Picoplankton community structure at a coastal front region in the northern part of the South China Sea

Abundances of picoplankton groups were determined by flow cytometryin the Northern South China Sea (SCS) in winter 2004 to studythe dynamics of picoplankton at a coastal front region. Prochlorococcusis more abundant in relatively high temperature and salinitywaters than in nearshore area. Heterotrophic bacteria dominatein total picoplanktonic biomass but keep rather stable in biomassand surface/bottom biomass ratio on both sides of the front.Increases of picophytoplanktonic biomass and their surface/bottombiomass ratio are remarkable mainly owing to the contributionof Synechococcus on the offshore open SCS waters. Temperatureis found to limit the growth of Synechococcus and Prochlorococcus.Picoeukaryotes and heterotrophic bacteria are less sensitiveto the change in hydrographic conditions across the front. Theautotrophic/heterotrophic biomass ratio of picoplankton is lowerin eutrophic coastal waters on the nearshore side relative tothe offshore and oligotrophic open SCS.     

Distribution of microbial populations and their relationship with environmental parameters in the coastal waters of Qingdao,China

In order to understand the large‐scale distribution of microbial populations simultaneously and their relationship with environmental parameters, flow cytometry was used to analyse samples collected from 46 stations in the coastal waters of Qingdao in spring, 2007. The distribution of virus was significantly and positively correlated with heterotrophic bacteria. Two groups of picophytoplankton (Synechococcus and picoeukaryotes) were detected; however, Prochlorococcus was not found. Picoeukaryotes and nanophytoplankton were abundant in the near‐shore waters, whereas Synechococcus was abundant in the off‐shore areas. No variation was found in vertical distribution of virus, heterotrophic bacteria, Synechococcus and nanophytoplankton abundances, except picoeukaryotes abundance in the bottom layer was dramatically lower than that in the upper layers. Correlation analyses indicated that the relationship between abiotic variables and heterotrophic bacteria, pico‐ and nanophytoplankton was closer than that between abiotic variables and virioplankton. Temperature and nutrients were the synchronous factors controlling the growth of heterotrophic bacteria, pico‐ and nanophytoplankton in the coastal waters of Qingdao in spring. The results suggested that synergistic and antagonistic effects existed among microbial groups.     

Seasonality of top-down control of bacterioplankton at two central Red Sea sites with different trophic status

The role of bottom-up (nutrient availability) and top-down (grazers and viruses mortality) controls on tropical bacterioplankton have been rarely investigated simultaneously from a seasonal perspective. We have assessed them through monthly samplings over 2 years in inshore and offshore waters of the central Red Sea differing in trophic status. Flow cytometric analysis allowed us to distinguish five groups of heterotrophic bacteria based on physiological properties (nucleic acid content, membrane integrity and active respiration), three groups of cyanobacteria (two populations of Synechococcus and Prochlorococcus), heterotrophic nanoflagellates (HNFs) and three groups of viruses based on nucleic acid content. The dynamics of bacterioplankton and their top-down controls varied with season and location, being more pronounced in inshore waters. HNFs abundances showed a strong preference for larger prey inshore (r = −0.62 to −0.59, p = 0.001–0.002). Positive relationships between viruses and heterotrophic bacterioplankton abundances were more marked inshore (r = 0.67, p < 0.001) than offshore (r = 0.44, p = 0.03). The negative correlation between HNFs and viruses abundances (r = −0.47, p = 0.02) in shallow waters indicates a persistent seasonal switch between protistan grazing and viral lysis that maintains the low bacterioplankton stocks in the central Red Sea area.     

Picoplankton community structure along the northern Iberian continental margin in late winter-early spring

