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We investigated picoplankton distribution patterns and environmental variables along an east-to-west transect in the northeastern South China Sea (SCS) during late winter 2016, giving us the opportunity to examine the impacts of the Kuroshio intrusion and the associated eddies. The results indicated that the subsurface (50–75 m) phytoplankton biomass chlorophyll (Chl a ) maximum (SCM) disappeared and was replaced by higher Chl a in the middle part of the transect due to the impacts of the Kuroshio intrusion and mesoscale eddies. Both flow cytometry and pyrosequencing data revealed that picoplankton abundance and community structure were significantly influenced by perturbations in complex physical processes. Picoeukaryotes represented most of the total phytoplankton biomass, and their maximum abundance (> 10 4 cells mL − 1 ) occurred within cyclonic eddy-affected regions (Stations 11 and 12), whereas the abundance of Prochlorococcus was the lowest in these regions. Prochlorococcus showed a higher abundance in the Kuroshio-affected area, while Synechococcus was mostly distributed at the upper well-lit depths, with its maximum abundance observed in surface waters (0–30 m) adjacent to the cyclonic eddy center. Heterotrophic bacteria (HBA) displayed high abundance along the transect, consistent with the total phytoplankton biomass. Phylogenetic analysis revealed 26 bacterial phyla, with major components belonging to Proteobacteria , Cyanobacteria , Actinobacteria , and Bacteroidetes , as well as SAR406. Notably, relatively more Rhodobacterales , Flavobacteriales , Alteromonadales , and Vibrionales that were distributed in surface waters of the cyclonic eddy center were specifically associated with the phytoplankton (mainly picoeukaryotes) bloom. Our study highlights the impacts of the Kuroshio intrusion in regulating the microbial ecology of the northeastern SCS and the potential coupling between phytoplankton and bacteria. [ABSTRACT FROM AUTHOR]

The abundance, biomass, and composition of phototrophic picoeukaryotes (PPE, cell size less than 3 μm) were studied in Onega Bay of the White Sea in June 2015. The highest PPE abundance and biomass were registered in the 0-5-m water layer. In the bay, in the 0-5-m water layer, the average abundance and biomass varied from 0 to 36.8 × 10 cell/L and from 0 to 117 μg С/m, respectively. The Illumina sequencing of V4 region of 18S rRNA gene revealed eight classes of PPE. Mamiellophyceae dominated both by number of reads and by operational taxonomic units. The green algae Bathycoccus prasinos, Ostreococcus tauri, and Micromonas pusila, as well as diatoms Skeletonema marinoi and Minidiscus trioculatus, were identified to the species level. [ABSTRACT FROM AUTHOR]

The standing stock of phytoplankton carbon is a fundamental property of oceanic ecosystems, and of critical importance to the development of Earth System models for assessing global carbon pools and cycles. Some methods to estimate phytoplankton carbon at large scales from ocean colour data rely on the parameterisation of carbon-to-chlorophyll ratio, which is known to depend on factors such as the phytoplankton community structure, whereas other methods are based on the estimation of total particulate organic carbon (POC), and rely on the assumption that a known fraction of POC is made up of phytoplankton carbon. The carbon-to-chlorophyll ratio is also used in marine ecosystem models to convert between carbon and chlorophyll, a common requirement. In this paper we present a novel bio-optical algorithm to estimate the carbon-to-chlorophyll ratio, and the standing stocks of phytoplankton carbon partitioned into various size classes, from ocean colour. The approach combines empirical allometric relationships of phytoplankton size structure with an absorption-based algorithm for estimating phytoplankton size spectra developed earlier. Applying the new algorithm to satellite ocean colour data from September 1997 to December 2013, the spatio-temporal variations of carbon-to-chlorophyll ratio and phytoplankton carbon across various size classes are computed on a global scale. The average annual stock of phytoplankton carbon, integrated over the oceanic mixed-layer depth, is estimated to be ∼0.26 gigatonnes, with the size-partitioned stocks of 0.14 gigatonnes for picoplankton, 0.08 gigatonnes for nanoplankton and 0.04 gigatonnes for microplankton. The root-mean-square error and the bias in the satellite-derived estimates of picoplankton carbon, when compared with corresponding in situ data, are found to be 36.23 mgC m −3 and −13.53 mgC m −3 , respectively, on individual pixels. The regional uncertainties in the estimates of phytoplankton carbon are calculated to be less than the relative uncertainties in other satellite-derived products, for most parts of the global ocean, and can amplify only for certain oceanographic regions. Although the new estimates of phytoplankton are of the same order of magnitude as those based on existing models, our study suggests that a consensus is yet to be built on the accurate sizes of the phytoplankton carbon pools; improved satellite chlorophyll products, and better estimates of inherent optical properties would be essential pre-requisites to minimising the uncertainties. [ABSTRACT FROM AUTHOR]

