Picophytoplankton Community Research Articles

Diversity of Marine Eukaryotic Picophytoplankton Communities with Emphasis on Mamiellophyceae in Northwestern Philippines

Eukaryotic picophytoplankton (EPP) play vital roles in primary productivity and biogeochemical cycling in the marine environment. In this study, we explored the diversity of EPP communities in two different embayments and the shifts in their community structuring during monsoonal reversal in the northwestern Philippines. Water samples were collected weekly from late northeast (NE) monsoon to intermonsoon (IM) or summer periods (February–April 2019) in Bolinao, Pangasinan, and once in January in Masinloc, Zambales. EPP community profiling was done through targeted sequencing of the V4 region of the 18S rRNA gene. Grouping of samples based on physicochemical parameters was consistent with that of community beta diversity, suggesting strong clustering between late NE and IM periods. This exhibits short- term community shifts of EPPs possibly associated with the monsoonal transition. Specifically, overall EPP alpha diversity increased towards summer coupled with increased temperature and lower nutrient concentrations. NE monsoon samples from Bolinao and Masinloc were dominated by Chlorophyta and Stramenopiles, while Prymnesiophyta, Rhizaria, and Picozoa dominated the IM period samples in Bolinao. Specifically, the prasinophytes (Chlorophyta) Ostreococcus and Nannochloris distinguished the late NE communities of Masinloc and Bolinao, respectively. Phylogenetic analysis of dominant photosynthetic EPP further revealed the presence of Clades B5 and A1 of Micromonas, as well as Clades B and E of Ostreococcus. Tree topology of Ostreococcus diversity suggests the presence of a clade distinct from other established clades, possibly indicating novel diversity in the West Philippine Sea. This is the first report of these major picophytoplankton in Philippine waters, suggesting their significance and potential “hidden” diversity, which warrants further studies.

Environmental influence on the picophytoplankton community structure in the central and northern Bay of Bengal

The Bay of Bengal (BoB) is a low productive, oligotrophic ecosystem. The picophytoplankton (Pico; <3μm) play a significant role in the biogeochemistry of such ecosystems. The information on the Pico is exiguous from this Bay. Here, the seasonal and spatial variations in abundance and carbon biomass of the Pico groups were investigated (July’08 to April’09) in the central and northern BoB in relation to the environmental conditions. Surface samples were collected from twenty-two stations across the Chennai–Port Blair–Kolkata sector. The Synechococcus group was dominant during most of the year, both in abundance and biomass, with few exceptions when Prochlorococcus dominated the Pico community. The Synechococcus abundance and carbon biomass were higher in the nutrient-rich coastal regions and correlated negatively with salinity. The picoeukaryotes exhibited a distribution pattern similar to Synechococcus, although relatively lower in numbers and biomass. The Prochlorococcus abundance and biomass were higher in the open ocean regions of the central BoB (CBoB). The relatively lower abundance of Prochlorococcus in the northern BoB implies an influence of the freshwater influx, as depicted from the positive correlation with salinity. During the winter monsoon, a cyclonic storm coincided with a deep mixed layer characterized by high concentrations of nutrients and chlorophyll-a in the CBoB. The Pico abundance and biomass were relatively higher during this period, with the Prochlorococcus dominating the former and Synechococcus, the latter. The real-time observations of the surface distribution of Pico abundance and carbon biomass in the BoB revealed seasonal variations that were modulated by the episodic mesoscale features.

Functional redundancy in natural pico-phytoplankton communities depends on temperature and biogeography.

Biodiversity affects ecosystem function, and how this relationship will change in a warming world is a major and well-examined question in ecology. Yet, it remains understudied for pico-phytoplankton communities, which contribute to carbon cycles and aquatic food webs year-round. Observational studies show a link between phytoplankton community diversity and ecosystem stability, but there is only scarce causal or empirical evidence. Here, we sampled phytoplankton communities from two geographically related regions with distinct thermal and biological properties in the Southern Baltic Sea and carried out a series of dilution/regrowth experiments across three assay temperatures. This allowed us to investigate the effects of loss of rare taxa and establish causal links in natural communities between species richness and several ecologically relevant traits (e.g. size, biomass production, and oxygen production), depending on sampling location and assay temperature. We found that the samples' biogeographical origin determined whether and how functional redundancy changed as a function of temperature for all traits under investigation. Samples obtained from the slightly warmer and more thermally variable regions showed overall high functional redundancy. Samples from the slightly cooler, less variable, stations showed little functional redundancy, i.e. function decreased when species were lost from the community. The differences between regions were more pronounced at elevated assay temperatures. Our results imply that the importance of rare species and the amount of species required to maintain ecosystem function even under short-term warming may differ drastically even within geographically closely related regions of the same ecosystem.

