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The Arctic warming leads to a decline in sea-ice extent and thickness, rapid warming and freshening of the sea surface which impact the distribution of phytoplankton size composition. Picophytoplankton is an ecologically important component of Arctic pelagic marine ecosystems, and its role may be altered by global warming. In this study, the abundance and biomass, the chlorophyll a (Chl-a) and primary production (PP) of picophytoplankton, and its spatial and temporal distribution were investigated in the Kara Sea during the ice-melt season in July 2019. Picophytoplankton played a major role in the surface PP in the southern and western areas of the Kara Sea. In the surface layer, the contribution of picophytoplankton to total Chl-a increased insignificantly, and the contribution of picophytoplankton to total PP decreased significantly with the time of sea ice retreat. In the euphotic zone, the Chl-a concentration of picophytoplankton and its contribution to total Chl-a decreased with the time of sea ice retreat. The average picophytoplankton biomass determined in the present study (2.72 ± 5.10 mg C m -3 ) corresponded to the biomass estimates in the Arctic. The picophytoplankton community was strongly dominated by eukaryotes, cyanobacteria were only detected at 3 out of 11 stations, with maximum abundances (0.07 × 10 9  cells m -3 ) observed at depths below 15 m. The obtained results contribute significantly to the study of the picophytoplankton dynamics during the ice-melting season in the hard-to-reach Kara Sea., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

In estuaries, phytoplankton are faced with strong environmental forcing (e.g. high turbidity, salinity gradients). Taxa that appear under such conditions may play a critical role in maintaining food webs and biological carbon pumping, but knowledge about estuarine biota remains limited. This is also the case in the Elbe estuary where the lower 70 km of the water body are largely unexplored. In the present study, we investigated the phytoplankton composition in the Elbe estuary via metabarcoding. Our aim was to identify key taxa in the unmonitored reaches of this ecosystem and compare our results from the monitored area with available microscopy data. Phytoplankton communities followed distinct seasonal and spatial patterns. Community composition was similar across methods. Contributions of key classes and genera were correlated to each other (p < 0.05) when obtained from reads and biovolume (R 2  = 0.59 and 0.33, respectively). Centric diatoms (e.g. Stephanodiscus) were the dominant group - comprising on average 55 % of the reads and 66-69 % of the biovolume. However, results from metabarcoding imply that microscopy underestimates the prevalence of picophytoplankton and flagellates with a potential for mixotrophy (e.g. cryptophytes). This might be due to their small size and sensitivity to fixation agents. We argue that mixotrophic flagellates are ecologically relevant in the mid to lower estuary, where, e.g., high turbidity render living conditions rather unfavorable, and skills such as phagotrophy provide fundamental advantages. Nevertheless, further findings - e.g. important taxa missing from the metabarcoding dataset - emphasize potential limitations of this method and quantitative biases can result from varying numbers of gene copies in different taxa. Further research should address these methodological issues but also shed light on the causal relationship of taxa with the environmental conditions, also with respect to active mixotrophic behavior., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

Picophytoplankton are a ubiquitous component of marine plankton communities and are expected to be favored by global increases in seawater temperature and stratification associated with climate change. Eukaryotic and prokaryotic picophytoplankton have distinct ecology, and global models predict that the two groups will respond differently to future climate scenarios. At a nearshore observatory on the Northeast US Shelf, however, decades of year-round monitoring have shown these two groups to be highly synchronized in their responses to environmental variability. To reconcile the differences between regional and global predictions for picophytoplankton dynamics, we here investigate the picophytoplankton community across the continental shelf gradient from the nearshore observatory to the continental slope. We analyze flow cytometry data from 22 research cruises, comparing the response of picoeukaryote and Synechococcus communities to environmental variability across time and space. We find that the mechanisms controlling picophytoplankton abundance differ across taxa, season, and distance from shore. Like the prokaryote, Synechococcus, picoeukaryote division rates are limited nearshore by low temperatures in winter and spring, and higher temperatures offshore lead to an earlier spring bloom. Unlike Synechococcus, picoeukaryote concentration in summer decreases dramatically in offshore surface waters and exhibits deeper subsurface maxima. The offshore picoeukaryote community appears to be nutrient limited in the summer and subject to much greater loss rates than Synechococcus. This work both produces and demonstrates the necessity of taxon- and site-specific knowledge for accurately predicting the responses of picophytoplankton to ongoing environmental change., (© 2024 The Authors. Global Change Biology published by John Wiley & Sons Ltd.)

Phytoplankton are usually considered autotrophs, but an increasing number of studies show that many taxa are able also to use organic carbon. Acquiring nutrients and energy from different sources might enable an efficient uptake of required substances and provide a strategy to deal with varying resource availability, especially in highly dynamic ecosystems such as estuaries. In our study, we investigated the effects of 31 organic carbon sources on the growth (proxied by differences in cell counts after 24 h exposure) of 17 phytoplankton strains from the Elbe estuary spanning four functional groups. All of our strains were able to make use of at least 1 and up to 26 organic compounds for growth. Pico-sized green algae such as Mychonastes , as well as the nano-sized green alga Monoraphidium in particular were positively affected by a high variety of substances. Reduced light availability, typically appearing in turbid estuaries and similar habitats, resulted in an overall poorer ability to use organic substances for growth, indicating that organic carbon acquisition was not primarily a strategy to deal with darkness. Our results give further evidence for mixotrophy being a ubiquitous ability of phytoplankton and highlight the importance to consider this trophic strategy in research.

We explored picophytoplankton in the surface (0 m) and bottom (2.3-8.7 m) layers of a shallow (<10 m) eutrophic coastal system (Isahaya Bay, Japan). We found that picophytoplankton (principally Synechococcus) constituted the major phytoplankton in spring and summer. The chlorophyll a (chl.a) concentration in the 0.7-2.0-μm picophytoplankton fraction (hereinafter 'pico-sized chl. a') and picophytoplankton abundance in Isahaya Bay were higher than those in other eutrophic coastal waters. The pico-sized chl. a concentration and the picophytoplankton abundance counted with an epifluorescence microscope was up to 49.31 μg L -1 and 1.9 × 10 6  cells mL -1 , respectively. Higher contributions of pico-sized chl. a to the total chl. a were evident in summer (up to 63.5%), relative to spring (up to 32.1%), at both depths. Picophytoplankton abundance and the pico-sized chl. a concentration was positively correlated with water temperature and dissolved inorganic phosphorus (DIP) concentrations. Thus, both temperature and DIP may be major controllers of picophytoplankton in Isahaya Bay. The pico-sized chl. a concentration and picophytoplankton cell number at the bottom layer were positively correlated with those in the surface layer, suggesting that picophytoplankton in bottom layers may have sunk from the surface layers. The results imply that the picophytoplankton affects the biogeochemical processes in the bottom of Isahaya Bay more than previously thought. This may be true not only for this estuary but also for other eutrophic coastal seas., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

