Mikhail V. Zubkov

High bacterivory by the smallest phytoplankton in the North Atlantic Ocean

Planktonic algae <5 μm in size are major fixers of inorganic carbon in the ocean. They dominate phytoplankton biomass in post-bloom, stratified oceanic temperate waters. Traditionally, large and small algae are viewed as having a critical growth dependence on inorganic nutrients, which the latter can better acquire at lower ambient concentrations owing to their higher surface area to volume ratios. Nonetheless, recent phosphate tracer experiments in the oligotrophic ocean have suggested that small algae obtain inorganic phosphate indirectly, possibly through feeding on bacterioplankton. There have been numerous microscopy-based studies of algae feeding mixotrophically in the laboratory and field, as well as mathematical modelling of the ecological importance of mixotrophy. However, because of methodological limitations there has not been a direct comparison of obligate heterotrophic and mixotrophic bacterivory. Here we present direct evidence that small algae carry out 40–95% of the bacterivory in the euphotic layer of the temperate North Atlantic Ocean in summer. A similar range of 37–70% was determined in the surface waters of the tropical North-East Atlantic Ocean, suggesting the global significance of mixotrophy. This finding reveals that even the smallest algae have less dependence on dissolved inorganic nutrients than previously thought, obtaining a quarter of their biomass from bacterivory. This has important implications for how we perceive nutrient acquisition and limitation of carbon-fixing protists as well as control of bacterioplankton in the ocean.

Picoplanktonic community structure on an Atlantic transect from 50°N to 50°S

Plankton samples were collected from 10 depths at 25 stations spaced at intervals of about 4° of latitude along a transect from the British Isles to the Falkland Islands. Four categories of picoplankton were discriminated: Synechococcus spp., Prochlorococcus spp., eukaryotic picophytoplankton and heterotrophic bacteria. The populations in each category in the samples were counted by flow cytometry and the mean size of bacterial cells was determined by fractionation through filters. Categories of phototrophic cells were discriminated by size and by the fluorescence of photosynthetic pigments; samples stained with the fluorochrome TOTO were used to enumerate heterotrophic bacteria (and Prochlorococcus in surface waters where their chlorophyll content was very small). The carbon biomass concentration of each category in each sample was calculated. Prochlorococcus was present at all stations between 47°N and 38°S, and reached peak population densities above 200,000 cells ml-1 in equatorial waters; the depth occupied by these cells increased in oligotrophic waters, where they dominated picophytoplankton biomass. Synechococcus reached high concentrations in the Mauritanian upwelling region and in the frontal region near the southern end of the transect, where they represented the largest single component of picophytoplankton biomass, but was almost absent in oligotrophic regions. Picoeukaryotes were present in low numbers at all latitudes, but they are larger cells and constituted a substantial part of the total picophytoplankton biomass at most latitudes. The depth-integrated (200 m) biomass of heterotrophic bacteria was nearly as great as that of the picophytoplankton at all latitudes, because substantial numbers of cells occurred at all depths. Numbers and biomass of these bacteria were maximal in the upwelling region and high at both ends of the transect. There was a clear contrast in the composition of the picoplankton community in both the North and South Atlantic between mesotrophic waters where Synechococcus and picoeukaryotes dominated the biomass, and oligotrophic waters where the smaller total biomass was dominated by Prochlorococcus.

Significant CO2 fixation by small prymnesiophytes in the subtropical and tropical northeast Atlantic Ocean.

Global estimates indicate the oceans are responsible for approximately half of the carbon dioxide fixed on Earth. Organisms ⩽5 μm in size dominate open ocean phytoplankton communities in terms of abundance and CO2 fixation, with the cyanobacterial genera Prochlorococcus and Synechococcus numerically the most abundant and more extensively studied compared with small eukaryotes. However, the contribution of specific taxonomic groups to marine CO2 fixation is still poorly known. In this study, we show that among the phytoplankton, small eukaryotes contribute significantly to CO2 fixation (44%) because of their larger cell volume and thereby higher cell-specific CO2 fixation rates. Within the eukaryotes, two groups, herein called Euk-A and Euk-B, were distinguished based on their flow cytometric signature. Euk-A, the most abundant group, contained cells 1.8±0.1 μm in size while Euk-B was the least abundant but cells were larger (2.8±0.2 μm). The Euk-B group comprising prymnesiophytes (73±13%) belonging largely to lineages with no close cultured counterparts accounted for up to 38% of the total primary production in the subtropical and tropical northeast Atlantic Ocean, suggesting a key role of this group in oceanic CO2 fixation.