Picocyanobacteria in the Sargasso Sea accumulate significant amounts of Si, a finding which questions how we interpret previous regional measurements of biogenic silica (bSi) production and the role of diatoms in the open ocean. The picoplankton (<3 μm cells) contributed a measurable, and at times significant, proportion of both the total bSi standing stock and its rate of production. The 100 m integrated bSi stock and bSi production rate in the <3 μm size fraction averaged 14% and 16%, respectively, of the total. At some stations, specific rates of bSi production in the <3 μm cells were up to threefold higher than for larger cells. But among all stations and depths, the two size fractions had statistically indistinguishable specific production rates (~0.35 day−1). The estimated contributions of Synechococcus alone to the 100 m integrated bSi stock and bSi production in cells <3 μm were 15% and 55%, respectively, suggesting that half of bSi production in this small size fraction could be sustained by Synechococcus, but a majority of the bSi was not associated with living Synechococcus. Our results suggest that picoplankton have a small but persistent regional contribution to bSi stocks, which is masked by a dynamic bSi pool driven by larger cells. While a significant fraction of bSi production is attributable to picoplankton, their contributions are likely to have been included in previous analyses, making prior regional budgets still relevant. However, our understanding of the factors controlling regional bSi production and our interpretations of particulate matter elemental ratios (e.g., Si:C) may require revision. [ABSTRACT FROM AUTHOR]

Productivity around the tropical coast of North Australia has been considered to be dominated by phytoplankton. The phytoplankton encompasses a large taxon which, dependent on the source definition, can vary significantly. Traditionally, they would only include pelagic photosynthetic organisms (autotrophic), yet more recently the definition is being revised to include heterotrophic organisms. Recent advancements in oceanography over the past three decades have found the prevalence in the global ocean of cyanobacteria of sizes ranging from 0.5–1.5 µm. Two cyanobacterial generaSynechoccocusandProchlorococcusare now recognised to dominate phytoplankton cell counts and biomass specifically in the oligotrophic tropical and subtropical oceans. This study is the first to identify the importance of picoplankton as a foremost contributor to productivity in a macro-tidal ecosystem in Northern Tropical Australia. Picoplankton was found to be the dominant group of primary producers (> 99% or 6 × 109cells/L) within the phytoplankton in Darwin Harbour during the dry season of 2013. The larger phytoplankton > 2 µm contributed (< 0.05% or 2 × 102cells/L) of the population. This study further defines the influences on this dynamic system that would allow the picoplankton community to thrive, highlighting the complexity of this nitrogen-limited system, where nutrient cycling, including nitrogen fixation mediated possibly through the microbial community, plays a pertinent role. Other environmental influences include spring tides, spatial variation and increased turbidity which benefits heterotrophy over autotrophy in this ecosystem. [ABSTRACT FROM AUTHOR]

Virio- and picoplankton mediate important biogeochemical processes and the environmental factors that regulate their dynamics, and the virus-host interactions are incompletely known, especially in the deep sea. Here we report on their distributions and relationships with environmental factors at 21 stations covering a latitudinal range (2-23° N) in the Western Pacific Ocean. This region is characterized by a complex western boundary current system. Synechococcus, autotrophic picoeukaryotes, heterotrophic prokaryotes and virus-like particles (VLPs) were high (<2.4×10²-6.3×104, <34-2.8×10³, 3.9×104-1.3×106 cellsmL-1 and 5.1×105-2.7×107 mL-1, respectively), and Prochlorococcus were low (<2.3×10²-1.0×105 cellsmL-1) in the Luzon Strait and the four most southerly stations, where upwelling occurs. Covariations in the abundances of VLPs with heterotrophic and autotrophic picoplankton, and their correlation (i.e. r² =0.63 and 0.52, respectively) suggested a strong host dependence in the epi- and mesopelagic zones. In the bathypelagic zone, only abiotic factors significantly influenced VLPs abundance variation (r² =0.12). This study shows that the dynamics of virio- and picoplankton in this Western Pacific are controlled by suite of complex and depth-dependent relationship among physical and biological factors that in turn link the physical hydrography of the western boundary current system with microbial-mediated biogeochemical processes. [ABSTRACT FROM AUTHOR]

