksna reservoir (Upper Volga) were studied in August 2007. For this purpose abundance and biomass of main components of the microbial food web: heterotrophic bacteria, phototrophic pico-and nanoplankton, heterotrophic nanoflagellates, ciliates and viruses, as well as the phytoplankton and bacterial production and bacterivory by nanoflagellates and bacterial lysis by viruses were determined. In the different parts of the reservoir the biomass of microbial community was varied from 170 to 282 (mean 221 mg C/m3), which was from 26.2 to 64.3% (mean 45.5%) of the total plankton biomass. Heterotrophic bacteria were the main component of the microbial community (mean 63.9% of the biomass) and the second most important component of plankton (mean 28.6%). Phytoplankton made the main contribution to the total plankton biomass (mean 39.2%), but the bacterial biomass exceeded the phytoplankton biomass in some parts of the reservoir. The ratio of areal heterotrophic bacterial and primary phytoplankton production was high and averaged 0.9. It suggests an importance of heterotrophic bacteria actively using allochthonous organic substances in the planktonic food webs of the reservoir. 34 species of heterotrophic nanoflagellates from 15 taxa and 15 species of ciliates from 4 classes were identified in the water column. On average, flagellates consumed 24.7% and viruses lysed 11.7% of the daily bacterial production.

This deliverable is composed of two reports which propose indice of phytoplankton composition, based respectively on the results of pigment and genetic diversity methods, and the results of microscopy method. Proposals based on the results of pigment and genetic diversity methods. The present work is a continuation of several studies carried out in the past (Siano and Delmas, 2013, 2015) on investigating an indicator of phytoplankton diversity using two methods complementary to optical microscopy: the genetic diversity analyzed with metabarcoding (exhaustive analysis of the diversity and relative abundance of OTUs, i.e. groups of sequences identified by their genetic resemblance) and the pigmentary diversity (analysis of the diversity and abundance of pigments produced by photosynthetic cells). As part of the present study we aim to (i) to deepen our knowledge on the spatio‐temporal structure of phytoplankton assemblages in relation with environmental forcings for three size classes (0.2‐3 μm, 3‐20μm, >20μm, here respectively defined by convention as picophytoplankton, nanophytoplankton and microphytoplankton), and (ii) to verify the hypothesis that certain groups of the phytoplankton could be representative and descriptor of their corresponding size class. This work relies on data collected during four sampling campaigns: three of them at spatial scale (Pelgas 2012, 2013, and Phytec campaigns) and one at temporal scale (Dynapse). During Pelgas 2012 and 2013 campaigns, the sampling was done in the Gironde plume during May, on three coast‐tooffshore transects. The Phytec campaign was carried out in the Iroise sea (Bay of Brest) and the south area of Britain, including the Loire plume, bays of Quiberon, Vilaine, and Concarneau. Finally, the Dynapse campaign was carried out in the bay of Concarneau, between March and July 2012. The statistical analyses performed on metarbarcoding data show a clear separation of the three size classes studied and suggest that picophytoplankton is less variable in the time space than nano‐ and microphytoplankton. This is related to the recurrent presence in this size fraction of some OTUs such as Micromonas sp, Ostreococcus sp and Chrysochromulina rotalis. Within picophytoplankton, we found a good correlation between certain OTUs of the Mamiellophyceae group and chlorophyll b, the pigment characteristic of this class of microalgae. This correspondence confirms the good intercalibration of pigment and genetic diversity data. Chlorophyll b appeared to be a good descriptor of biomass and variability of picophytoplankton. Indeed, within the picophytoplankton, chlorophyll b is positively correlated to the total chlorophyll a, the latter being an indicator of the total picophytoplankton biomass. The phytoplankton variability, studied through the genetic and pigmentary analysis, is not explained by measured environmental parameters, certainly because of insufficiently pronounced environmental gradients during the sampling campaigns. Given the results obtained, we recommend testing the relevance of OTUs (especially those identified within picophytoplankton) and chlorophyll b as descriptors of the state of water masses on a more contrasted gradient of pressures. A sampling should be carried out on a well‐established environmental gradient beforehand in order to test the hypothesis that chlorophyll b and relative abundances of OTUs Micromonas and Bathycoccus prasinos could be used for the detecting and quantifying environmental conditions that fall anthropogenic influences. Proposals based on the results of microscopy method. As part of the implementation of the Water Framework Directive (WFD), the evaluation of the ecological status of coastal waters is based in particular on the monitoring of phytoplankton composition, indice