Environmental factors controlling the dynamics of phytoplankton communities during spring and fall seasons in the southern Sunda Shelf.

Phytoplankton play important roles in marine ecosystems and have major impacts on the global biogeochemical cycles, yet the knowledge of the processes for oceanic environments controlling the phytoplankton dynamics still needs to be improved, particularly in the coastal ecosystems. Monitoring diversity of phytoplankton communities, including micro/nano- and pico-forms, as well as their dynamics in relation to environmental factors, is thus of critical significance. Here, we conducted two cruises in the southern Sunda Shelf during spring and fall 2016 to investigate the community structure and dynamics of phytoplankton assemblages in relation to environmental factors combining microscopy and flow cytometry. Micro/nanophytoplankton communities changed from Trichodesmium spp. and diatoms co-dominance in spring to communities dominated by Trichodesmium spp. in fall. Diatoms were a highly diverse group, with 78 species, followed by dinoflagellates (35 species). Differences in average abundance of micro/nanophytoplankton between spring and fall might be attributed to the shift in dominance of Trichodesmium spp. and diatoms. Picophytoplankton characterized by a great abundance of picocyanobacteria were dominated by Synechococcus. The dynamics of picophytoplankton during spring and fall might be associated with the differences in dominant Synechococcus strains. Environmental variables had significant influence on the dynamics of phytoplankton communities, especially nitrate and silicate. Silicate was a major driver responsible for the variations of micro/nanophytoplankton. Low silicate potentially limited the diatom growth during fall, leading to the overwhelming dominance of Trichodesmium spp. Picophytoplankton showed positive correlations with nitrate and silicate, suggesting that these nutrients collectively controlled variations in picophytoplankton communities during spring and fall.

Picophytoplankton variability: Influence of Rossby wave propagation in the southeastern Arabian Sea

The westward propagating Rossby waves and associated currents transport low salinity nutrient-rich water mass from the Bay of Bengal into the southeastern Arabian Sea (SEAS) during winter monsoon, and ensuing changes in physicochemical properties of water column regulate phytoplankton distribution in the latter region. Herein, we elucidate the influence of Rossby wave propagation on picophytoplankton community composition in the SEAS. We compare the picophytoplankton community structure between the Rossby front (low saline, high nitrate concentration and strong current) and the surrounding non-frontal regions (high saline, low nitrate and weak current) along a cruise track in the SEAS during early winter monsoon (November 2016). The higher abundance of phycoerythrin-rich Synechococcus (‘dim’ and ‘bright’ types) and lower abundance of Prochlorococcus in the frontal region than the non-frontal regions suggest that salinity and nitrate concentration in the water column significantly influence the picophytoplankton community dynamics. We argue that the advection of low saline water mass from the Bay of Bengal associated with Rossby wave propagation introduces a ‘dim’ phycoerythrin-rich Synechococcus group in the frontal region, and the ensuing physicochemical changes in water column favor their growth, whereas low cell abundance of Prochlorococcus and Phycocyanin-rich Synechococcus suggests restricted growth of both the groups. These inferences indicate that Rossby wave associated circulation pattern and physicochemical changes in the water column regulate picophytoplankton community composition and its contribution to food web dynamics in the SEAS.

Synthesis of mycosporine-like amino acids by a size-fractionated marine phytoplankton community of the arctic beaufort sea