The composition, biomass and dynamics of autotrophic picoplankton (APP, size range of 0.2–2 μm) in a subtropical reservoir (Cerrillos, Puerto Rico) were examined from January to December 2013. Samples were collected monthly in the limnetic zone. Because of their small size, APP were identified and quantified using flow cytometry and epifluorescence microscopy. The warm monomictic nature of this reservoir resulted in thermal stratification during most of the year. Near surface dissolved oxygen was relatively high (7.61 mg/L). However, hypoxic and anoxic conditions were present in the hypolimnion during the whole period of study. Flow cytometry analyses allowed the identification of two populations of picoplanktonic organisms. Picocyanobacteria groups (phycoerythrin-richSynechococcustype) were dominant throughout the study, with maximum summer abundances of 6.6 × 104cells/mL, although picoeukaryotes were temporally important. Annual mean total chlorophyll-a(Chl-a) in the euphotic zone was 7.54 ± 3.41 μg/L. Fractionated Chl-aexhibited a relatively constant pattern: ranging from 0.24 to 1.99 μg/L. The average contribution of APP to total Chl-aconcentration was 11%. Chl-aconcentration reflected the annual variation in picoplankton, being higher during summer when solar radiation was also higher. The picoplanktonic community was structured in accordance to thermal stratification, nutrients and light availability. Understanding the dynamics of picoplankton and their response to environmental change are important because they are a good indicator of the trophic status of reservoirs, especially drinking water reservoirs, and can guide management decisions. [ABSTRACT FROM PUBLISHER]

The Proterozoic–Phanerozoic transition recorded a general trend of decrease in phytoplankton cell size, in contrast to the earlier and much larger Ediacaran acritarchs. Particularly minute, unornamented but sculptured organic-walled microfossils have been recovered from the lower Cambrian Lükati Formation in northern Estonia. The lack of any significant thermal alteration in this formation allowed for excellent preservation of fine microstructures on these microfossils. Among the rich palynomorph assemblages in the Lükati Formation, a new species of tiny, spheroidal eukaryotic microfossil is recorded:Reticella corrugatagen. et sp. nov. It is characterised by a corrugated and flexible vesicle wall that is densely perforated by nano-scale pores. Despite its unique morphology, the new species shares diagnostic characters with fossil and extant prasinophyte algae.Reticella corrugatais among the smallest microfossils with typical eukaryotic morphology (conspicuous wall sculpture) and contributes to the diversity of the size class of small acritarchs. Size, abundance, inferred prasinophyte affinity and eukaryotic wall sculpture make this new taxon a likely member of the early eukaryotic picoplankton. [ABSTRACT FROM AUTHOR]

Sequencing of rRNA gene polymerase chain reaction amplicons (rRNA tags) is the most common approach for investigating microbial diversity. The recent development of high-throughput sequencing (HTS) technologies has enabled the exploration of microbial biodiversity at an unprecedented scale, greatly expanding our knowledge on the microbiomes of marine ecosystems. These approaches provide accurate, fast and cost efficient observations of the marine communities, and thus, may be suitable tools in biodiversity monitoring programs. To reach this goal, consistent and comparable methodologies must be used over time and within sites. Here, we have performed a cross-platform study of the two most common HTS methodologies, i.e. 454-pyrosequencing and Illumina tags to evaluate their usefulness in biodiversity monitoring and assessment of environmental status. Plankton biodiversity has been compared through both methodologies by sequencing the 16S and 18S rRNA genes of a set of samples collected in the coast of Barcelona (NW Mediterranean). The results show that, despite differences observed in the rare OTUs retrieved, both platforms provide a comparable view of the marine picoplankton communities. On a taxonomic level, there was an accurate overlap in the detected phyla between the two methods and the overall estimates of alpha- and betadiversity were comparable. In addition, we explored the concept of ‘indicator species’ and found that certain taxa (i.e. members of the Gammaproteobacteria among others) as well as the ratio between some phylogenetic groups (i.e. the ratio of Alphaproteobacteria / Gammaproteobacteria, Alteromonas / SAR11 and Alteromonas + Oceanospirillales / SAR11) have potential for being useful indicators of environmental status. The data show that implementing new protocols and identifying indicators of environmental status based on rRNA amplicon sequencing is feasible, and that is worth exploring whether the identified indices are universally applicable.