not defined to date in France. As part of the observation and surveillance network for phytoplankton and hydrology (REPHY), established in 1984, counts of all phytoplankton species sampled in water are made by optical microscopy. This study aims to propose one or more indices of phytoplankton composition, based on counts made in microscope, to evaluate the quality of coastal waters of the Channel‐Atlantic coast, as part of the WFD. In their article, Spatharis and Tsirtsis (2010) propose a method for the selection of composition indices, based on the study of the monotonicity and linearity behavior of the relationship between the indice and the logarithm of the total abundance. In this study, the list of indices and the selection method proposed by Spatharis and Tsirtsis (2010) are taken and applied to the data from 16 monitoring locations spread across the Channel‐Atlantic coast. This study selected six indices: the indices "Odum", "Menhinick", "Camargo", "Sheldon," "E3" and "Simpson E." Further study of the relationship indice vs pressures, other than total abundance, using other statistical tools would be needed to confirm the relevance of these indices. In addition, for the indice to be operational within the WFD framework, additional work on temporal aggregation of measurements, on the definition of reference values and therefore on the calculation of EQR, are still required in connection with thematic experts., Ce livrable est composé de deux rapports faisant des propositions pour un indice de composition du phytoplancton, basés respectivement sur les résultats des méthodes de diversité pigmentaire et génétique, et sur les résultats de la méthode microscopie. Propositions basées sur les résultats des méthodes de diversité pigmentaire et génétique (19 pages, pages 13 à 31 du document pdf). Ce travail s’inscrit dans la continuité des études effectuées précédemment (Siano et Delmas, 2013, 2015) sur la recherche d’un indicateur de diversité du phytoplancton en utilisant deux méthodes complémentaires à la microscopie optique : la diversité génétique analysée par approche de metabarcoding (analyse exhaustive de la diversité et abondance relative des OTUs, i.e. groupes des séquences identifiés pour leur ressemblance génétique) et la diversité pigmentaire (analyse de la diversité et de l’abondance des pigments produits par les cellules photosynthétiques). Dans le cadre de cette étude nous avons cherché à : (i) approfondir nos connaissances sur la structure spatiotemporelle des assemblages phytoplanctoniques en rapport aux forçages environnementaux pour trois classes de taille (0,2‐3 μm, 3‐20μm, >20μm, ici définies par convention respectivement picophytoplancton, nanophytoplancton et microphytoplancton) et (ii) vérifier l’hypothèse que certains groupes du phytoplancton pourraient être représentatifs et descripteurs de leur classe de taille correspondante. Cette étude s’est basée sur les données recueillies lors de quatre campagnes d’échantillonnage : trois d’entre elles à l’échelle spatiale (campagnes Pelgas 2012, 2013 et campagne Phytec) et une à l’échelle temporelle (Dynapse). Lors des campagnes Pelgas 2012 et 2013, l’échantillonnage a été réalisé dans le panache de la Gironde au mois de mai, sur trois radiales côte‐large. La campagne Phytec a été effectuée en juin dans la mer d’Iroise (rade de Brest) et la zone Bretagne sud, incluant le panache de la Loire, les Baies de Quiberon et de la Vilaine et la Baie de Concarneau. Enfin, la campagne Dynapse a été réalisée dans la Baie de Concarneau, entre mars et juillet 2012. Les analyses statistiques effectuées sur les données de metabarcoding montrent une nette séparation des trois classes de taille étudiées et suggèrent que le picophytoplancton est moins variable dans l’espace‐temps que le nano‐ et le microphytoplancton. Ceci est lié à la présence récurrente et proportionnellement plus abondante dans cette fraction de taille de certains OTUs tels que Micromonas, Ostreococcus (Mamiellophyceae) et Chrysochromulina rotalis (Haptophyceae). Au sein du picophytoplancton nous avons retrouvé une bonne correspondance entre certains OTUs du groupe des Mamiellophyceae et la chlorophylle b, le pigment caractéristique de cette classe de microalgues. Cette correspondance conforte la bonne inter‐calibration des données de diversité pigmentaire et génétique. La chlorophylle b s’est avéré être un bon descripteur de la biomasse et de la variabilité du picophytoplancton. En effet, au sein du picophytoplancton, la chlorophylle b est positivement corrélée à la chlorophylle a totale, cette dernière étant elle‐même indicatrice de la biomasse picophytoplanctonique totale. La variabilité du phytoplancton, étudiée par l’analyse génétique et des pigments, n’est pas expliquée par les paramètres environnementaux mesurés, certainement à cause de gradients environnementaux insuffisamment prononcés retrouvés au moment de l’échantillonnage des campagnes. Au vue des résultats obtenus, nous recommandons de tester la pertinence des OTUs (notamment ceux identifiés au sein du picophytoplancton) et de la chlorophylle b en tant que descripteurs de l’état des masses d’eau sur un gradient plus contrasté en termes de pressions. Il faudrait