The surface distribution of autotrophic and heterotrophic picoplanktonwas assessed in 24 transects perpendicular to the coast alongthe N and NW Iberian peninsula shelf in late winter and earlyspring 2002. Community structure was analyzed by flow cytometry(FC) and found to be strongly influenced by hydrography. Typicallate winter conditions were found during the survey, characterizedby the presence of the poleward Portugal coastal counter current(PCCC) in the west and an increasing stratification eastwards.Cyanobacteria (mostly Synechococcus) dominated at low chlorophylla (Chl a) concentration whereas both the total and relativeabundance of picoeukaryotes generally increased with total phytoplanktonbiomass. Differences in the cell size of most FC-defined picoplanktonicgroups were also observed along the longitudinal and coastal–offshoregradients. The presence of Prochlorococcus (<10³ cells mL^–1)coincided with the core of the PCCC and its significant correlationwith salinity suggests its possible use as a tracer of thiscurrent. Two groups of heterotrophic bacteria were distinguishedaccording to their relative DNA content. High DNA bacteria dominatedthe community (60 ± 1% SE of total numbers), reachingmaximum values in areas under riverine influence with presumedhigher inputs of organic matter. Picoplankton biomass was dominatedby heterotrophic bacteria in the western region (58 ±3%) while autotrophic groups contributed on average 66 ±2% in the southern Bay of Biscay. The heterotrophic bacteriato phytoplankton biomass ratio decreased significantly alongthe measured range. Yet showing regional differences, the estimatedcontribution of picophytoplankton to total algal biomass washigh (mean 59 ± 4%), indicating the important role ofsmall cells at the onset of the spring bloom in these temperateshelf waters.     

Ocean time-series reveals recurring seasonal patterns of virioplankton dynamics in the northwestern Sargasso Sea

There are an estimated 10³⁰ virioplankton in the world oceans, the majority of which are phages (viruses that infect bacteria). Marine phages encompass enormous genetic diversity, affect biogeochemical cycling of elements, and partially control aspects of prokaryotic production and diversity. Despite their importance, there is a paucity of data describing virioplankton distributions over time and depth in oceanic systems. A decade of high-resolution time-series data collected from the upper 300 m in the northwestern Sargasso Sea revealed recurring temporal and vertical patterns of virioplankton abundance in unprecedented detail. An annual virioplankton maximum developed between 60 and 100 m during periods of summer stratification and eroded during winter convective mixing. The timing and vertical positioning of this seasonal pattern was related to variability in water column stability and the dynamics of specific picophytoplankton and heterotrophic bacterioplankton lineages. Between 60 and 100 m, virioplankton abundance was negatively correlated to the dominant heterotrophic bacterioplankton lineage SAR11, as well as the less abundant picophytoplankton, Synechococcus. In contrast, virioplankton abundance was positively correlated to the dominant picophytoplankton lineage Prochlorococcus, and the less abundant alpha-proteobacteria, Rhodobacteraceae. Seasonally, virioplankton abundances were highly synchronous with Prochlorococcus distributions and the virioplankton to Prochlorococcus ratio remained remarkably constant during periods of water column stratification. The data suggest that a significant fraction of viruses in the mid-euphotic zone of the subtropical gyres may be cyanophages and patterns in their abundance are largely determined by Prochlorococcus dynamics in response to water column stability. This high-resolution, decadal survey of virioplankton abundance provides insight into the possible controls of virioplankton dynamics in the open ocean.     

Abundance,growth and grazing loss rates of picophytoplankton in Barguzin Bay,Lake Baikal

The abundance, growth, and grazing loss rates of picophytoplankton were investigated in August 2002 in Barguzin Bay, Lake Baikal. Water samples for incubation were taken once at a near-shore station and twice at an offshore station. Contributions of picophytoplankton to total phytoplankton were high (56.9–83.9%) at the offshore station and low (5.8–6.8%) at the near-shore station. The picophytoplankton community in the offshore station comprised mainly phycoerythrin (PE)-rich cyanobacteria, with eukaryotic picophytoplankton being less abundant. In contrast, as well as PE-rich cyanobacteria and eukaryotic picophytoplankton, phycocyanin (PC)-rich cyanobacteria were found in the near-shore station. At the offshore station, growth and grazing loss rates on 25 August were 0.56 and 0.43 day⁻¹, respectively, and on 29 August, 0.69 and 0.83 day⁻¹, respectively. At the near-shore station, growth and grazing loss rates were 1.61 and 0.70 day⁻¹, respectively. These results show that there is a difference in the abundance, composition, and ecological role in the microbial food web of picophytoplankton between the near-shore and the offshore areas in Barguzin Bay.     