Picocyanobacteria are extremely important organisms in the world’s oceans and freshwater ecosystems. They play an essential role in primary production and their domination in phytoplankton biomass is common in both oligotrophic and eutrophic waters. Their role is expected to become even more relevant with the effect of climate change. However, this group of photoautotrophic organisms still remains insufficiently recognized. Only a few works have focused in detail on the occurrence of massive blooms of picocyanobacteria, their toxicity and allelopathic activity. Filling the gap in our knowledge about the mechanisms involved in the proliferation of these organisms could provide a better understanding of aquatic environments. In this review, we gathered and described recent information about allelopathic activity of picocyanobacteria and occurrence of their massive blooms in many aquatic ecosystems. We also examined the relationships between climate change and representative picocyanobacterial genera from freshwater, brackish and marine ecosystems. This work emphasizes the importance of studying the smallest picoplanktonic fractions of cyanobacteria.

We measured the vertical and seasonal distribution of picoplankton (0.2-2 µm) and nanoplankton (2-20 µm) in the photic layer of Cuban southern oceanic and coastal waters. The concentration of the different fractions was estimated by epifluorescence microscopy. Heterotrophic components from the different fractions showed higher vertical stratification in the oceanic station in comparison to the coastal one. The autotrophic components showed an irregular vertical distribution pattern, both in coastal and oceanic stations. In all the analyzed stations, the heterotrophic bacteria showed an inverse correlation with the autotrophic (r= -0.98), and the heterotrophic nanoplankton (r= -0.96). Auto and heterotrophic nanoplankton probably regulate bacteria abundance by predation, although autotrophic nanoplankton may represent a source of organic matter for microorganisms. Rev. Biol. Trop. 55 (2): 449-457. Epub 2007 June, 29.Determinamos la distribución vertical y estacional del picoplancton (0.2-2 µm) y el nanoplancton (2-20 µm) en la capa fótica de las aguas oceánicas y costeras al sur de Cuba. La concentración de las diferentes fracciones fue estimada mediante microscopía de epifluorescencia. Los componentes heterotróficos de las diferentes fracciones mostraron una elevada estratificación vertical en la estación oceánica en comparación con la estación costera. Por otro lado, los componentes autotróficos presentaron un patrón de distribución vertical irregular tanto en la estación costera como en la oceánica. En los análisis realizados las bacterias heterótrofas mostraron una correlación inversa con el nanoplancton autótrofo (r= -0.98), y con el nanoplancton heterótrofo (r= -0.96). Estos resultados sugieren que el nanoplancton (autótrofo y heterótrofo) probablemente regula la abundancia de bacterias mediante la depredación, además que el nanoplancton autótrofo pudiera representar una fuente de materia orgánica para los microorganismos.

Sharp boundaries in the physical environment are usually associated with abrupt shifts in organism’s abundance, activity and diversity. Aquatic surface microlayers (SML) form a steep gradient between two contrasted environments, the atmosphere and surface waters, where they regulate the gas exchange between both environments. They usually harbor an abundant and active microbial life: the neuston. Few ecosystems are subjected to such a high UVR regime as high altitude lakes during summer. Here, we measured bulk estimates of heterotrophic activity, community structure and single-cell physiological properties by flow cytometry in 19 high-altitude remote Pyrenean lakes and compared the biological processes in the SML with those in the underlying surface waters. Phototrophic picoplankton (PPP) populations, were generally present in high abundances and in those lakes containing PPP populations with phycoerythrin (PE), total PPP abundance was higher at the SML. Heterotrophic nanoflagellates (HNF) were also more abundant in the SML. Bacteria in the SµL had lower leucine incorporation rates, lower percentages of live cells, and higher numbers of highly-respiring cells, likely resulting in a lower growth efficiency. No simple and direct linear relationships could be found between microbial abundances or activities and environmental variables, but factor analysis revealed that, despite their physical proximity, microbial life in SML and underlying waters was governed by different and independent processes. Overall, we demonstrate that piconeuston in high altitude lakes has specific features different from those of the picoplankton, and that they are highly affected by potential stressful environmental factors, such as high UVR radiation.

An intense bloom of red Noctiluca scintillans (NS) occurred off the Rushikulya estuarine region along the east coast of India, an important site for mass nesting events of the vulnerable Olive Ridley sea turtle. At its peak, densities of NS were 3.3 × 10 5 cells-l − 1 , with low relative abundance of other phytoplankton. The peak bloom coincided with high abundance of gelatinous planktivores which may have facilitated bloom development by their grazing on other zooplankton, particularly copepods. Ammonium concentrations increased by approximately 4-fold in the later stages of bloom, coincident with stable NS abundance and chlorophyll concentrations in the nano- and microplankton. This increase likely was attributable to release of intracellular ammonium accumulated through NS grazing. Dissolved oxygen concentrations decreased in sub-surface waters to near hypoxia. Micro-phytoplankton increasingly dominated chlorophyll- a biomass as the bloom declined, with diminishing picoplankton abundance likely the result of high predation by the ciliate Mesodinium rubrum . Together, these data illustrate factors that can disrupt ecosystem balance in this critically important Indian coastal region. [ABSTRACT FROM AUTHOR]