donc réaliser un échantillonnage sur un gradient environnemental bien établi au préalable afin de tester l’hypothèse selon laquelle la chlorophylle b et les abondances relatives des OTUs Micromonas et Bathycoccus prasinos peuvent être utilisées pour la détection et la quantification des conditions environnementales qui relèvent des influences anthropiques. Propositions basées sur les résultats de la méthode microscopie (37 pages, pages 32 à 68 du document pdf). Dans le cadre de la mise en oeuvre de la Directive Cadre sur l'Eau (DCE), l'évaluation de l'état écologique des eaux littorales repose notamment sur le suivi de la composition du phytoplancton, indice non défini à ce jour en France. Dans le cadre du réseau d'observation et de surveillance du phytoplancton et de l’hydrologie (REPHY), mis en place en 1984, des dénombrements de l'ensemble des espèces phytoplanctoniques échantillonnées dans l'eau sont réalisés par microscopie optique. La présente étude a pour objectif de proposer un ou plusieurs indice(s) de composition phytoplanctonique, basé(s) sur les dénombrements réalisés au microscope, pour évaluer la qualité des eaux littorales de la façade Manche‐Atlantique, dans le cadre de la DCE. Dans leur article, Spatharis et Tsirtsis (2010) proposent une méthode pour la sélection d'indices de composition, basée sur l'étude de la monotonicité et de la linéarité de la relation entre l'indice et le logarithme de l'abondance totale. Dans la présente étude, la liste d'indices et la méthode de sélection proposées par Spatharis et Tsirtsis (2010) sont reprises et appliquées aux données issues de 16 lieux de surveillance, répartis sur l'ensemble de la façade Manche‐Atlantique. Cette étude a permis de sélectionner six indices : les indices « Odum », « Menhinick », « Camargo », « Sheldon », « E3 » et « Simpson E ». Une étude plus approfondie de la relation indice vs pressions, autre que l'abondance totale, au moyen d'autres outils statistiques serait nécessaire pour confirmer la pertinence de ces indices. De plus, pour que l'indice soit opérationnel dans le cadre de la DCE, des travaux complémentaires relatifs à l’agrégation temporelle des mesures, à la définition des valeurs de référence et par conséquent au calcul des EQR restent à faire, en lien avec les experts thématiques.

Studies on phytoplankton community diversity cannot be limited microphytoplankton (cells>20 ìm), it is indeed necessary to take into account also the nano (3-20ìm) picophytoplankton (, Afin d'appréhender la diversité du phytoplancton dans toutes ses composantes, on ne peut plus se limiter au seul micro-phytoplancton, il est indispensable de considérer le nano et le pico-phytoplancton. Pour l’étude de la biodiversité des communautés phytoplanctoniques, deux types d’approches alternatives et complémentaires à l’approche morphologique classique obtenue par microscope optique ont été utilisés : diversité pigmentaire et diversité génétique par approche de metabarcoding. Le point REPHY (047-P-016 Concarneau Large) situé en baie de Concarneau a été échantillonné pendant la période la plus importante de développement du phytoplancton (mars- juillet 2012) avec une fréquence bihebdomadaire. Les résultats obtenus par les méthodes d’analyses pigmentaires et de metabarcoding montrent une bonne résolution de la biodiversité phytoplanctonique, pouvant représenter au niveau des groupes ou familles (diversité pigmentaires) ou au niveau des espèces ou des clades génétiques (metabarcoding) toutes les classe de taille du phytoplancton y compris celles du pico (

Flow cytometry was used to describe seasonal cycles of Prochlorococcus (Prochl), Synechococcus (Syn), picoeukaryotes and heterotrophic bacteria in the central Adriatic Sea along the trophic gradient from January to December 2010. All picoplankton parameters decreased from eutrophic to oligotrophic areas, while the biomass ratio of bacterial to autotrophic picoplankton showed an increase along the trophic gradient. Bacterial biomass ranged from 5.28 to 21.20 μg C l-1. Increased values were present during warmer seasons with the domination of the low nucleic acid (LNA) group of bacteria. The high nucleic acid (HNA) bacterial group dominated during winter and spring. Bacterial production ranged from 0.09 -0.45 × 104 cells ml-1 h-1 . At coastal stations, increased production was present during the winter and spring and was more or less uniform at open sea stations. The biomass of Syn and Prochl ranged from 0.16 to 11.47 µg C-1 l-1 and from 0.01 to 3.08 µg C l-1, respectively. They were elevated during the summer and the autumn at coastal stations and during late winter at the open sea. Syn biomass always dominated over Prochl, with 61.6-97.2% participation in the biomass of cyanobacteria. The biomass of picoeukaryotes ranged from 1.21 to 21.85 µg C l-1 and was the highest during the winter. Their biomass notably prevailed in autotrophic picoplankton (APP) biomass over that of picocyanobacteria during the whole year. Autotrophic components (Prochl, Syn and picoeukaryotes) made a greater contribution to picoplankton biomass in mesotrophic and eutrophic areas, while heterotrophic bacteria became more important under oligotrophic conditions.