Rapid Diversification of Marine Picophytoplankton with Dissimilar Light-Harvesting Structures Inferred from Sequences of Prochlorococcus and Synechococcus (Cyanobacteria)

Cultured isolates of the unicellular planktonic cyanobacteria Prochlorococcus and marine Synechococcus belong to a single marine picophytoplankton clade. Within this clade, two deeply branching lineages of Prochlorococcus, two lineages of marine A Synechococcus and one lineage of marine B Synechococcus exhibit closely spaced divergence points with low bootstrap support. This pattern is consistent with a near-simultaneous diversification of marine lineages with divinyl chlorophyll b and phycobilisomes as photosynthetic antennae. Inferences from 16S ribosomal RNA sequences including data for 18 marine picophytoplankton clade members were congruent with results of psbB and petB and D sequence analyses focusing on five strains of Prochlorococcus and one strain of marine A Synechococcus. Third codon position and intergenic region nucleotide frequencies vary widely among members of the marine picophytoplankton group, suggesting that substitution biases differ among the lineages. Nonetheless, standard phylogenetic methods and newer algorithms insensitive to such biases did not recover different branching patterns within the group, and failed to cluster Prochlorococcus with chloroplasts or other chlorophyll b-containing prokaryotes. Prochlorococcus isolated from surface waters of stratified, oligotrophic ocean provinces predominate in a lineage exhibiting low G + C nucleotide frequencies at highly variable positions. Received: 18 January 1997 / Accepted: 18 May 1997     

Distribution of picophytoplankton communities from brackish to hypersaline waters in a South Australian coastal lagoon

Background

Picophytoplankton (i.e. cyanobacteria and pico-eukaryotes) are abundant and ecologically critical components of the autotrophic communities in the pelagic realm. These micro-organisms colonized a variety of extreme environments including high salinity waters. However, the distribution of these organisms along strong salinity gradient has barely been investigated. The abundance and community structure of cyanobacteria and pico-eukaryotes were investigated along a natural continuous salinity gradient (1.8% to 15.5%) using flow cytometry.

Results

Highest picophytoplankton abundances were recorded under salinity conditions ranging between 8.0% and 11.0% (1.3 × 10⁶ to 1.4 × 10⁶ cells ml^-1). Two populations of picocyanobacteria (likely Synechococcus and Prochlorococcus) and 5 distinct populations of pico-eukaryotes were identified along the salinity gradient. The picophytoplankton cytometric-richness decreased with salinity and the most cytometrically diversified community (4 to 7 populations) was observed in the brackish-marine part of the lagoon (i.e. salinity below 3.5%). One population of pico-eukaryote dominated the community throughout the salinity gradient and was responsible for the bloom observed between 8.0% and 11.0%. Finally only this halotolerant population and Prochlorococcus-like picocyanobacteria were identified in hypersaline waters (i.e. above 14.0%). Salinity was identified as the main factor structuring the distribution of picophytoplankton along the lagoon. However, nutritive conditions, viral lysis and microzooplankton grazing are also suggested as potentially important players in controlling the abundance and diversity of picophytoplankton along the lagoon.

Conclusions

The complex patterns described here represent the first observation of picophytoplankton dynamics along a continuous gradient where salinity increases from 1.8% to 15.5%. This result provides new insight into the distribution of pico-autotrophic organisms along strong salinity gradients and allows for a better understanding of the overall pelagic functioning in saline systems which is critical for the management of these precious and climatically-stress ecosystems.     

Pico-, nano- and microplankton communities in hydrothermal marine coastal environments of the Eolian Islands (Panarea and Vulcano) in the Mediterranean Sea

This paper reports the relationship among pico-, nano- and microplanktoniccommunities observed in two different shallow marine hydrothermalenvironments. Seawater samples from five stations in the coastalarea of the ‘Porto di Levante’ (Vulcano Island)and from three stations off the Island of Panarea (Eolian Islands,Italy) were collected in May and July of 1989, respectively.Microbiological investigations were carried out in order todetermine the density of: (i) the total picoplankton (both autotrophicand heterotrophic): (ii) the total picophytoplankton (autofluorescentpicoplanktonic cells); (iii) the larger phytoplankton (>2µm): (iv) the ‘metabolically active’ cellsof total picoplankton and cyanobacteria; (v) the heterotrophicaerobic bacteria. The peak values of picoplankton and picophytoplanktoncomponents, with an order of magnitude of 10⁹ and 10⁷ cells1¹, respectively, were registered in the wannest water samples(30–75°C) collected from the Vulcano area. At Panarea.eukaryotic picophytoplankton and ‘metabolically active’coccoid cyanobacteria showed an opposite trend. A possible competitionbetween the two groups is to be considered. Cyanobacteria, diatomsand the genus Licmophora. in particular, were prevalent in thewarmest hydrothermal vents of Vulcano. Nano- and microphytoplanktoniccommunities in the offshore waters of the Island of Panareashowed more variability than in Vulcano. Moreover, in the Panareawaters the prevalence of phytoflagellates above the thermoclinewas observed, whereas diatoms were predominant below this layer.     