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. TiO 2 and ZnO nanoparticles (NPs) are commonly used as inorganic UV filters. In this study, we explored the effects of TiO 2 and ZnO NPs on natural phytoplankton assemblages in coastal seawater. Growth rates of natural phytoplankton assemblages were significantly decreased by 10 mg L -1 TiO 2 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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

The phytoplankton community structure exhibits seasonal and spatial variations in response to the environmental conditions, which aids in understanding the ecosystem's health. Given this, four samplings were conducted between October 2013 to April 2015, encompassing the monsoon, post-monsoon, and pre-monsoon seasons, from the Haldia port ecosystem of India. The samples were collected from the flowing estuary and an extended semi-enclosed dock. We hypothesized that the seasonal phytoplankton community (diversity, abundance, and carbon biomass) response will differ based on the environmental and hydrographical characteristics of the study site. Picophytoplankton and nano-microphytoplankton dominated the phytoplankton community in terms of numbers and biomass, respectively. Bacillariophytes dominated the nano-microphytoplankton abundance and total biomass, except during the monsoon when Dinophytes contributed (inner-zone). The dominance of Bacillariophytes and Chlorophytes in the outer-zone with picophytoplankton and Dinophytes in the inner-zone indicated group-specific hydrographic preferences that supported the hypothesis. The positive correlation of the picophytoplankton abundance (pre-monsoon) with Secchi disc depth and the negative correlation of diatoms (post-monsoon) with temperature signify the seasonal role of light and temperature, respectively. The highest nano-microphytoplankton species diversity (Shannon-Wiener's index) during the pre-monsoon (inner-zone) with more rare species indicated the probable influence of stable waters with increased water transparency. However, the community was unevenly distributed in the estuary due to the high abundance of the diatom, Aulacoseira granulata. Although harmful algal blooms were not detected, the higher temperature and nutrient concentrations could have favoured potentially harmful species (Pseudonitzschia delicatissima, Dinophysis acuta) during the monsoon. The system ranged from oligo- to mesotrophic state with moderate pollution levels (Carlson's Trophic State Index and Shannon's Index), indicating a reduction of the nutrient accumulation effects by the existing water renewal frequency. This study recommends incorporating qualitative and quantitative phytoplankton assessment in ecological monitoring of the stable coastal sites to prevent future harmful algal episodes., (© 2021. The Author(s), under exclusive licence to Springer Nature Switzerland AG.)

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.

Based on the field surveys aimed at understanding the variations of Synechococcus (Syn) abundance in the Pearl River Estuary during different seasons. We found that heavy terrestrial precipitation result in significant riverine runoffs and promote Syn growth, extension and blooms during warm and wet seasons. To understand the ecological role of Syn play in this estuary during wet seasons, we combined flow cytometry and high throughput sequencing (HTS) of 16S rDNA to investigate the phytoplankton distribution patterns and the potential shaping mechanisms during a typical wet season. During the cruise, picophytoplankton, especially Syn, and Nano-eukaryotes contributed importantly to the total phytoplankton biomass of the estuary. Syn can be further divided into phycoerythrin (PE)-rich Syn and phycocyanin (PC)-rich Syn, with PC-rich Syn about 1.5 times higher than PE-rich Syn in abundance. Both PE-rich Syn (60.75 × 10 3  cells ml -1 ) and PC-rich Syn (604.05 × 10 3  cells ml -1 ) reach the highest abundance at the lower part of the estuary. Moreover, PE-rich Syn can be divided into two subgroups which showed different salinity preference, with PE1 distributed in the high salinity area (with salinity >25) while PE2 in the middle salinity area (with salinity 7-20). Our results from the 16S rDNA sequencing also indicated abundant diversity and different niche adaptation of Syn with the operational taxonomic units (OTUs) along the estuary. Besides, analysis also indicated a tight correlation between estuarine Syn and active heterotrophic bacteria, especially groups of Rhodobacteria and Actionobacteria., (Copyright © 2019. Published by Elsevier B.V.)

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., (© 2019 The Authors. MicrobiologyOpen published by John Wiley & Sons Ltd.)

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., (© 2018 The Authors. MicrobiologyOpen published by John Wiley & Sons Ltd.)

A comprehensive analysis on the phytoplankton ecology with special reference to different phytoplankton size classes was carried out at green Noctiluca scintillans (hereafter Noctiluca) bloom and non-bloom locations in offshore waters of the northern Arabian Sea. At the bloom locations, green Noctiluca represented a dense mono-specific proliferation with average cell density of 10.16 ± 5.806 × 10 4  cells-L -1 and relative abundance share of 98.63%. Active photosynthesis through prasinophytic endosymbiont was depicted from net community production magnitude reaching 85.26 mgC/m 3 /Day under low prey abundance. Parallel swarming of Porpita porpita, a voracious copepod feeder signified the competitive advantage of Noctiluca to have the phytoplankton prey. Average concentration of picophytoplankton biomass was eleven times lower in surface waters of non-bloom stations in comparison to bloom. Higher N:P ratio in subsurface waters of non-bloom stations signified non-utilization of nitrogenous nutrients. Green Noctiluca bloom onset subsequent to diatom rich conditions was evident from spatio-temporal ocean colour satellite imageries., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

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 × 10 7 ) 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.

In order to better understand the picophytoplankton (PP) dynamics in tropical monsoon influenced coastal regions, samples were collected daily (June-September 2008: monsoon, December 2008: post-monsoon and April 2009: pre-monsoon) from a fixed station in Dona Paula Bay, India. Eight PP abundance peaks comprising Prochlorococcus-like cells, picoeukaryotes, and three groups of Synechococcus occurred. The chlorophyll biomass and PP abundance were negatively influenced by reduced solar radiation, salinity and water transparency due to precipitation and positively influenced by the stabilized waters during precipitation break/non-monsoon periods. Responses to environmental conditions differed with PP groups, wherein the presence of Synechococcus-PEI (phycoerythrin) throughout the year suggested its ability to tolerate salinity and temperature variations and low light conditions. Synechococcus-PEII appearance toward monsoon end and non-monsoon during high water transparency suggests its tidal advection from offshore waters. Dominance of Synechococcus-PC (phycocyanin) at intermediate salinities under low water transparency during MON and high salinities in PrM coinciding with high nitrate concentrations implies a greater influence of light quality or nutrients. Cyanobacteria and not picoeukaryotes were the dominant picophytoplankton in terms of numbers as well as biomass. This study suggests that PP could be used as tracers of environmental forcing driven by tides and freshwater influx and also highlights the importance of high-frequency samplings in dynamic coastal regions through which transient responses can be captured.