The understanding of ecosystem functioning occurs, in large part, through knowledge of their interactions within food webs, in which occurs the flow of matter and energy. Studies of interactions between its components are fundamental to the understanding of food chains, given its importance to nutrient cycling, accumulation of biomass and carbon flux. These organisms can be strongly affected by the availability of resources and the effect of predation on aquatic environments. We evaluated the influence of bottom-up and top-down control mechanisms on the abundance and size structure of planktonic protists (flagellates and ciliates), and hypothesized that bottom-up would control the abundance while top-down would affect the size of those organisms. As predicted, resources were the most important variables determining the abundance of protists, mostly by picophytoplankton and bacteria. The size structure was influenced by both resources and predation. Besides, resource contribution to protist biomass was mainly through herbivory in all phytoplankton sizes, both at the surface and the bottom of the environments. A compreensão do funcionamento dos ecossistemas ocorre, em grande parte, por meio do conhecimento das interações dentro de suas teias alimentares, pelas quais ocorre o fluxo de matéria e energia. As abordagens das interações entre seus componentes são fundamentais para a compreensão das cadeias alimentares, dada a sua importância para a ciclagem de nutrientes, acumulo de biomassa e fluxo de carbono. Estes organismos podem ser fortemente afetados pela disponibilidade dos recursos e pelo efeito da predação em ambientes aquáticos. A influência dos mecanismos de controle ascendente e descendente sobre a abundância e a estrutura de tamanho das comunidades de protozoários planctônicos (flagelados e ciliados) foi avaliada, com a hipótese de que os mecanismos de controle ascendentes controlam a abundância das comunidades de protozoários enquanto que mecanismos de controle descendentes controlam o padrão de tamanho destes organismos. Como foi predito, os recursos parecem ser as variáveis mais importantes na determinação da abundância dos protozoários, representados principalmente pelo picofitoplâncton e bactérias. Já para o tamanho, tanto os recursos quanto os predadores tiveram importante papel na estruturação das comunidades. Além disso, a contribuição de recursos para a biomassa dos protozoários se deu principalmente pela herbivoria em todas as frações do fitoplâncton, tanto da superfície quanto no fundo dos ambientes. 45 f

The abundance and distribution of total autotrophic picophytoplankton (PFP), temperature, salinity, PAR, and chlorophyll a were determined in two presumably contrasting environments: (1) two coastal areas (close to the mouths of three rivers), and (2) one oceanic area (Campeche Canyon), of the southern Gulf of Mexico, during the "dry season" (June-July, 2004). The picoprokaryotes Prochlorococcus and Synechococcus were identified by TEM, whereas Synechococcus and picoeukaryotes populations were also recognized by flow cytometry. The highest PFP abundance (1.67×105 cells ml-1) was found in shallow waters (~10 m depth) around the Grijalva-Usumacinta river mouth, followed by that found at a station close to the Coatzacoalcos River (1.19×105 cells ml-1); PFP abundances in the Campeche Canyon were usually lower (maximum 1.53×104 cells ml-1). Greater variability in PFP abundances was found in coastal stations than in oceanic waters, and weak relationships appeared between the patterns of chlorophyll a and PFP abundance. Peaks of PFP were detected in both coastal and more oceanic areas, but in the Campeche Canyon they were located deeper (60 m), relatively closer to the deep maximum of chlorophyll (located at about 75 m). Results suggest that PFP populations include a substantial photosynthetic component in both coastal and oceanic waters of the southern Gulf of Mexico. Abundância e distribuição do picofitoplâncton autotrófico total (PFP), temperatura, salinidade, PAR e clorofila-a, foram determinados em dois ambientes presumivelmente diferentes: (1) duas áreas costeiras (perto da foz de três rios) e (2) uma área oceânica (Campeche