Productivity, photosynthetic pigments and hydrology in the coastal front of the Eastern English Channel

In the French area of the Eastern English Channel, the coastalwaters 3 or 4 miles wide are separated from offshore watersby a coastal front. The phytoplanktonic biomass (B) and production(P) are higher in coastal waters, and an increase of biomasshas been observed at the interface. This frontal biomass, lessproductive, resulted from hydrodynamic accumulation, not dueto local growth. The productivity (P/B) deficit (50%) spreadsover the external area of the front, probably because the biomassis partly constituted of exported inactive coastal phytoplankton.The productivity is higher again in offshore waters. The variationof the pigment pool indicates a stress on phytoplankton in thefront and this ergocline was not productive at the time of thetwo cruises.     

Temporal and spatial distribution of the ichthyotoxic dinoflagellate Gyrodinium aureolum Hulburt in the St Lawrence, Canada

The occurrence of the potentially ichthyotoxic dinoflagellateGyrodinium aureolum Hulburt is reported for the first time inthe Lower Estuary and Gulf of St Lawrence. Taxonomic identificationusing an iramunochemical tagging method suggests a close taxonomicproximity between the species found in the St Lawrence and G.aureolumpresent in European waters. In 1993, G.aureolum was observedalong the entire coast of Quebec and in offshore waters of theGulf of St Lawrence. At the coastal stations, G.aureolum showedlarge week-to-week variations, but higher densities were observedduring late August and early September. Bloom concentrations(10⁶ cells 1^–1) were only observed in the GaspéCurrent at Mont-Louis, on the north coast of the Gaspd peninsula.In the central part of the Gulf, G.aureolum was concentratedin the upper 5 m of the stations directly influenced by thefreshwater run-off of the St Lawrence Estuary. Results fromprincipal component analysis indicate that G.aureolum favorsenvironments with high nitrogen recycling activity.     

Tidal influence upon appendicularian abundance in North Inlet estuary, South Carolina

Tidal influences on appendicularian densities were observedat North Inlet, South Carolina, by sampling along a transectwhich ran from a tidal creek to a station 5 km offshore. Oikopleuradioica was the dominant species in North Inlet, while Oikopleuralongicauda and Appendicularia sicula contributed marginallyto appendicularian numbers during midsummer and fall. A strongtide-dependent density pattern was clear for inshore waters.Low-tide densities of all three species showed a dramatic increasein an offshore direction. At high tide, densities were similarbetween all stations for O. dioica, while O. longicauda andA. sicula showed a less pronounced density gradient than atlow tide. Population densities within the inlet were greateron spring tides than neap tides and tidal influences were generallyconsistent between seasons. Appendicularians enter the estuaryin densities as high as 20 072 animals m^–3, indicatingthat tidal currents may be an important mechanism for exchangeof appendicularian biomass between coastal and estuarine waters. ¹Present address: Allan Hancock Foundation, University of SouthernCalifornia, Los Angeles, CA 90007, USA.     

Aggregation of avian predators and zooplankton prey in Otago shelf waters, New Zealand