The marine phytoplankton community responds to latitudinal, longitudinal (coast to ocean), and vertical environmental gradients, an important subject in Oxygen minimum zones (OMZ). The phytoplankton structure and the effect of environmental gradients along the central Mexican Pacific, an area within an OMZ, were studied, especially the importance of size fractions and taxonomic groups. A combination of various methods and protocols, such as microscopic analysis, flow cytometer, and pigment analysis, were followed in this study. Oceanographic conditions included thermal gradients along the study area and unreported evidence of a weak upwelling in the southern zone (Acapulco). Vertical distribution of chlorophyll a showed subsurface maxima (SCM, 15-45 m depth) in all stations, and deeper chlorophyll-a maxima (DCM, 85-95 m depth) in more oceanic stations. Chain-forming diatoms dominated in the northern zone stations. Prochlorococcus, Synechococcus, and picoeukaryotes abundances ranged between 0.01 to 21.7 cells×104 mL-1, although most samples showed the highest contribution to biomass (47.95 µg C L-1) by picoeukaryotes. Expected tendencies of Prochlorococcus distribution were observed: highest densities coincided with the DCM and divinyl chlorophyll-a distribution at oceanic stations. Fucoxanthin had the highest concentrations, whereas fucoxanthin and zeaxanthin concentrations were higher at SCM depths. We documented the co-dominance of the pico- and microplankton: picoplankton was important at the DCM, related to oligotrophic and more stratified water column, whereas microplankton prevailed in coastal stations, with mixed water column, high nutrient concentrations, and diatoms as the dominant group. Picoeukaryotes abundances were related to the concentration of prasinoxanthin, which suggests an important mamiellophyte component not previously revealed. [ABSTRACT FROM AUTHOR]

Phytoplankton are usually considered autotrophs, but an increasing number of studies show that many taxa are able also to use organic carbon. Acquiring nutrients and energy from different sources might enable an efficient uptake of required substances and provide a strategy to deal with varying resource availability, especially in highly dynamic ecosystems such as estuaries. In our study, we investigated the effects of 31 organic carbon sources on the growth (proxied by differences in cell counts after 24 h exposure) of 17 phytoplankton strains from the Elbe estuary spanning four functional groups. All of our strains were able to make use of at least 1 and up to 26 organic compounds for growth. Pico-sized green algae such as Mychonastes, as well as the nano-sized green alga Monoraphidium in particular were positively affected by a high variety of substances. Reduced light availability, typically appearing in turbid estuaries and similar habitats, resulted in an overall poorer ability to use organic substances for growth, indicating that organic carbon acquisition was not primarily a strategy to deal with darkness. Our results give further evidence for mixotrophy being a ubiquitous ability of phytoplankton and highlight the importance to consider this trophic strategy in research. [ABSTRACT FROM AUTHOR]

Herein, we conducted surveys during the 2018–2022 summers to investigate the impact of climate change-related changes in the phytoplankton community structure on the marine ecosystem in the South Sea of Korea. The average surface water temperature increased by ~1.07 °C at 0.0195 °C·yr−1 between 1968 and 2022. During the summers, the rate was 0.0211 °C·yr−1, with a total increase of ~1.16 °C, indicating a stronger increase in summer surface water temperature. Over the last 30 years, nutrient levels in the South Sea have decreased, particularly at the surface. Moreover, 29.3–90.0% of the phytoplankton community structure was dominated by nanoflagellates (≤20 μm). Based on the size of the phytoplankton chl-a, the average contribution rate of picophytoplankton was the highest (60.1%). Redundancy analysis revealed negative correlations between nutrients and water depth, excluding NH4. Increased stratification due to climate change is causing reduced nutrient availability at the surface mixed layer, and the size of the phytoplankton structure is progressively reducing. These changes are expected to manifest in a complex microbial food web centered on smaller phytoplankton with low primary productivity. This can reduce the efficiency of carbon transfer to higher consumer levels, suggesting a potential decrease in marine productivity. [ABSTRACT FROM AUTHOR]

Marine picophytoplankton plays a major role in marine cycling and energy conversion, and its effects on the carbon cycle and global climate change have been well documented. In this study, we investigated the response of picophytoplankton across a broad range of physicochemical conditions in two distinct regions of the tropical western Pacific. Our analysis considered the abundance, carbon biomass, size fraction, distribution, and regulatory factors of the picophytoplankton community, which included the cyanobacteria Prochlorococcus and Synechococcus, and small eukaryotic phytoplankton (picoeukaryotes). The first region was a latitudinal transect along the equator (142–163° E, 0° N), characterized by stratified oligotrophic conditions. The second region was a meridional transect (143° E, 0–22° N) known for its high‐nutrient and low‐chlorophyll (HNLC) conditions. Results showed that picophytoplankton contributed >80% of the chlorophyll a (Chl a), and was mainly distributed above 100 m. Prochlorococcus was the dominant organism in terms of cell abundance and estimated carbon biomass in both latitudinal and meridional transects, followed by Synechococcus and picoeukaryotes. In the warm pool, Prochlorococcus was primarily distributed below the isothermal layer, with the maximum subsurface abundance forming below it. The maximum Synechococcus abundance was restricted to the west‐warm pool, due to the high temperature, and the second‐highest Synechococcus abundance was associated with frontal interaction between the east‐warm pool and the westward advance of Middle East Pacific water. In contrast, picoeukaryotes formed a maximum subsurface abundance corresponding to the subsurface Chl a maximum. In the mixed HNLC waters, the cell abundance and biomass of the three picophytoplankton groups were slightly lower than those in the warm pool. Due to a cyclonic eddy, the contours of the maximum subsurface Prochlorococcus abundance were uplifted, evidently with a lower value than the surrounding water. Synechococcus abundance varied greatly in patches, forming a weakly high subsurface peak when the isothermal layer rose to the near‐surface (<50 m). The subsurface maximum picoeukaryote abundance was also highly consistent with that of the subsurface Chl a maximum. Correlation analysis and generalized additive models of environmental factors showed that nutrient availability had a two‐faceted role in regulating the spatial patterns of picophytoplankton in diverse latitudinal and meridional environments. We concluded through regression that temperature and light irradiance were the key determinants of picophytoplankton variability in the tropical western Pacific. This study provides insights into the changing picophytoplankton community structure with potential future changing hydroclimatic force. [ABSTRACT FROM AUTHOR]

The port ecosystems are prone to deterioration due to the maritime and coastal activities and as a major source of the country's economy need efficient management. Phytoplankton communities can serve as reliable indicators of the prevailing environmental conditions due to their short life cycles. Seasonal sampling was conducted at 26 stations from October 2014 to February 2016 at Kandla port situated in a creek, along the west coast of India. The post-monsoon and monsoon water temperatures were higher (30 °C) whereas pre-monsoon were lower (21 °C). The salinity varied from polyhaline (18–30; monsoon) to euhaline (30 to 45; non-monsoon). The strong currents, high tidal activity, shallow depth areas, and the creek backwater systems make this ecosystem well-mixed and turbid. The annual average trophic index (TRIX) scores indicated very good water quality and low eutrophication, except during pre-monsoon (2.3 ± 0.7 to 4.1 ± 0.2). Based on the cell size, the phytoplankton community was classified into two main groups, i.e., nano-microphytoplankton, which comprised forty-seven species (represented by diatoms, dinoflagellates, and silicoflagellates) and picophytoplankton including two groups (picocyanophytes and picoeukaryotes). The diatoms and picophytoplankton dominated the total biomass and cell abundance, respectively. Only the picophytoplankton exhibited significant seasonal variations in cell abundance and carbon biomass. The lowest monsoon phytoplankton abundance coincided with high turbidity and vice versa during the post-monsoon. The hypersaline pre-monsoon environment with lower annual temperature, relatively lower turbid waters, and increased nutrients favoured higher diatom diversity. These conditions also supported potentially harmful Gymnodinium sp. and bloom-forming Tripos furca and Pyrophacus sp. Overall, ten non-toxic but bloom-forming species were observed. The study provides insights into the phytoplankton community's response to environmental conditions that can have repercussions on the ecosystem's functioning. [ABSTRACT FROM AUTHOR]