In oligotrophic regions, picophytoplankton can play a key role in total carbon production and energy transfer. Since the mesoscale hydrographic variability can influence the resource availability and therefore the biological communities, here we studied the linkage between hydrography, resource supply, abundance, and biomass contribution of prokaryotic picoplankton in the south Balearic Sea during the stratified season. The sampling area covered the confluence of two different water masses, the fresher new Atlantic water, and the saltier resident Atlantic water. Our results showed higher Synechococcus abundances in the more oligotrophic new Atlantic water mass and suggest that the spatial patterns of prokaryotic picophytoplankton are dictated by the mesoscale processes in this region. The summer stratification condition separated clearly the surface mixed layer (ML) from the deep layer (DL); our results support different limiting factors for picophytoplankton in the two layers: nutrient and light availability in the ML and DL, respectively. We also obtained no significant difference in the Synechococcus biomass contribution to total autotrophic biomass within the water column, but higher contribution in the new Atlantic water mass. These results demonstrate the general importance of picophytoplankton as carbon producers in oligotrophic waters and particularly their variability as biomass source at the mesoscale. [ABSTRACT FROM AUTHOR]

Spatial distribution of phaeopigment and size-fractionated chlorophyll a (Chl a) concentrations were examined in relation to hydrographic conditions in the northern South China Sea (NSCS) during a survey from 20 August to 12 September, 2014. The total Chl a concentration varied from 0.006 to 1.488 µg/L with a mean value of 0.259±0.247 (mean±standard deviation) µg/L. Chl a concentration was generally higher in shallow water (<200 m) than in deep water (>200 m), with mean values of 0.364±0.311 µg/L and 0.206±0.192 µg/L respectively. Vertically, the maximum total Chl a concentration appeared at depths of 30-50 m and gradually decreased below 100 m. The size-fractionated Chl a concentrations of grid stations and time-series stations (SEATS and J4) were determined, with values of pico- (0.7-2 µm), nano- (2-20 µm) and micro-plankton (20-200 µm) ranging from 0.001-0.287 (0.093±0.071 µg/L), 0.004-1.149 (0.148±0.192 µg/L) and 0.001-0.208 (0.023±0.036 µg/L), respectively. Phaeopigment concentrations were determined at specific depths at ten stations, except for at station A9, and varied from 0.007 to 0.572 (0.127±0.164) µg/L. Nano-and pico-plankton were the major contributors to total phytoplankton biomass, accounting for 50.99%±15.01% and 39.30%±15.41%, respectively, whereas microplankton only accounted for 9.39%±8.66%. The results indicate that the contributions of microplankton to total Chl a biomass were less important than picoplankton or nanoplankton in the surveyed NSCS. Different sized-Chl a had similar spatial patterns, with peak values all observed in subsurface waters (30-50 m). The summer monsoon, Kuroshio waters, Zhujiang (Pearl) River plume, and hydrological conditions are speculated to be the factors controlling the abundance and spatial heterogeneity of Chl a biomass in the NSCS. [ABSTRACT FROM AUTHOR]

We observed reproductive swarmer cells of the nassellarian and spumellarian polycystine radiolarians Didymocyrtis ceratospyris, Pterocanium praetextum , Tetrapyle sp., and Triastrum aurivillii using light, scanning and transmission electron microscopy. The swarmer cells had subspherical to ovoid or spindle shapes with two unequal flagella tapered to whip-like ends. The cell size was approximately 2.5–5.5 μm long and 1.6–2.2 μm wide, which is significantly smaller than that of the collodarian (colonial or naked) polycystine radiolarians. Transmission electron microscopy revealed that the swarmer cells possessed a nucleus, mitochondria with tubular cristae, Golgi body, and small lipid droplets in the cytoplasm; they also had a large vacuole in which a single crystalline inclusion (approx. 1.0–1.5 μm) that was probably celestite (SrSO 4 ) was enclosed. The swarmer cells were released directly from the parent cells. At that time, morphological change such as encystment was not observed in the parent cells, and the axopodia remained extended in a period of swarmer reproduction for floating existence. This may have prevented the polycystine swarmers from rapidly sinking down to great depths. Thus, we concluded that the polycystine radiolarians release the swarmer cells into the photic layer in the same way as the symbiotic acantharians. [ABSTRACT FROM AUTHOR]