Canyon), ambas situadas ao sul do Golfo do México, durante a "estação seca" (Junho-Julho, 2004). Os picoprocariontes Prochlorococcus e Synechococcus foram identificados por TEM, e as populações de Synechococcus e de picoeucariontes também foram reconhecidas por citometria de fluxo. A maior abundância de PFP (1,67 × 105 células ml-1) foi encontrada em águas rasas (~ 10 m de profundidade) em torno dos rios Grijalva Usumacinta, seguida de uma estação perto do Rio Coatzacoalcos (1,19 × 105 células ml-1). As abundâncias de PFP em Campeche Canyon foram geralmente menores (máximo 1,53 × 104 células ml-1). A maior variabilidade em abundâncias de PFP foi encontrada em estações costeiras quando comparado às águas oceânicas, e quase não houve correlação entre os padrões de clorofila-a e abundância de PFP. Picos de PFP foram detectados nas áreas costeiras e oceânicas, mas em Campeche Canyon localizaram-se em maior profundidade (60 m), relativamente mais perto do local onde se registrou o máximo de clorofila (cerca de 75 m). Os resultados sugerem que as populações de PFP englobam um componente fotossintético substancial em ambas as águas costeiras e oceânicas do sul do Golfo do México.

Four surveys (Jun’06 and Nov’06; Feb’07 and May’07) were carried out on the northeastern shelf of the Gulf of Cádiz (southwest Iberian Peninsula) to relate the spatio-temporal distribution of the carbon cycle parameters (dissolved inorganic carbon and dissolved organic carbon) to picophytoplankton biomass and community composition. In addition, the net ecosystem production and the picophytoplankton contribution to the air-sea CO2 exchange process were investigated. The results showed that chlorophyll-a, carbon cycle parameters and picophytoplankton composition showed large seasonality, and the Guadalquivir Estuary plays an important role in the contribution of nutrient and suspended particular material over the year. Regarding picophytoplankton composition, the flow cytometry analysis demonstrated that Prochlorococcus and Synechococcus were the main populations in the studied area and their temporal and spatial distributions were complementary: the Prochlorococcus population showed its maximum concentration in May’07 and Jun’06 and in the surface oceanic water, whereas the Synechococcus population was at its maximum during Feb’07 and Nov’06, and off the Guadalquivir Estuary and Bay of Cádiz. In addition, a relationship between the studied parameters and the fugacity of CO2 was also observed, suggesting that primary production is an important factor in the regulation of this parameter in the studied area. The calculated carbon budget showed that the area acts as a carbon sink on an annual basis., Se llevaron a cabo cuatro campañas oceanográficas (Jun’06 y Nov’06; Feb’07 y May’07) en la plataforma continental noreste del golfo de Cádiz (suroeste de la península Ibérica) para relacionar la distribución espacio-temporal de los parámetros del ciclo del carbono (carbono inorgánico disuelto y carbono orgánico disuelto) con la biomasa picofitoplanctónica y la composición de la comunidad. Además, se investigó la producción neta del ecosistema y la contribución del picofitoplancton al proceso de intercambio atmósfera-agua de CO2. Los resultados mostraron que la clorofila-a, los parámetros del ciclo del carbono y la composición del picofitoplancton presentaron una gran estacionalidad, jugando el estuario del Guadalquivir un papel importante en la contribución de nutrientes y material particulado en suspensión a lo largo del año. Los análisis de citometría demostraron que Prochlorococcus y Synechococcus fueron las principales poblaciones en el área de estudio y que su distribución espacial y temporal fue complementaria: Prochlorococcus presentó la máxima concentración en primavera y verano en aguas superficiales oceánicas, mientras que Synechococcus en invierno y otoño en aguas someras. Además, se observó una relación entre los parámetros estudiados y fugacidad de CO2, lo que sugiere que la producción primaria es un factor importante en la regulación de este parámetro en el área de estudio. El balance de carbono calculado indicó que el área actúa como sumidero de carbono a escala anual.