Marine picophytoplankton have crucial ecological value and make an important contribution to marine primary productivity. While biomass of phytoplankton in general is projected to decline as a result of global warming, picophytoplankton will likely dominate in the future oceans due to their growth advantages in an oligotrophic environment. To better understand the biography of picophytoplankton, we undertook a comprehensive study of the distribution patterns of picophytoplankton, carbon biomass, and Chl a concentrations, etc. based on large-scale sampling in the tropical Western Pacific Ocean. In terms of cellular abundance, Prochlorococcus was the most abundant group (averaging [1.03 ± 0.40] × 104 cells/mL), followed by Synechococcus (averaging [1.31 ± 1.22] × 103 cells/mL) and then picoeucaryote (averaging [4.83 ± 2.84] × 102 cells/mL). The picophytoplankton size-fractionated chlorophyll a (Pico-Chl a) accounted for about 30% of the total Chl a , with Prochlorococcus and picoeukaryotes contributing 41 and 35%, respectively, of the Pico-Chl a -normalized carbon biomass, indicating the ecological importance of picophytoplankton as the primary producers. In terms of biogeographic distribution, the picophytoplankton communities exhibited contrasting patterns. The surface distribution of Prochlorococcus and Synechococcus was concentrated in the low latitude of the 142°E section, while picoeucaryote was more abundant near the 130°E and equator sections. Synechococcus was higher in the shallow layer at 25 m, and it was extremely tolerant of high-light irradiation, while Prochlorococcus and picoeucaryote were distributed in the deep Chlorophyll maximum layer (DCM) (about 100 m). From the carbon-to-Chlorophyll a ratios, which was derived from Prochlorococcus and picoeucaryote population groups, we found that the ratio varied widely, from 0.19 to 75.56, and was highest at the depth of 200 m. Of these, Prochlorococcus had an important contribution. The correlation analysis of environmental factors showed that Prochlorococcus , Synechococcus , and picoeucaryote were negatively correlated with nutrient concentration. We concluded that Prochlorococcus group was dominant in the WPO, both in abundance and biomass, and the various abiotic factors such as temperature, salinity, and nutrient concentrations were closely correlated with the spatial variation in the picophytoplankton community. These findings aid our understanding of how contrasting environmental conditions influence picophytoplankton community and the importance of picophytoplankton in contributing the carbon pool in the oligotrophic ocean. [ABSTRACT FROM AUTHOR]

Species diversity of phototrophic eukaryotes (PEs) with cell size <3 µm in autumn phytoplankton of the Kara and Laptev seas was studied. High-throughput sequencing of the 18S rRNA gene V4 region revealed 15 classes of algae in the plankton pico-sized fraction, which belonged to five divisions: Haptophyta, Cryptophyta, Chlorophyta, Ochrophyta, and Dinoflagellata. Dinoflagellata and green algae of the class Mamiellophyceae were the main contributors to total PE sequences. Chlorophyta was the most diverse section, represented by seven classes: Mamiellophyceae, Trebouxiophyceae, Nephroselmidophycea, Palmophyllophyceae, Pyramimonadophyceae, Chlorodendrophyceae, and Chlorophyceae. Bolidophyceaea species Triparma strigata and T. laevis, as well as the diatom Skeletonema marinoi, were first identified in the Kara and Laptev Seas, respectively. The hydrological conditions on the stations determined the PE taxonomic composition. The diversity indices were higher at the stations located in the upper estuary of river Khatanga and at the areas of both seas adjacent to the Khatanga and Ob estuaries than at the northernmost Laptev Sea station. The obtained data showed that the taxonomic composition of the smallest phytoplankton fraction of the two shelf seas of the Russian Arctic was similar to that found in other Arctic regions. [ABSTRACT FROM AUTHOR]

Seasonal dynamics of phytoplankton production and structural characteristics was studied at two closely located stations in the open part of the Sevastopol Bay mouth, one of which was a mussel farm. The net primary production, chlorophyll a content, total phytoplankton biomass, the quantitative ratio between small and large algal cells, and their seasonal dynamics were the same for both stations. Linear dependencies were observed between the relevant parameters of the stations, with the angular regression coefficients close to 1. The phytoplankton production and biomass peaked during the summer period, reaching 350 ± 43 mg C/(m3 day) and 450 ± 50 mg C/m3, respectively. Chlorophyll a content varied from 0.5 to 3 mg/m3, with the minimum in January‒February and the maximum in summer. The assimilation number values for chlorophyll a during the light period corresponded to those reported for production waters and correlated with the temperatures during the year. Total oxygen consumption by bacterio- and phytoplankton during the summer period was 30–70% of net photosynthesis. The data were obtained on seasonal biomass variations in three groups of algae (Synechococcus, picoeukaryotic phytoplankton, and nanophytoplanktom); on average, picoalgae constituted 30% of the phytoplankton biomass, with maxima during the winter-spring and summer periods. Bacterioplankton abundance was determined using flow cytometry and the SYBR Green I vital fluorochrome. A positive relation between bacterial abundance and water temperature was shown. [ABSTRACT FROM AUTHOR]

The purpose of this study was to determine phytoplankton community structure in February 2018 at Laguna Ojo de Liebre, a relevant site for biodiversity at El Vizcaino Biosphere Reserve. The parameters: diversity and abundance indices of the main phytoplankton groups were used to relate them to the oceanographic variables temperature and salinity. Phytoplankton was quantified in three size groups: microphytoplankton (diatoms, dinoflagellates), nanophytoplankton (Chromophytes, Cryptophytes), and picophytoplankton (Cyanobacteria, Prochlorophytes) using optical microscopy and chemotaxonomy (HPLC) analyses. This last method indicated fucoxanthin (pigment characteristic of diatoms) was the most abundant and predominant photosynthetic carotenoid in the whole lagoon. Temperature showed an inverse relationship with microphytoplankton and a direct relationship with nanophytoplankton and picophytoplankton; saline gradient showed a direct association with the distribution of total chlorophyll. [ABSTRACT FROM AUTHOR]