In Otago shelf waters surface swarms of krill (Nyctiphanes australis),hyperiid amphipods (Parathemisto spp.) and galatheid crab larvae(Munida gregaria) provide an abundant summer food source forplanktivores. We tested the hypothesis that aggregation of avianplanktivores depends upon the spatial distribution of theirprey. Gulls (Larus scopulinus, L.bulleri, L.dominicanus), sootyshearwaters (Puffinus griseus) and white-fronted terns (Sternastriata) showed significantly aggregated distributions. Thedistribution of birds sitting on the sea surface was correlatedwith the abundance pattern of krill but was not correlated withthe distribution of smaller hyperiid amphipods. The distributionof flying red-billed gulls (L.scopulinus), black-billed gulls(L.bulleri) and sooty shearwaters was correlated with the krilldistribution but black-backed gulls (L.dominicanus) were not.Stomach contents of black-billed gulls were dominated by krill,in contrast to sooty shearwaters, which ate a higher proportionof Minida, and black-backed gulls, which contained Munida andfish. No amphipods were found in bird stomachs. There was nosignificant correlation between bird distributions and the hydrographicregime, water depth or distance offshore. The distribution ofprey rather than hydrographic regime was a more important determinantof bird distributions at this spatial scale (2.6–12 km)and location.     

Ecology of uncultured Prochlorococcus clades revealed through single-cell genomics and biogeographic analysis

Prochlorococcus is the numerically dominant photosynthetic organism throughout much of the world''s oceans, yet little is known about the ecology and genetic diversity of populations inhabiting tropical waters. To help close this gap, we examined natural Prochlorococcus communities in the tropical Pacific Ocean using a single-cell whole-genome amplification and sequencing. Analysis of the gene content of just 10 single cells from these waters added 394 new genes to the Prochlorococcus pan-genome—that is, genes never before seen in a Prochlorococcus cell. Analysis of marker genes, including the ribosomal internal transcribed sequence, from dozens of individual cells revealed several representatives from two uncultivated clades of Prochlorococcus previously identified as HNLC1 and HNLC2. While the HNLC clades can dominate Prochlorococcus communities under certain conditions, their overall geographic distribution was highly restricted compared with other clades of Prochlorococcus. In the Atlantic and Pacific oceans, these clades were only found in warm waters with low Fe and high inorganic P levels. Genomic analysis suggests that at least one of these clades thrives in low Fe environments by scavenging organic-bound Fe, a process previously unknown in Prochlorococcus. Furthermore, the capacity to utilize organic-bound Fe appears to have been acquired horizontally and may be exchanged among other clades of Prochlorococcus. Finally, one of the single Prochlorococcus cells sequenced contained a partial genome of what appears to be a prophage integrated into the genome.     

Picophytoplankton dynamics in the equatorial Pacific: diel cycling from flow-cytometer observations

The French JGOFS cruise FLUPAC was conducted in October–November 1994 in the western and central equatorial Pacific Ocean. During a 7-day time series at 150 °W, in addition to conventional sampling (four times per day from discrete depths between 0 and 150 m), a high frequency (hourly) experiment was performed by continuously pumping water at 5 m below the surface over 24 h. Flow cytometric measurements allowed us to recognise and to follow separately the two major components of equatorial picophytoplankton, the Prochlorococcus and the picoeukaryotes. The hourly surface experiment confirmed the synchrony of Prochlorococcus cell division and showed that picoeukaryotes exhibited a similar behaviour. The main consequence is that the maximum potential growth rate of picophytoplankton is one doubling per day for both cell groups. The vertical profiles indicated that the diel cycling extends throughout the surface layer for both algal groups. The cells were observed to divide daily from late afternoon to the middle of the night, and then to disappear, probably as a result of grazing. In the surface layer, variations of abundance allowed us to estimate a growth rate of about 0.6 day¹. Mean cell light scattering parFS as measured by the cytometer indicated a decrease in cell size concurrent with cell division and an increase during the photosynthetic growth phase.     

Ecological Genomics of Marine Picocyanobacteria

Summary: Marine picocyanobacteria of the genera Prochlorococcus and Synechococcus numerically dominate the picophytoplankton of the world ocean, making a key contribution to global primary production. Prochlorococcus was isolated around 20 years ago and is probably the most abundant photosynthetic organism on Earth. The genus comprises specific ecotypes which are phylogenetically distinct and differ markedly in their photophysiology, allowing growth over a broad range of light and nutrient conditions within the 45°N to 40°S latitudinal belt that they occupy. Synechococcus and Prochlorococcus are closely related, together forming a discrete picophytoplankton clade, but are distinguishable by their possession of dissimilar light-harvesting apparatuses and differences in cell size and elemental composition. Synechococcus strains have a ubiquitous oceanic distribution compared to that of Prochlorococcus strains and are characterized by phylogenetically discrete lineages with a wide range of pigmentation. In this review, we put our current knowledge of marine picocyanobacterial genomics into an environmental context and present previously unpublished genomic information arising from extensive genomic comparisons in order to provide insights into the adaptations of these marine microbes to their environment and how they are reflected at the genomic level.     