During the RV-ARAON cruise, a comparative study on the biosynthesis of mycosporine-like amino acids (MAAs) was conducted for the size-fractionated phytoplankton of the Beaufort Sea (Arctic). The MAAs contents in the micro-phytoplankton community (>20 μm size) is considerably higher than that observed in the nano- (20–2 μm size) and pico-phytoplankton (<2 μm size) communities. The micro-phytoplankton of the Mackenzie Shelf had a relatively higher Chlorophyll a (Chl a) concentration. Considering the total phytoplankton community, the MAAs concentration as well as net production of individual MAAs (such as shinorine and palythine) were higher at the Mackenzie Shelf rather than at the sites located beyond the Beaufort Sea; precisely, the highest net production rates of shinorine and palythine were 0.211 (±0.02) ng C L−1 d−1 and 0.136 (±0.001) ng C L−1 d−1 respectively (No other MAAs were detected). The micro-phytoplankton used around 0.5% of the total carbon uptake for the synthesis of MAAs. Compared to the smaller phytoplankton community, the micro-phytoplankton utilized more of their energy for the biosynthesis of MAAs; on the other hand, nano- and pico-phytoplankton focused on cellular activity and had poor biosynthesis of MAAs. This clearly indicates the phytoplankton size-dependent variation in the biosynthesis of MAA in the natural phytoplankton community. This study revealed the environmental adaptation of the various sizes of phytoplankton community as well as their physiological response in the Arctic Beaufort Sea.

A metabarcoding survey for seasonal picophytoplankton composition in two coral reefs around Sesoko Island, Okinawa, Japan

Okinawa is the southwesternmost part of Japan and represents the northern limit of coral reefs that are influenced by the Kuroshio warm current. We have been studying two coral reefs off Sesoko Island, in northern Okinawa, to evaluate coral reef environments since 2014. In this study, we applied a metagenomic approach to analyze the seasonal composition of picophytoplankton at these two reef sites (Sesoko Minami and Ishikawabaru). The two sites were selected for their differences: Sesoko Minami faces the open ocean and experiences relatively small land effects, while Ishikawabaru experiences relatively high land effects and is surrounded by barrier reefs. Seawater samples were collected from these two sites and filtered through a 1.6 μm pore filter followed by a 0.2 μm membrane filter. DNA was extracted from the 0.2 μm filters, and the V1-V2 region of the 16S rRNA gene (260 bp) was amplified before sequencing using next-generation sequencing techniques. Cyanobacteria or chloroplast sequences were selected for further metabarcording analyses. Interestingly, almost no differences were indicated between the two sites and more than 95% of the picophytoplankton at both sampling sites was composed of only 5 to 7 OTUs. The seasonal composition of picophytoplankton clearly changed; in summer (Jul 2014, May to Jul 2015, and May to Aug 2016) OTU 1 (Synechococcus sp. NC007513) was most abundant. OTU 4 (Prochlorococcus marinus NC009091) also reached its maximum abundance in summer (May to Aug 2016), while OTU 6 (Bathycoccous prasinos FN563099) increased in winter. OTU 8 (Micromonas commoda RCC299 FJ858267) was present at every sampling period and accounted for 5–20% of the total picophytoplankton contig numbers. Our results indicate that seasonal changes clearly influence fluctuations in picophytoplankton community compositions.

Spatial variability in picophytoplankton, bacteria and viruses in waters of the Great Australian Bight (southern Australia)

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.

Responses of the picophytoplankton community to temperature fronts in the northeastern Arabian Sea during the northeast monsoon

We investigated the responses of the picophytoplankton (< 3 µm) community to a temperature filament and front through high resolution spatial (~ 1 NM) sampling (November-23 to December-11, 2012) in the northeastern Arabian Sea (69°E, 18.85°N to 20.25°N). Samples were collected at discrete depths within the 100 m water column. Synechococcus dominated the picophytoplankton community numerically and in terms of biomass along the entire transect. To investigate the patterns of variability in picophytoplankton distribution, depending on the water mass characteristics, the entire transect was divided into four zones (1) south of filament (SFIL) with warm oligotrophic waters, (2) filament (FIL) with cooler and low saline waters, (3) north of filament (NFIL) with relatively cooler waters than the SFIL and (4) front (FRO) with relatively cooler and less saline waters than the FIL. Depth-integrated abundance and biomass of Synechococcus were relatively higher within the FIL and FRO whereas Prochlorococcus and picoeukaryotes were abundant in SFIL and NFIL. Redundancy analysis of environmental variables and picophytoplankton abundance showed that lower saline water mass within the mesoscale features harbored relatively higher Synechococcus abundance and biomass. Two Synechococcus ecotypes were distinguished based on the fluorescence intensity of the accessory pigment, phycoerythrin; the one with higher intensity (open ocean ecotype) dominating in the FIL and the other with lower intensity in the FRO (coastal ecotype). The relatively lower saline surface water mass at the FRO, probably a result of coastal advection, could have introduced the latter ecotype. Vertically, a positive correlation of Prochlorococcus with nutrients and Synechococcus with temperature corroborates their higher and lower abundance and biomass, respectively in the deeper waters. The positive correlation of Synechococcus with the total chlorophyll biomass indicates a similar response to environmental variables within the mesoscale features. This study shows that picophytoplankton contribution (16–24%) to the total phytoplankton carbon biomass in tropical mesoscale features is likely to have important consequences on the planktonic food web function.