Abundance and biomass of small photosynthetic flagellates (SPF; 3–10 µm), chlorophyll a concentration, and the contribution of SPF to total phytoplankton biomass at different stages of seasonal succession in ice and in surface water were studied in Kandalaksha Bay of the White Sea in September 2016 and February and July 2017. In summer, SPF biomass in the photic layer averaged 38.36 ± 9.77 mg C/m3, 2.22 ± 1.43 mg C/m3 in autumn, 2.6 ± 1.72 mg C/m3 in under-ice water, and 14.79 ± 11.25 mg C/m3 in ice. The contribution of SPF to total phytoplankton biomass depended on the season and ranged from 29 to 95%; the contribution of SPF to sympagic communities averaged 66%. The size structure of photosynthetic flagellates varied by seasons. Flagellates with a cell size of 6–10 µm dominated in summer plankton and in the ice. Flagellates with a cell size of 3–6 µm prevailed in autumn and in the under-ice water. The obtained data of SPF abundance showed higher values of the total phytoplankton biomass of the White Sea in the autumn–winter period compared with the estimates obtained previously. Applying the method of epifluorescence microscopy confirmed the idea that photosynthetic flagellates were the main producers in the winter period as well as in the summer when the biomass of planktonic algae with cell size exceeding 10 µm is low. [ABSTRACT FROM AUTHOR]

The spatial distribution of picophytoplankton abundance, biomass, chlorophyll a and the contribution of picoalgae to the total chlorophyll a was studied in the outer Ob River estuary with the adjacent shelf and in the western part of the Kara Sea. In August–September, the picophytoplankton abundance and biomass varied from 0.1 to 17.3 × 106 cell/L and from 0.06 to 9.20 mg С/m3, respectively. Photosynthetic picoeukaryotes dominated in the plankton picofraction; the contribution of cyanobacteria to the total picophytoplankton biomass did not exceed 11%. The highest contribution of picophytoplankton to the total phytoplankton abundance was observed at a lower (<11 mg/m2) euphotic zone integrated chlorophyll a. The spatial heterogeneity of picoforms contribution was determined by the silicon concentration. [ABSTRACT FROM AUTHOR]

Abstract Seasonal dynamic of Utermöhl fraction of phytoplankton (UFP, cell size > 3 μm) and picophytoplankton (Pico, cell size < 3 μm) were compared in three eutrophic coastal lagoons located in the same geographical area (Western Mediterranean Sea). Considering data on density, biomass, volume and composition of each size class, we focused on three aspects: i) the contribution of UFP and Pico to total phytoplankton; ii) the responses of UFP and Pico to seasonal variations of selected environmental variables; iii) the identification of the main environmental drivers of UFP and Pico seasonal dynamics. Pico contribution to total phytoplankton density was from 20% to 90%, highlighting the potential importance of Pico also in eutrophic ecosystems. Pico contribution to total phytoplankton biomass was lower, from 9% to 16%. Statistically, temperature resulted the strongest environmental factor controlling both the UFP and Pico seasonal variations in all the studied lagoons. In addition, the role of salinity, water transparency and nutrient concentrations was important in relation to the study site and the size class considered. In this work, the effects of temperature, salinity, transparency and nutrient availability on two different phytoplankton size classes have been described in Mediterranean shallow coastal lagoons for the first time, providing new insight for this ecosystem typology worldwide. Graphical abstract Image 1 Highlights • Potential importance of picophytoplankton in eutrophic ecosystems was established. • Temperature was the strongest driver of seasonal cell size structure. • Role of salinity, transparency and nutrients was size- and site-specific. • Synergic interaction of several drivers affected seasonal cell size structure. [ABSTRACT FROM AUTHOR]

Abstract The Great Australian Bight (GAB) is a region of great economic and ecological importance to southern Australia, supporting fisheries of national and international importance, and large populations of small pelagic fish and iconic apex predators. There is, however, limited information on the plankton forming the base of the food web and their links to higher trophic levels in the GAB. Bacteria [inclusive of Archaea ], picophytoplankton (Prochlorococcus , Synechococcus and picoeukaryotes) and viruses were investigated in the central through to the eastern GAB for the first time, across five shelf edge to offshore transects in the austral Autumn of 2013. Downwelling conditions were evident over the region prior to and during the study period. Prochlorococcus numerically dominated picophytoplankton, exceeding Synechococcus and picoeukaryotes on average, by 5-fold and 30-fold respectively. Synechococcus and picoeukaryotes showed a clear shelf edge to offshore trend with highest abundances occurring at the shelf edge. Bacterial abundances showed low variation over the region, but with highest percent of high DNA bacteria (%HDNA) occurring at the shelf edge. Viral abundances closely followed bacteria, however individual viral groups were positively correlated with picophytoplankton groups, suggesting all picoplankton groups were subject to viral infection in GAB waters. Highest total picoplankton biomass occurred in surface waters at the shelf edge (26.5 ± 1.49 µg C L−1), with picophytoplankton carbon biomass (PB) and bacterial carbon biomass (BB) each accounting for ~50% of the total picoplankton biomass in the upper 60 m of the water column. Total picoplankton biomass was ~2-fold lower offshore waters, with BB comprising the bulk of the biomass (~72%). For the upper 60 m of the water column, picoeukaryotes accounted for ~51% of PB at the shelf edge, compared to ~24% in offshore waters. Based on our estimated C:Chl a ratios, picoeukaryotes accounted for 32.7 ± 3.0% of Chl a at the shelf edge, compared to 9.1 ± 0.6% in offshore waters. Prochlorococcus dominated PB in offshore waters (~67%), accounting for ~28% of Chl a. Multivariate analyses showed that the best predictors of the picoplankton community were a combination of nitrate and salinity (ρ = 0.704), indicating strong environmental forcing of the picophytoplankton community across the GAB. The elevated PB at the shelf edge, due to the dominance of picoeukaryotes, was a regional feature of the central and eastern GAB during the time of this study. We hypothesize that picoeukaryotes play a critical role in supporting higher trophic levels in GAB shelf edge waters. This study highlights the importance of small plankton in food web dynamics in GAB waters. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Yellow Sea Cold Water Mass (YSCWM) is the most prominent oceanographic phenomenon in the bottom layer of the Yellow Sea. A subsurface chlorophyll maximum (SCM) has been widely observed in this area, yet it has only been briefly described in previous research. In this study, we present the first in-depth analysis of the spatial distribution and intrinsic characteristics of the SCM in the YSCWM, based on two comprehensive cruises conducted in the central Yellow Sea during August 2011 and July 2013. In general, the spatial distribution of the SCM coincides approximately with the YSCWM area during the stratified seasons, with the southern boundary at around 33.5°N and the western boundary at approximately 122.5°E. The vertical position of the SCM varied between 12 and 29 m and 13–25 m during August 2011 and July 2013 respectively, which is mainly located within or at the bottom of the thermocline and the upper part of the nitracline. The averaged Chl a concentration in the SCM was 1.24 ± 0.27 mg m −3 and 1.92 ± 0.65 mg m −3 respectively during the two cruises. The shallower but stronger intensity characteristics are consistent with the meso-trophic status of the Yellow Sea. Picophytoplankton, composed of Synechococcus and picoeukaryotes, dominated the phytoplankton community in cell abundance and carbon biomass, contributing > 90%. The micro- and nanophytoplankton were almost completely dominated by dinoflagellate species. The phytoplankton community at the SCM depth was not significantly different from that in the surface mixed layer, while the relatively higher photosynthetic yields ( F v /F m ) indicated the active growth of the phytoplankton assemblage in the SCM layer. Significantly lower carbon-to-chlorophyll a ratios estimated in the SCM layer suggest that the main contribution originates from photoacclimation. Our results demonstrate that the active photosynthesis of the phytoplankton combined with their physiological photo-acclimation are the main formation mechanisms of the SCM. We also suggest that the vertical migration of motile dinoflagellates is an additional contributing factor to the SCM in the YSCWM area. A higher vertical sampling resolution and 1-D physical-biological modeling are needed in future studies to confirm our hypothesis and quantify the contributions of the different formation mechanisms in the YSCWM area. [ABSTRACT FROM AUTHOR]