Comparison of Cellular and Biomass Specific Activities of Dominant Bacterioplankton Groups in Stratified Waters of the Celtic Sea

ABSTRACT A flow-sorting technique was developed to determine unperturbed metabolic activities of phylogenetically characterized bacterioplankton groups with incorporation rates of [35S]methionine tracer. According to fluorescence in situ hybridization with rRNA targeted oligonucleotide probes, a clade of α-proteobacteria, related to Roseobacter spp., and aCytophaga-Flavobacterium cluster dominated the different groups. Cytometric characterization revealed both these groups to have high DNA (HNA) content, while the α-proteobacteria exhibited high light scatter (hs) and the Cytophaga-Flavobacteriumcluster exhibited low light scatter (ls). A third abundant group with low DNA (LNA) content contained cells from a SAR86 cluster of γ-proteobacteria. Cellular specific activities of the HNA-hs group were 4- and 1.7-fold higher than the activities in the HNA-ls and LNA groups, respectively. However, the higher cellular protein synthesis by the HNA-hs could simply be explained by their maintenance of a larger cellular protein biomass. Similar biomass specific activities of the different groups strongly support the main assumption that underlies the determination of bacterial production: different bacteria in a complex community incorporate amino acids at a rate proportional to their protein synthesis. The fact that the highest growth-specific rates were determined for the smallest cells of the LNA group can explain the dominance of this group in nutrient-limited waters. The metabolic activities of the three groups accounted for almost the total bacterioplankton activity, indicating their key biogeochemical role in the planktonic ecosystem of the Celtic Sea.

Picoplankton community structure on the Atlantic Meridional Transect: a comparison between seasons

Abstract Samples collected from 10 depths at 25 stations in September–October 1996 and 12 depths at 28 stations in April–May 1997 on an Atlantic Meridional Transect between the British Isles and the Falkland Islands were analysed by flow cytometry to determine the numbers and biomass of four categories of picoplankton: Prochlorococcus spp, Synechococcus spp, picoeukaryotic phytoplankton and heterotrophic bacteria. The composition of the picoplankton communities confirmed earlier findings ( Zubkov, Sleigh, Tarran, Burkill & Leakey, 1998 ) about distinctive regions along the transect and indicated that the stations should be grouped into five provinces: northern temperate, northern Atlantic gyre, equatorial, southern Atlantic gyre and southern temperate, with an intrusion of upwelling water off the coast of Mauritania between the northern Atlantic gyre and equatorial waters. Prochlorococcus was the most numerous phototrophic organism in waters of both northern and southern gyres and in the equatorial region, at concentrations in excess of 0.1×106ml−1; it also dominated plant biomass in the gyres, but the biomass of the larger picoeukaryotic algae equalled that of Prochlorococcus in the equatorial region; higher standing stocks of both Prochlorococcus and picoeukaryotes were present in spring than in autumn in waters of both gyres. In temperate waters at both ends of the transect the numbers and biomass of picoeukaryotes and, more locally, of Synechococcus increased, and the Synechococcus, particularly, were more numerous in spring than in autumn. There was a pronounced southward shift of the main populations of both Synechococcus and Prochlorococcus in April–May in comparison to those of September–October, associated with seasonal changes in solar radiation, the abundance of Prochlorococcus dropping sharply near the 17°C contour, while Synechococcus was still present at temperatures below 10°C. Picoeukaryotes were more tolerant of low temperatures and lower light levels, often being more abundant in samples from greater depths, where they contributed to the deep chlorophyll maximum. Heterotrophic bacterial numbers and biomass tended to be highest in those samples where phototrophic biomass was greatest, with peaks in temperate and equatorial waters, which were shifted southwards in April–May compared with September–October.