Dynamics of size-fractionated phytoplankton biomass in a monsoonal estuary: Patterns and drivers for seasonal and spatial variability

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.

Picophytoplankton variability: Influence of winter convective mixing and advection in the northeastern Arabian Sea

Abstract 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.

Increased risk of cyanobacterial blooms in northern high‐latitude lakes through climate warming and phosphorus enrichment

Abstract Harmful cyanobacterial blooms are an increasing problem at many locations throughout the world but are rarely reported in aquatic habitats at high latitudes. Shallow lakes are a major feature of northern permafrost landscapes and are likely to experience large‐scale changes in their limnological properties in the future as a consequence of climate warming. In the present study, we addressed the question of what preconditions would be necessary to stimulate the growth and dominance of bloom‐forming cyanobacteria in northern fresh waters. We analysed the summer phytoplankton of 18 lakes on eroding permafrost (thaw lakes) and on glacier‐scoured rock (rock basin lakes) in subarctic Quebec, Canada, to determine their phytoplankton community structure and the biomass contribution of cyanobacteria. This survey was complemented with an incubation experiment to evaluate the direct warming and indirect phosphorus (P) enrichment effects of climate change on cyanobacterial bloom development. All lakes contained diverse phytoplankton communities, often dominated by chrysophytes, dinoflagellates and chlorophytes. Cyanobacteria were present in all waterbodies, but their contribution to the total community biovolume was highly variable (mean of 8.7%, range 0.1%–47%). Cyanobacterial community biovolumes correlated positively with surface water temperatures, and negatively with dissolved organic carbon, soluble reactive phosphorus, iron and manganese concentrations in the surface waters. Phosphorus enrichment of water from a thaw lake resulted in a fourfold increase of chlorophyll a (Chl‐a) and an increase in the cyanobacterial pigments echinenone and zeaxanthin. The phytoplankton counts showed that there was a sharp decrease in diversity (expressed as decline of the Shannon–Wiener index from 1.69 to 0.16), accompanied by a shift to cyanobacterial dominance, notably by the heterocystous, potentially toxic species Dolichospermum cf. planctonicum. Increased temperature led to an initial doubling of cyanobacterial biovolume, followed by the development of a chrysophyte bloom. Combined warming and P enrichment led to reduced phytoplankton biodiversity, with a community composed of cyanobacteria and chrysophytes. There was also a pronounced response by the picophytoplankton community; picocyanobacteria were strongly stimulated by P enrichment, while picoeukaryotes increased in response to warming. The current inoculum levels of cyanobacteria in subarctic lakes and their responsiveness to temperature and phosphorus indicate the potential for an abrupt increase in their abundance, accompanied by a decrease in phytoplankton diversity. Ongoing climate change will increase the risk of noxious cyanobacterial blooms in northern lakes and ponds, with potentially negative consequences for higher trophic levels.

Seasonal variability of picophytoplankton under contrasting environments in northern Tunisian coasts, southwestern Mediterranean Sea

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

Influence of short-term hydrographic variations during the north-east monsoon on picophytoplankton community structure in the eastern Arabian Sea

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 3h 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.

CO2 alters picophytoplankton community structure in freshwater ecosystems

CO2 alters picophytoplankton community structure in freshwater ecosystems

Synechococcus in the Atlantic Gateway to the Arctic Ocean

Increasing temperatures, with pronounced effects at high latitudes, have raised questions about potential changes in species composition, as well as possible increased importance of small-celled phytoplankton in marine systems. In this study, we mapped out one of the smallest and globally most widespread primary producers, the picocyanobacterium Synechococcus, within the Atlantic inflow to the Arctic Ocean. In contrast to the general understanding that Synechococcus is almost absent in polar oceans due to low temperatures, we encountered high abundances (up to 21,000 cells mL-1) at 79 °N, and documented their presence as far north as 82.5 °N. Covering an annual cycle in 2014, we found that during autumn and winter, Synechococcus was often more abundant than picoeukaryotes, which usually dominate the picophytoplankton communities in the Arctic. Synechococcus community composition shifted from a quite high genetic diversity during the spring bloom to a clear dominance of two specific operational taxonomic units (OTUs) in autumn and winter. We observed abundances higher than 1,000 cells mL-1 in water colder than 2 °C at seven distinct stations and size-fractionation experiments demonstrated a net growth of Synechococcus at 2 °C in the absence of nano-sized grazers at certain periods of the year. Phylogenetic analysis of petB sequences demonstrated that these high latitude Synechococcus group within the previously described cold-adapted clades I and IV, but also contributed to unveil novel genetic diversity, especially within clade I.