Phytoplankton size-fractionated biomass is an important determinant of the type of food web functioning in aquatic ecosystems. Knowledge about the effect of seasonal salinity gradient on the size-fractionated biomass dynamics is still lacking, especially in tropical estuaries experiencing monsoon. The phytoplankton size-fractionated chlorophyll a biomass (>3 μm and <3 μm) and picophytoplankton community structure were characterized in the monsoonal Zuari estuary, along the west coast of India, from October 2010 to September 2011 across the salinity gradient (0–35). On an annual scale, >3 μm size-fraction was the major contributor to the total phytoplankton chlorophyll a biomass with the ephemeral dominance of <3 μm size-fraction. During monsoon season, freshwater runoff and shorter water residence time resulted in a size-independent response. The lowest annual chlorophyll a biomass concentration of both size-fractions showed signs of recovery with increasing salinity downstream towards the end of the monsoon season. In contrast, the chlorophyll a biomass response was size-dependent during the non-monsoon seasons with the sporadic dominance (>50%) of <3 μm chlorophyll a biomass during high water temperature episodes from downstream to middle estuary during pre-monsoon and at low salinity and high nutrient conditions upstream during post-monsoon. These conditions also influenced the picophytoplankton community structure with picoeukaryotes dominating during the pre-monsoon, phycoerythrin containing Synechococcus during the monsoon and phycocyanin containing Synechococcus during the post-monsoon. This study highlights switching over of dominance in size-fractionated phytoplankton chlorophyll a biomass at intra, inter-seasonal and spatial scales which will likely govern the estuarine trophodynamics. [ABSTRACT FROM AUTHOR]

Biomass of photosynthetic picoplankton (Bpic), its contribution to the total phytoplankton biomass (Bpic%), chlorophyll a concentration (Chl), and associated hydrophysical characteristics of the water masses in the White Sea were estimated in June 2015 at 47 stations located in the open parts and in the inlets of the Onega and Kandalaksha Bays and in the western part of the Basin. Spatial variability of mean values of Bpic in the photic layer (0.01-1.91 mg C/m3) was preconditioned by sub- and mesoscale heterogeneity of the hydrological conditions. The values of Bpic were higher near the frontal zones than those in the water masses characterized by quasi-homogeneous distribution of the thermohaline characteristics. The relative contribution of Bpic did not exceed 1% at half of the stations and varied from 1 to 8% for the rest of the studied water area. The value of Bpic% reached 40.5% in the Basin and did not exceed 2% during the phytoplankton bloom in Knyazhaya Inlet (Chl > 3 mg/m3) accompanied by the dominance of Skeletonema costatum sensu lato. [ABSTRACT FROM AUTHOR]

The deepening of mixed layer and ensuing changes in optical and physicochemical properties of euphotic zone can influence phytoplankton community dynamics in the northeastern Arabian Sea during winter monsoon. The response of picophytoplankton community to such changes during winter convective mixing is not well understood. Herein, we have compared variations in the picophytoplankton community structure during early (November–December 2012), peak (end-January 2014) and late (mid-February 2015) winter monsoon from three separate cruises in the southern northeastern Arabian Sea. The higher Synechococcus abundance owing to entrainment of nutrients in mixed layer was observed during peak winter monsoon, while the concomitant changes in nitrate concentration, light and oxygen environment restricted Prochlorococcus growth resulting in lower abundance during the same period. This highlights the diverse responses of picophytoplankton groups to physicochemical changes of water column during winter convective mixing. The divinyl chlorophyll b / a ratio (marker for Prochlorococcus ecotypes) indicated prevalence of one low-light adapted ecotype (sensitive to light shock) in sub-surface water, one high-light adapted ecotype in surface water during early winter monsoon and both disappeared during intense mixing period in peak winter monsoon. Subsequently, a distinct low-light adapted ecotype, capable to tolerate light shock, was noticed during late winter monsoon and we argue that this ecotype is introduced to southern northeastern Arabian Sea through advection from north by sub-surface circulation. The total picophytoplankton biomass available to microbial loop is restored during late winter monsoon, when stratification begins, with a higher abundance of Synechococcus and the re-occurrence of Prochlorococcus population in the region. These inferences indicate that variability in picophytoplankton community structure and their contribution to the microbial loop are driven by convective mixing and advection, which in turn influence ecosystem functioning and trophodynamics of the southern northeastern Arabian Sea. [ABSTRACT FROM AUTHOR]

Picophytoplankton are acknowledged to contribute significantly to primary production (PP) in the ocean while now the method to measure PP of picophytoplankton (PPPico) 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 PPPico, 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 PPPico whose principle is based on the group-specific abundance, cellular carbon conversion factor (CCF), and temperature-derived growth rate of picophytoplankton. Comparative analysis showed that the estimated PPPico using CbPM method is significantly and positively related (r² D 0.53, P < 0.001, n D 171) to the measured 14C uptake. This significant relationship suggests that CbPM has the potential to estimate the PPPico over large spatial and temporal scales. Currently this model application may be limited by the use of invariant cellular CCF and the relatively small data sets to validate the model which may introduce some uncertainties and biases. Model performance will be improved by the use of variable conversion factors and the larger data sets representing diverse growth conditions. Finally, we apply the CbPM-based model on the collected data during four cruises in the Bohai Sea in 2005. Model-estimated PPPico ranged from 0.1 to 11.9, 29.9 to 432.8, 5.5 to 214.9, and 2.4 to 65.8 mg C m-2 d-1 during March, June, September, and December, respectively. This study shed light on the estimation of global PPPico using carbon-based production model. [ABSTRACT FROM AUTHOR]

The eastern Arabian Sea over the continental margin is a dynamic region subjected to short-term variability in hydrography as a result of various physical forcing such as coastal advection and vertical mixing. In order to assess the influence of hydrography on the picophytoplankton community, a temporal high resolution (every 3 h for nine days) study was carried out at a fixed location (15° 18′ 46″N, 72° 41′ 53″E) in the eastern Arabian Sea during the early north-east monsoon (November 2011). The picophytoplankton community comprised of Synechococcus , Prochlorococcus , and picoeukaryotes. Based on the temperature and salinity distribution, the study period was divided into phase I representing a stratified water column and phase II representing a vertically mixed water column. Phase I had higher picophytoplankton abundance with the initial dominance of Prochlorococcus which was later taken over by picoeukaryotes. Towards the end of phase I, with the initiation of vertical mixing, picoeukaryotes were the first to respond to the nutrient influx. As the vertical mixing intensified during phase II, the picophytoplankton abundance declined. Picophytoplankton carbon biomass and their contribution to total phytoplankton biomass was relatively higher during phase I with picoeukaryotes as the major contributors. These transient variations in picophytoplankton abundance highlights the importance of high frequency observations at the single cell level for better understanding the population dynamics in such environments. [ABSTRACT FROM AUTHOR]