The Arabian Sea experiences moderate to weak upwelling along the south-west coast of India, which subsequently propagates towards the north. This causes variation in plankton community composition, which is addressed in the present study. Here we report the spatial variations in distribution of phytoplankton groups along the north-south transect in the eastern Arabian Sea based on marker pigments supported with flow-cytometric and microscopic analyses. 15 phytoplankton pigments were identified using High-performance liquid chromatography (HPLC) and the chemotaxonomic software (CHEMTAX) analysis associated these to seven major group of phytoplankton. The phytoplankton biomass, chlorophyll a (Chl a ) was higher in southern stations with dominance of fucoxanthin whereas, divinyl chlorophyll a (divinyl Chl a ), marker pigment of Prochlorococcus was present only in the northern region. Microscopic observation revealed the dominance of larger forms; diatoms ( Chaetoceros coarctatum and Nitzschia sp.) and dinoflagellates ( Scrippsiella sp., Oxytoxum nanum and Oxytoxum sp.) in the southern region. Furthermore, a study of plankton size distribution showed dominance of picoplankton ( f pico ) followed by nanoplankton ( f nano ) along the northern stations with comparatively higher microplankton ( f micro ) in the south. This study clearly showed the influence of different environmental conditions on the phytoplankton community as reflected in dominance of diatoms in the southern (south of 12 °N) and that of picoplankton in the northern (north of 12 °N) region. [ABSTRACT FROM AUTHOR]

We investigated the effect of diel variation in cell properties of picoplankton prey and the intrinsic grazing rhythm of heterotrophic nanoflagellates (HNF) on the diel periodicity of HNF grazing. To achieve this aim, 2-point dilution experiments with short incubation time were performed over 24 h in the South China Sea and Hong Kong (HK) coastal water to assess the prey cell properties and HNF grazing. Fluorescently labeled bacteria (FLB) were added for inference of the intrinsic diel rhythm of HNF feeding. Cell sizes of in situ prey at both study sites and the carbon to nitrogen molar ratio (C:N) of picoplankton prey in HK coastal water exhibited an increasing trend during the day, indicating cellular growth of prey during the period. Higher grazing rates on FLB during the day compared with night at both study sites suggested higher intrinsic HNF feeding during the day. Diel patterns of HNF grazing with increasing trend during the day and decreasing trend at night were observed for all in situ prey at both study sites. Statistical analysis suggests that both time of day and prey C:N could have significant effects on the diel variation of HNF feeding. [ABSTRACT FROM AUTHOR]

The detection of plankton DNA is one of the important methods for the diagnosis of drowning from postmortem tissues. This study investigated the quantities of picoplankton (Cyanobacteria) DNA in the lung, liver, kidney tissues and blood of drowned and non-drowned rabbits, and the sensitivity of detection of picoplankton DNA by polymerase chain reaction (PCR) detect for the diagnosis of death from drowning. For this purpose, the DNA of the 16S ribosomal RNA gene of picoplankton was quantitatively assayed from the tissues of drowned and non-drowned rabbits immersed in water after death. Each of the liver, kidney and lung tissues and blood were obtained from drowned and non-drowned rabbits. Picoplankton DNA in the tissues was extracted using the DNeasy® Blood & Tissue kit to determine the yield of picoplankton DNA from each tissue. TaqMan real-time PCR was performed for quantitative analysis of picoplankton DNA. Target DNA was detected in the liver, kidney and lung samples obtained from the drowned rabbits, while no picoplankton DNA was detected in the non-drowned rabbit tissues (except in lung samples). The results verified that direct PCR for the detection of picoplankton DNA is useful for the diagnosis of drowning. Although we observed seasonal changes in the quantity of picoplankton in river water, we were able to detect DNA from various organs of drowned bodies during the season when picoplankton were not the most abundant. [ABSTRACT FROM AUTHOR]