Size-fractionated phytoplankton carboxylase activities in the Indian sector of the Southern Ocean

During the ANTARES 3 cruise in the Indian sector of the SouthernOcean in October–November 1995, the surface waters ofKerguelen Islands plume, and the surface and deeper waters (30–60m) along a transect on 62°E from 48°36'S to the iceedge (58°50'S), were sampled. The phytoplankton communitywas size-fractionated (2 µm) and cell numbers, chlorophyllbiomass and carbon assimilation, through Rubisco and ß-carboxylaseactivities, were characterized. The highest contribution of<2 µm cells to total biomass and total Rubisco activitywas reported in the waters of the Permanent Open Ocean Zone(POOZ) located between 52°S and 55°S along 62°E.In this zone, the picophytoplankton contributed from 26 to 50%of the total chlorophyll (a + b + c) with an average of 0.09± 0.02 µg Chl l^–1 for <2 µm cells.Picophytoplankton also contributed 36 to 64% of the total Rubiscoactivity, with an average of 0.80 ± 0.30 mg C mg Chla^–1 h^–1 for <2 µm cells. The picophytoplanktoncells had a higher ß-carboxylase activity than largercells >2 µm. The mixotrophic capacity of these smallcells is proposed. From sampling stations of the Kerguelen plume,a relationship was observed between the Rubisco activity perpicophytoplankton cell and apparent cell size, which variedwith the sampled water masses. Moreover, a depth-dependent photoperiodicityof Rubisco activity per cell for <2 µm phytoplanktonwas observed during the day/night cycle in the POOZ. In thenear ice zone, a physiological change in picophytoplankton cellsfavouring phosphoenolpyruvate carboxykinase (PEPCK) activitywas reported. A species succession, or an adaptation to unfavourableenvironmental conditions such as low temperature and/or availableirradiance levels, may have provoked this change. The high contributionof picophytoplankton to the total biomass, and its high CO₂fixation capacity via autotrophy and mixotrophy, emphasize thestrong regeneration of organic materials in the euphotic layerin the Southern Ocean.     

Modeling Selective Pressures on Phytoplankton in the Global Ocean

Our view of marine microbes is transforming, as culture-independent methods facilitate rapid characterization of microbial diversity. It is difficult to assimilate this information into our understanding of marine microbe ecology and evolution, because their distributions, traits, and genomes are shaped by forces that are complex and dynamic. Here we incorporate diverse forces—physical, biogeochemical, ecological, and mutational—into a global ocean model to study selective pressures on a simple trait in a widely distributed lineage of picophytoplankton: the nitrogen use abilities of Synechococcus and Prochlorococcus cyanobacteria. Some Prochlorococcus ecotypes have lost the ability to use nitrate, whereas their close relatives, marine Synechococcus, typically retain it. We impose mutations for the loss of nitrogen use abilities in modeled picophytoplankton, and ask: in which parts of the ocean are mutants most disadvantaged by losing the ability to use nitrate, and in which parts are they least disadvantaged? Our model predicts that this selective disadvantage is smallest for picophytoplankton that live in tropical regions where Prochlorococcus are abundant in the real ocean. Conversely, the selective disadvantage of losing the ability to use nitrate is larger for modeled picophytoplankton that live at higher latitudes, where Synechococcus are abundant. In regions where we expect Prochlorococcus and Synechococcus populations to cycle seasonally in the real ocean, we find that model ecotypes with seasonal population dynamics similar to Prochlorococcus are less disadvantaged by losing the ability to use nitrate than model ecotypes with seasonal population dynamics similar to Synechococcus. The model predictions for the selective advantage associated with nitrate use are broadly consistent with the distribution of this ability among marine picocyanobacteria, and at finer scales, can provide insights into interactions between temporally varying ocean processes and selective pressures that may be difficult or impossible to study by other means. More generally, and perhaps more importantly, this study introduces an approach for testing hypotheses about the processes that underlie genetic variation among marine microbes, embedded in the dynamic physical, chemical, and biological forces that generate and shape this diversity.