The temporal β diversity of small eukaryotic phytoplankton in a tropical urban coastal lagoon (Rodrigo de Freitas Lagoon, Brazil) was characterized throughout two annual cycles (2012-2103) with twice-weekly sampling. Small eukaryotic phytoplankton (<20 μm) were very abundant (above 1 × 10³ and up to 6.2 × 105 cells mL-1). Small eukaryotes were also the most diverse phytoplankton group, showing high temporal variability at the species level that was represented by seven taxonomic divisions. Temporal β diversity was positively related to environmental heterogeneity on a short-time scale. Although abundance peaks of the main species were positively related to NH4 inputs, selection of species ultimately depended on the simultaneous interaction among several physical-chemical variables acting hierarchically on different temporal scales (first seasonally and then on a short-time scale). Considering the numerical importance of small size fraction phytoplankton, information relevant to the management and conservation of coastal lagoons could be incomplete if the floristic composition and temporal species turnover of pico- and nanophytoplankton species are not properly determined. [ABSTRACT FROM AUTHOR]

Picophytoplankton (Pico) seasonal dynamics and ecology were investigated in three eutrophic coastal lagoons (Calich, Santa Giusta, and Corru S'Ittiri) located in the same geographic region (Sardinia, Mediterranean Sea). Pico dynamics were examined to determine a relationship with variability in environmental conditions characterizing the three sites, and identify which parameters affected observed Pico dynamics. Results showed Pico density reached 10 cells L and biomass values did not exceed 72.8 μg C L. Statistical analyses detected significant differences in hydrographic and nutrient conditions among lagoons, which were also characterized by variable Pico seasonal dynamics and composition. Calich and Santa Giusta were dominated all year around by picocyanobacteria. Maximum picocyanobacteria abundance was observed during summer in both lagoons, associated with the highest temperatures recorded. Picocyanobacteria abundance and nutrient concentrations showed a significant negative and positive correlation in Calich and Santa Giusta, respectively. In contrast, picoeukaryotes dominated Pico assemblages in Corru S'Ittiri, where the highest transparency and salinity and the lowest seasonal variation in salinity were recorded. In Corru S'Ittiri, the maximum Pico abundance was observed in autumn, when lower temperature and the highest nutrient concentrations were observed. Our study provides information on Pico assemblage ecology, which remains scarce in coastal and transitional environments, particularly in Mediterranean climate regimes. [ABSTRACT FROM AUTHOR]

Phytoplankton photosynthesis is often inhibited by ultraviolet ( UV) and intense photosynthetically available radiation ( PAR), but the effects on ocean productivity have received little consideration aside from polar areas subject to periodic enhanced UV-B due to depletion of stratospheric ozone. A more comprehensive assessment is important for understanding the contribution of phytoplankton production to the global carbon budget, present and future. Here, we consider responses in the temperate and tropical mid-ocean regions typically dominated by picophytoplankton including the prokaryotic lineages, Prochlorococcus and Synechococcus. Spectral models of photosynthetic response for each lineage were constructed using model strains cultured at different growth irradiances and temperatures. In the model, inhibition becomes more severe once exposure exceeds a threshold ( Emax) related to repair capacity. Model parameters are presented for Prochlorococcus adding to those previously presented for Synechococcus. The models were applied to estimate midday, water column photosynthesis based on an atmospheric model of spectral radiation, satellite-derived spectral water transparency and temperature. Based on a global survey of inhibitory exposure severity, a full-latitude section of the mid-Pacific and near-equatorial region of the east Pacific were identified as representative regions for prediction of responses over the entire water column. Comparing predictions integrated over the water column including versus excluding inhibition, production was 7-28% lower due to inhibition depending on strain and site conditions. Inhibition was consistently greater for Prochlorococcus compared to two strains of Synechococcus. Considering only the surface mixed layer, production was inhibited 7-73%. On average, including inhibition lowered estimates of midday productivity around 20% for the modeled region of the Pacific with UV accounting for two-thirds of the reduction. In contrast, most other productivity models either ignore inhibition or only include PAR inhibition. Incorporation of Emax model responses into an existing spectral model of depth-integrated, daily production will enable efficient global predictions of picophytoplankton productivity including inhibition. [ABSTRACT FROM AUTHOR]

šIn this study, we investigated the food sources of eastern oysters Crassostrea virginica cultivated in Atlantic Canada. Stable isotopes (13Cand 15N) and fatty acid biomarkerswere used to identify these sources under in situ conditions for suspended (~0.5mbelow surface) and bottom(~2m) 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<0.05). Bottom oysters did not show significant preference for detrital or bacterial organic matter. Near-surface waters contained an elevated picophytoplankton biomass (PPP, 0.2-2mm, 1.93 ± 0.16µg l-1, mean ± SEM) compared to nanophytoplankton biomass (NPP, >2mm, 1.05 ± 0.15µg 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 (d13C) 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 FA22:2 (non-methylene-interrupted orNMI) 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. [ABSTRACT FROM AUTHOR]

Abundance and seasonal trophodynamics (specific growth rate, daily production, and grazing mortality) of the major picophytoplankton components, Synechococcus cyanobacteria (Syn) and picoeukaryotes (Pico-E), were studied at three stations in Sevastopol Bay and adjacent coastal waters (the Black Sea) in 2014 by flow cytometry and the dilution method. Pico-E abundance was shown to increase along the nutrient and pollution gradient from the coastal waters outside the bay (annual average of 7.3 ± 5.4 × 10 cells mL) to the eastern corner of the bay (28.7 ± 11.4 × 10 cells mL), while no relation was found between the water pollution status and Syn abundance (9.9 ± 8.7 × 10 cells mL; at all the stations, n = 27). Matter flows through the communities (daily production for Syn and Pico-E 06.6 and 09.3 µg C L day, respectively; grazing mortality for Syn and PicoE 0-3.6 and 0-21.2 µg C L day, respectively) were comparable to or even exceeded their biomass stocks (>0.05-6.8 and 0.9-26.5 µg C L for Syn and PicoE, respectively), indicating high biomass turnover rates. The highest flow-to-stock ratio (up to 6 for Syn) and a significant imbalance between daily production (P) and grazing mortality ( G) were observed in the most polluted and eutrophic waters of the bay in spring (Pico-E: P/G < 1) and late summer (Syn: P/G > 1). Black River inflow to the bay was hypothesized to be among the mechanisms maintaining this pronounced and long-term imbalance in the open system without any negative consequences for the picophytoplankton assemblages. [ABSTRACT FROM AUTHOR]