The Sargasso Sea is a dynamic physical environment in which strong seasonal variability combines with forcing by mesoscale (~100 km) eddies. These drivers determine nutrient, light, and temperature regimes and, ultimately, the composition and productivity of the phytoplankton community. On four cruises (2011 and 2012; one eddy per cruise), we investigated links between water column structure and phytoplankton community composition in the Sargasso at a range of time and space scales. On all cruises, cyanobacteria ( Prochlorococcus and Synechococcus ) dominated the phytoplankton numerically, while haptophytes were the dominant eukaryotes (up to 60% of total chl- a ). There were substantial effects of mesoscale and sub-mesoscale forcing on phytoplankton community composition in both spring and summer. Downwelling (in anticyclones) resulted in Prochlorococcus abundances that were 22−66% higher than at ‘outside’ stations. Upwelling (in cyclones) was associated with significantly higher abundances and POC biomass of nanoeukaryotes. In general, however, each eddy had its own unique characteristics. The center of anticyclone AC1 (spring 2011) had the lowest phytoplankton biomass (chl- a ) of any eddy we studied and had lower nitrate+nitrite (N+N <5 mmol m −2 ) and eukaryote chl- a biomass as compared to its edge and to the Bermuda Atlantic Time-Series station (BATS). At the center of cyclone C1 (summer 2011), we observed uplift of the 26.5 kg m −3 isopycnal and high nutrient inventories (N+N=74±46 mmol m −2 ). We also observed significantly higher haptophyte chl- a (non-coccolithophores) and lower cyanobacterial chl- a at the center and edge of C1 as compared to outside the eddy at BATS. Cyclone C2 (spring 2012) exhibited a deep mixed layer, yet had relatively low nutrient concentrations. We observed a shift in the taxonomic composition of haptophytes between a coccolithophore-dominated community in C2 (98% of total haptophyte chl- a ) and a non-coccolithophore community at BATS. In summer 2012, downwelling associated with anticyclone AC2 occurred at the edge of the eddy (not at the center), where AC2 interacted with a nearby cyclone. At the edge, we found significantly lower Synechococcus abundances and higher eukaryote chl- a compared to the center of AC2 and BATS. These along-transect nuances demonstrate the significance of small-scale perturbations that substantially alter phytoplankton community structure. Therefore, while seasonality in the North Atlantic is the primary driver of broad-scale trends in phytoplankton community composition, the effects of transient events must be considered when studying planktonic food webs and biogeochemical cycling in the Sargasso Sea. [ABSTRACT FROM AUTHOR]

Effects of nutrient enrichment on 400-L coastal phytoplankton community cultures were examined under 3 pCO levels [ambient (400), 800 and 1200 μatm]. Three days after addition of the nutrients, rapid increases in phytoplankton pigments and cell numbers were noticed. Relative growth rates of diatoms and dinoflagellates were higher than prasinophytes and haptophytes during early stages of the culture experiment, and only limited effects of increased CO were observed on nutrient consumption, biomass and cell numbers. Ocean acidification showed significant effects on phytoplankton composition during the post-blooming period with negligible dissolved nutrients; up to 70 % of total cells were picoplankton in the 1200-μatm condition as compared to 20 % in the ambient condition. An increase in chlorophyll b and a flow cytometry analysis of the cultured strains strongly suggested Micromonas-like (Prasinophyceae) picoplankton dominated in the acidified conditions. It is likely that the effects of ocean acidification are significant in low-nutrient conditions such as during the post-blooming period. [ABSTRACT FROM AUTHOR]

The ecological importance and diversity of pico/nanoplanktonic algae remains poorly studied in marine waters, in part because many are tiny and without distinctive morphological features. Amongst green algae, Mamiellophyceae such as Micromonas or Bathycoccus are dominant in coastal waters while prasinophytes clade VII, yet not formerly described, appear to be major players in open oceanic waters. The pigment composition of 14 strains representative of different subclades of clade VII was analyzed using a method that improves the separation of loroxanthin and neoxanthin. All the prasinophytes clade VII analyzed here showed a pigment composition similar to that previously reported for RCC287 corresponding to pigment group prasino-2A. However, we detected in addition astaxanthin for which it is the first report in prasinophytes. Among the strains analyzed, the pigment signature is qualitatively similar within subclades A and B. By contrast, RCC3402 from subclade C ( Picocystis) lacks loroxanthin, astaxanthin, and antheraxanthin but contains alloxanthin, diatoxanthin, and monadoxanthin that are usually found in diatoms or cryptophytes. For subclades A and B, loroxanthin was lowest at highest light irradiance suggesting a light-harvesting role of this pigment in clade VII as in Tetraselmis. [ABSTRACT FROM AUTHOR]