Latitudinal distribution of prokaryotic picoplankton populations in the Atlantic Ocean

Members of the prokaryotic picoplankton are the main drivers of the biogeochemical cycles over large areas of the worlds oceans. In order to ascertain changes in picoplankton composition in the euphotic and twilight zones at an ocean basin scale we determined the distribution of 11 marine bacterial and archaeal phyla in three different water layers along a transect across the Atlantic Ocean from South Africa (32.9 degrees S) to the UK (46.4 degrees N) during boreal spring. Depth profiles down to 500 m at 65 stations were analysed by catalysed reporter deposition fluorescence in situ hybridization (CARD-FISH) and automated epifluorescence microscopy. There was no obvious overall difference in microbial community composition between the surface water layer and the deep chlorophyll maximum (DCM) layer. There were, however, significant differences between the two photic water layers and the mesopelagic zone. SAR11 (35 +/- 9%) and Prochlorococcus (12 +/- 8%) together dominated the surface waters, whereas SAR11 and Crenarchaeota of the marine group I formed equal proportions of the picoplankton community below the DCM (both approximately 15%). However, due to their small cell sizes Crenarchaeota contributed distinctly less to total microbial biomass than SAR11 in this mesopelagic water layer. Bacteria from the uncultured Chloroflexi-related clade SAR202 occurred preferentially below the DCM (4-6%). Distinct latitudinal distribution patterns were found both in the photic zone and in the mesopelagic waters: in the photic zone, SAR11 was more abundant in the Northern Atlantic Ocean (up to 45%) than in the Southern Atlantic gyre (approximately 25%), the biomass of Prochlorococcus peaked in the tropical Atlantic Ocean, and Bacteroidetes and Gammaproteobacteria bloomed in the nutrient-rich northern temperate waters and in the Benguela upwelling. In mesopelagic waters, higher proportions of SAR202 were present in both central gyre regions, whereas Crenarchaeota were clearly more abundant in the upwelling regions and in higher latitudes. Other phylogenetic groups such as the Planctomycetes, marine group II Euryarchaeota and the uncultured clades SAR406, SAR324 and SAR86 rarely exceeded more than 5% of relative abundance.

Mixotrophic basis of Atlantic oligotrophic ecosystems

Oligotrophic subtropical gyres are the largest oceanic ecosystems, covering >40% of the Earths surface. Unicellular cyanobacteria and the smallest algae (plastidic protists) dominate CO2 fixation in these ecosystems, competing for dissolved inorganic nutrients. Here we present direct evidence from the surface mixed layer of the subtropical gyres and adjacent equatorial and temperate regions of the Atlantic Ocean, collected on three Atlantic Meridional Transect cruises on consecutive years, that bacterioplankton are fed on by plastidic and aplastidic protists at comparable rates. Rates of bacterivory were similar in the light and dark. Furthermore, because of their higher abundance, it is the plastidic protists, rather than the aplastidic forms, that control bacterivory in these waters. These findings change our basic understanding of food web function in the open ocean, because plastidic protists should now be considered as the main bacterivores as well as the main CO2 fixers in the oligotrophic gyres.

Reconciliation of the carbon budget in the ocean’s twilight zone

Photosynthesis in the surface ocean produces approximately 100 gigatonnes of organic carbon per year, of which 5 to 15 per cent is exported to the deep ocean. The rate at which the sinking carbon is converted into carbon dioxide by heterotrophic organisms at depth is important in controlling oceanic carbon storage. It remains uncertain, however, to what extent surface ocean carbon supply meets the demand of water-column biota; the discrepancy between known carbon sources and sinks is as much as two orders of magnitude. Here we present field measurements, respiration rate estimates and a steady-state model that allow us to balance carbon sources and sinks to within observational uncertainties at the Porcupine Abyssal Plain site in the eastern North Atlantic Ocean. We find that prokaryotes are responsible for 70 to 92 per cent of the estimated remineralization in the twilight zone (depths of 50 to 1,000 metres) despite the fact that much of the organic carbon is exported in the form of large, fast-sinking particles accessible to larger zooplankton. We suggest that this occurs because zooplankton fragment and ingest half of the fast-sinking particles, of which more than 30 per cent may be released as suspended and slowly sinking matter, stimulating the deep-ocean microbial loop. The synergy between microbes and zooplankton in the twilight zone is important to our understanding of the processes controlling the oceanic carbon sink.