Picophytoplankton are important primary producers in oligotrophic waters and sensitive to environmental changes. However, little is known about their ecophysiology in the northern Bering Sea with complex water masses. The distribution of picophytoplankton with their optical physiological features and physicochemical characteristics as well as water masses were investigated on the northern Bering Sea Shelf during the summer of 2008. Flow cytometry and statistical analysis including Pearson correlation analysis and cluster analysis were the main methods used. Synechococcus and picoeukaryotes were the primary existing picophytoplankton assemblages, with their respective abundances ranging from 0 to 2.69 × 10 and 0.47 × 10 to 13.20 × 10 cells l and peaking at water depths of 10-30 m with surface light levels of 30-50 %. Larger Synechococcus cells contained larger amounts of chlorophyll a (Chl a) and phycoerythrin. Low temperature, high nutrient concentrations and a high N/P value (>7) might limit cell abundance of picoeukaryotes, as shown by their close relationships. With an increase of temperature and oligotrophic freshwater input, smaller picophytoplankton would thrive and the cellular Chl a (Chl a/cell) increase with higher contribution to total Chl a. Seven water types with different physicochemical and biological characteristics were differentiated using the mini-ecosystem (a descriptor of a composite of parameters defining a composite water type community, including temperature, salinity, macronutrients and community abundance of picophytoplankton) as an indicator. They are Alaska Coastal Water, Bering Shelf Surface Water, 20-40 m of Bering Shelf Water, Bering Shelf Cold Water, Anadyr Water, Anadyr-Bering Shelf Water (Bering Sea Water) and Cold Pool. [ABSTRACT FROM AUTHOR]

The species composition and phytoplankton biomass, concentrations of chlorophyll ' a' (Chl) and nutrients in the surface water layer, and accompanying hydrophysical conditions were studied in Onega Bay of the White Sea in June 2015. The temperature and salinity of surface water layer and the water column stability varied greatly in the bay. The nutrients' concentrations exceeded the limiting threshold necessary for the phytoplankton development. The phytoplankton abundance was relatively low, averaged as 13.46 ± 9.00 mg C/m (total phytoplankton biomass), 0.78 ± 0.43 mg/m (concentration of chlorophyll ' a'), and 0.18 ± 0.27 mg C/m (picophytoplankton biomass). The highest phytoplankton biomass has been registered along the frontal zones. Three phytoplankton communities that differed significantly in their structure have been found. [ABSTRACT FROM AUTHOR]

The Costa Rica Dome (CRD) is a unique open-ocean upwelling system, with picophytoplankton dominance of phytoplankton biomass and suppressed diatoms, yet paradoxically high export of biogenic silica. As a part of Flux and Zinc Experiments cruise in summer ( June-July 2010), we conducted shipboard incubation experiments in the CRD to examine the potential roles of Si, Zn, Fe and light as regulating factors of phytoplankton biomass and community structure. Estimates of photosynthetic quantum yields revealed an extremely stressed phytoplankton population that responded positively to additions of silicic acid, iron and zinc and higher light conditions. Size-fractioned Chl a yielded the surprising result that picophytoplankton, as well as larger phytoplankton, responded most to treatments with added silicic acid incubated at high incident light (HL + Si). The combination of Si and HL also led to increases in cell sizes of picoplankton, notably in Synechococcus. Such a response, coupled with the recent discovery of significant intracellular accumulation of Si in some picophytoplankton, suggests that small phytoplankton could play a potentially important role in Si cycling in the CRD, which may help to explain its peculiar export characteristics. [ABSTRACT FROM AUTHOR]

'The plankton paradox' view which contradicted competitive exclusion principle has been changing since the last decade. Current outlook based on the evidence from size scaling studies suggests that in a given nutrient regime, phytoplankton that are capable of efficient nutrient intake increases in biomass, perhaps leading to the exclusion of those with inefficient nutrient intake ability. Commonly observed higher biomass of larger cells (e.g., diatoms and dianoflagellates) in eutrophic environment and that of smaller cells (e.g.,picophytoplankton) in oligotrophic environment has been viewed as the competitive success of higher and lower size strata in respective nutrient gradients. We analyse the above view point sceptically, and share the opinion as follows. Conventional approach of using relative biomass as a measure of competitive success in phytoplankton could lead to underestimate the ecological success of smaller phytoplankton in eutrophic and mesotrophic waters. We also suggest that picophytoplankton that require very low nutrient quota can co-exist with larger cells by sharing a negligible part of the community niche, without exerting competition related pressure, irrespective of the nutrient status of the system. Similarly, large cells that are confined to the nutrient rich waters, can avoid competition with smaller algae by their exceptional nutrient storage ability. Consequently, there is a limitation in using relative biomass as a measure of competitive success in phytoplankton community. We also emphasis that in aquatic microbial world where all interactions occur at cellular level, difference in particle size could provide an additional mechanism to prevent competitive exclusion by facilitating resource partitioning and selective predation pressure. • Generally, higher relative biomass (ie. biomass ratio of a particular size fraction in relation with other size fractions/ percentage contribution of biomass) of larger phytoplankton (e.g.,diatoms and dianoflagellates) in eutrophic environment and that of smaller ones (e.g., Picophytoplankton) in oligotrophic environment has been viewed as the competitive success of higher and lower size strata in respective nutrient gradients. • However, difference in particle size and physiology can facilitate positive interaction in phytoplankton to prevent competitive exclusion. • Hence conventional approach of using relative biomass as an index of competitive success in phytoplankton could lead to the underestimation of ecological success of lower size fractions. [ABSTRACT FROM AUTHOR]

Protistan grazers and viruses are major agents of mortality in marine microbial communities with substantially different implications for food-web dynamics, carbon cycling and diversity maintenance. While grazers and viruses are typically 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, picoeukaryotes) 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 methodological 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. [ABSTRACT FROM AUTHOR]

Environmental changes driven by intense human disturbance and natural processes govern the abundance and distribution characteristics of two picophytoplankton groups (Synechococcus and picoeukaryotes). Picophytoplancton and high DNA (HDNA) and low DNA (LDNA) bacterial groups, identified by flow cytometry, were assessed during the autumn monsoon transition period in Daya Bay (South China Sea). The abundance of Synechococcus and picoeukaryotes ranged from 2.16 x 104 to 1.45 x 105 cell mL-1 and from 0.78 x 10³ to 7.95 x 10³ cell mL-1, respectively. The abundance of HDNA bacteria in surface water (mean: 5.58 x 105 cell mL-1) was greater than in bottom water (mean: 3.74 x 105 cell mL-1), with significant difference (n = 12, P = 0.05). The difference in LDNA abundances between surface (mean: 7.06 x 105 cell mL-1) and bottom (mean: 4.83 x 105 cell mL-1) waters was insignificant (n = 12, P > 0.05). The results of the principal component analysis showed that both picophytoplankton (Synechococcus and picoeukaryotes) and bacteria (HDNA and LDNA) were positively related to nutrients (NO3-N, NH4-N, and SiO3-Si). Three subsystems in the bay were identified as follows: the west and east parts, the central and northwest parts, and the mouth and central part. [ABSTRACT FROM AUTHOR]