The Physiological Response of Picophytoplankton to Temperature and Its Model Representation

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

Succession of picophytoplankton during the spring bloom 2012 in Disko Bay (West Greenland)—an unexpectedly low abundance of green algae

Picoplankton are an ecologically important component of pelagic Arctic marine ecosystems that may be heavily impacted by climate change. In order to assess potential impacts of a changing environment on this group, it is necessary to develop a better understanding of their population dynamics and seasonal distribution. This study, carried out in Disko Bay, West Greenland, during spring 2012, demonstrates that fuco-algae (e.g. chrysophytes, cryptophytes, diatoms and pelagophytes) dominated the picophytoplankton during the spring bloom with minor contributions from haptophytes. In the post-bloom phase, fuco-algae were replaced by haptophytes. In contrast to total chlorophyll a, which varied dramatically over the study period, the picoplanktonic chlorophyll a remained relatively stable despite the variability in picophytoplankton community composition. Based on mostly molecular studies, a general picture has emerged from the literature that mamiellophytes (a group within the green algae) dominate Arctic picophytoplankton. Here, however, green algae were found to contribute with only about 10 % of the picoplanktonic chlorophyll a. We suggest here that differences in cell size may offer a plausible explanation for the contrast between results obtained from molecular studies and those obtained from pigment- and microscopy-based studies.

In situ associations between marine photosynthetic picoeukaryotes and potential parasites – a role for fungi?

Photosynthetic picoeukaryotes (PPEs) are important components of the marine picophytoplankton community playing a critical role in CO2 fixation but also as bacterivores, particularly in the oligotrophic gyres. Despite an increased interest in these organisms and an improved understanding of the genetic diversity of this group, we still know little of the environmental factors controlling the abundance of these organisms. Here, we investigated the quantitative importance of eukaryotic parasites in the free-living fraction as well as in associations with PPEs along a transect in the South Atlantic. Using tyramide signal amplification-fluorescence in situ hybridization (TSA-FISH), we provide quantitative evidence of the occurrence of free-living fungi in open ocean marine systems, while the Perkinsozoa and Syndiniales parasites were not abundant in these waters. Using flow cytometric cell sorting of different PPE populations followed by a dual-labelled TSA-FISH approach, we also demonstrate fungal associations, potentially parasitic, occurring with both pico-Prymnesiophyceae and pico-Chrysophyceae. These data highlight the necessity for further work investigating the specific role of marine fungi as parasites of phytoplankton to improve understanding of carbon flow in marine ecosystems.

Eukaryotic picophytoplankton community response to copper enrichment in a metal‐perturbed coastal environment

SummaryCopper is an essential micronutrient, especially for photosynthetic organisms, but can be toxic at high concentrations. In the past years, coastal waters have been exposed to an increase in copper concentration due to anthropogenic inputs. One well known case is the Chañaral area (Easter South Pacific coast), where a long term coastal copper enrichment event has occurred. That event strongly affected benthic marine diversity, including microbial communities. In this work, microcosm experiments were carried out to address the changes on picophytoplankton community composition of the disturbed area, when challenged to copper additions. Eukaryotic picophytoplankton communities from two areas were analyzed: one in the most copper‐perturbed area and another at the north edge of the perturbed area. Flow cytometry data showed that 25 μg L−1 of copper addition exerted a positive effect in the growth kinetics on part of the eukaryotic picophytoplankton communities, independently of the site. 16S‐plastid terminal restriction fragment length polymorphisms analysis suggested that eukaryotic picophytoplankton display a short and directional response to high copper levels. Members of the Prasinophyceae class, a Coscinodiscophyceae diatom, as well as Phaeocystis, respond in a short time to the environmental disturbance, making them excellent candidates for further studies to evaluate phytoplanktonic species as sentinels for copper disturbances in coastal marine ecosystems.