Rapid turnover of dissolved DMS and DMSP by defined bacterioplankton communities in the stratified euphotic zone of the North Sea

Bacterioplankton-driven turnover of the algal osmolyte, dimethylsulphoniopropionate (DMSP), and its degradation product, dimethylsulphide (DMS) the major natural source of atmospheric sulphur, were studied during a Lagrangian SF6-tracer experiment in the North Sea (60°N, 3°E). The water mass sampled within the euphotic zone was characterised by a surface mixed layer (from 0 m to 13–30 m) and a subsurface layer (from 13–30 m to 45–58 m) separated by a 2°C thermocline spanning 2 m. The fluxes of dissolved DMSP (DMSPd) and DMS were determined using radioactive tracer techniques. Rates of the simultaneous incorporation of 14C-leucine and 3H-thymidine were measured to estimate bacterioplankton production. Flow cytometry was employed to discriminate subpopulations and to determine the numbers and biomass of bacterioplankton by staining for nucleic acids and proteins. Bacterioplankton subpopulations were separated by flow cytometric sorting and their composition determined using 16S ribosomal gene cloning/sequencing and fluorescence in situ hybridisation with designed group-specific oligonucleotide probes. A subpopulation, dominated by bacteria related to Roseobacter-(α-proteobacteria), constituted 26–33% of total bacterioplankton numbers and 45–48% of biomass in both surface and subsurface layers. The other abundant prokaryotes were a group within the SAR86 cluster of γ-proteobacteria and bacteria from the Cytophaga–Flavobacterium—cluster. Bacterial consumption of DMSPd was greater in the subsurface layer (41 nM d−1) than in the surface layer (20 nM d−1). Bacterioplankton tightly controlled the DMSPd pool, particularly in the subsurface layer, with a turnover time of 2 h, whereas the turnover time of DMSPd in the surface layer was 10 h. Consumed DMSP satisfied the majority of sulphur demands of bacterioplankton, even though bacterioplankton assimilated only about 2.5% and 6.0% of consumed DMSPd sulphur in the surface and subsurface layers, respectively. Bacterioplankton turnover of DMS was also faster in the subsurface layer (12 h) compared to the surface layer (24 h). However, absolute DMS consumption rates were higher in the surface layer, due to higher DMS concentrations in this layer. The majority of DMS was metabolised into dissolved non-volatile products, and bacteria could satisfy only 1–3% of their sulphur demands from DMS. Thus, structurally similar bacterioplankton communities exerted strong control over DMSPd and DMS concentrations both in the subsurface layer and surface mixed layer.

Plankton respiration in the Eastern Atlantic Ocean

Concurrent measurements of dark community respiration (DCR), gross production (GP), size fractionated primary production (14C PP), nitrogen uptake, nutrients, chlorophyll a concentration, and heterotrophic and autotrophic bacterial abundance were collected from the upper 200 m of a latitudinal (32°S–48°N) transect in the Eastern Atlantic Ocean during May/June 1998. The mean mixed layer respiration rate was 2.5±2.1 mmol O2 m−3 d−1 (n=119) for the whole transect, 2.2±1.1 mmol O2 m−3 d−1 (n=32) in areas where chlorophyll a was <0.5 mg m−3 and 1.5±0.7 mmol O2 m−3 d−1 (n=10) where chlorophyll a was <0.2 mg m−3. These values lie within the range of published data collected in comparable waters, they co-vary with indicators of heterotrophic and autotrophic biomass (heterotrophic bacterial abundance, chlorophyll a concentration, beam attenuation and particulate organic carbon concentration) and they can be reconciled with accepted estimates of total respiratory activity. The mean and median respiratory quotient (RQ), calculated as the ratio of dissolved inorganic carbon production to dissolved oxygen consumption, was 0.8 (n=11). At the time of the study, plankton community respiration exceeded GP in the picoautotroph dominated oligotrophic regions (Eastern Tropical Atlantic [15.5°S–14.2°N] and North Atlantic Subtropical Gyre [21.5–42.5°N]), which amounted to 50% of the stations sampled along the 12,100 km transect. These regions also exhibited high heterotrophic: autotrophic biomass ratios, higher turnover rates of phytoplankton than of bacteria and low f ratios. However, the carbon supply mechanisms required to sustain the rates of respiration higher than GP could not be fully quantified. Future research should aim to determine the temporal balance of respiration and GP together with substrate supply mechanisms in these ocean regions.