The recent interest in the plankton structures and dynamics in tropical and subtropical lakes has revealed important trends that set these lakes apart from temperate lakes, and one of the main differences is the enhanced importance of the microbial food web with respect to net plankton. Ciliates are a key component of subtropical and tropical microbial webs because of their role as dominant picoplankton grazers and their ability to channel picoplankton production to the uppermost trophic levels. Plankton ciliates have been found to play a crucial role in the survival of fish larvae in lakes that share several features with Lake Catemaco, a eutrophic tropical Mexican lake. Therefore, the plankton ciliate composition, abundance, and biomass of Lake Catemaco were studied to assess their role in the microbial food web. The data were obtained from surface and bottom water samples collected at eleven points during three surveys in 2011 and an additional survey in 2013, with the surveys covering the local climatic seasons. The most abundant components of the plankton ciliate assemblages were small prostomatids (Urotricha spp.), choreotrichs (Rimostrombidium spp.), cyclotrichs (Mesodinium and Askenasia), and scuticociliates (Cyclidium, Cinetochilum, Pleuronema, and Uronema). Other important ciliates in terms of abundance and/or biomass were haptorids (Actinobolina, Belonophrya, Monodinium, Paradileptus, and Laginophrya), Halteria, oligotrichs (Limnostrombidium and Pelagostrombidium), Linostomella, Bursaridium, Cyrtolophosis, and Litonotus. The ciliate abundance averaged 57 cells mL-1 and ranged from 14 to 113 cells mL-1. The mean ciliate biomass was 71 µg C L-1 and ranged from 10 to 202 µg C L-1. Differences were not detected in ciliate abundance or biomass between the sampling points or sampling depths (surface to bottom); however, significant differences were observed between seasons for both variables. Nano-sized filamentous cyanobacteria were the most abundant component of the plankton, and their abundance was assessed through epifluorescence microscopy counts. The autotrophic and heterotrophic picoplankton abundance was measured through epifluorescence, and their abundance and biomass were higher at the study site relative to other shallow freshwater ecosystems. The total ciliate biomass distribution patterns were similar to those of filamentous cyanobacteria and autotrophic or heterotrophic picoplankton, although the nanociliate biomasses peaked when the picoplankton and filamentous cyanobacteria were least abundant. The consequences of this increased importance of ciliates on the structure of the plankton at Lake Catemaco will be discussed along with the probable causes. [ABSTRACT FROM AUTHOR]

The Lake Michigan ecosystem has undergone numerous, systemic changes (reduced nutrient, changing climate, invasive mussels) that have altered portions of the food web and thus, appear to have changed the lake's trophic state. That said, little is known about the components of the microbial food web (MFW, heterotrophic and phototrophic pico, nano, and micro-plankton), which we hypothesized have compensated as a food source for crustacean zooplankton given the recent declines in the biomass of large phytoplankton (mainly diatoms). Therefore, we measured the abundance of the entire MFW using complementary microscopic techniques, flow cytometry, and size fractionated chlorophyll concentrations at sites in northern and southern Lake Michigan, and one site in Lake Superior; the latter site served as a benchmark for oligotrophic conditions. In addition, a historic comparison was made between 1987 and 2013 for the southern Lake Michigan site. Ppico numbers (i.e., picocyanobacteria) in 2013 were lower compared with those in the 1980s; however, the percent contribution of the < 2 µm fraction increased 2-fold (> 50% of total chlorophyll). The abundance of small, pigmented chrysomonads and cryptomonads (Pnano size category) was not significantly different between 1987 and 2013 at the same time Pmicro did decline; this shift towards Ppico and Pnano dominance may be related to the recent oligotrophication of Lake Michigan. The abundance of ciliated protists (Hmicro size class) was 3-fold lower in 2013 compared with levels in 1987, while the abundance of both Hpico (eubacteria, range 0.24-1.36 x 106 cells mL- 1) and Hnano (mainly colorless chrysomonads; range 0.11-6.4 x 10³ cells mL-1) remained stable and reflected the resilience of bacteria-flagellate trophic linkage. [ABSTRACT FROM AUTHOR]

The structure of the microbial food web was studied in six estuary areas along the eastern Adriatic coast during March, July and October 2012. Limitation by phosphorus, not nitrogen, was a common feature for all studied estuaries. Heterotrophic bacteria and autotrophic picoplankton (APP) (particularly picoeukaryotes and Synechococcus) can reach notable abundances and biomasses, suggesting potential importance of the picoplankton community in P-limited estuarine environments. The main features of the microbial community structure in these environments included: (1) higher heterotrophic biomass in comparison to autotrophic biomass within the picoplankton community; (2) general domination of picoeukaryotes within the APP community, and increase of absolute and relative biomass of prokaryotic autotrophs (Prochlorococcus and Synechococcus) in the total APP in P-limited conditions; (3) domination of Synechococcus over Prochlorococcus biomass in all studied conditions, and common spatial distribution of these two groups of cyanobacteria, which was mostly determined by concentration of phosphorus; (4) relatively high contribution (about 50%) of LNA bacteria in the total bacterial abundance; and (5) relatively high contribution (about 33%) of heterotrophic pico-flagellates in the total flagellate abundance. [ABSTRACT FROM PUBLISHER]