Seasonal shifts in Fe-acquisition strategies in Southern Ocean microbial communities revealed by metagenomics and autonomous sampling

Iron (Fe) governs the cycling of organic carbon in large parts of the Southern Ocean. The strategies of diverse microbes to acquire the different chemical forms of Fe under seasonally changing organic carbon regimes remain, however, poorly understood. Here, we report high-resolution seasonal metagenomic observations from the region off Kerguelen Island (Indian Sector of the Southern Ocean) where natural Fe-fertilization induces consecutive spring and summer phytoplankton blooms. Our data illustrate pronounced, but distinct seasonal patterns in the abundance of genes implicated in the transport of different forms of Fe and organic substrates, of siderophore biosynthesis and carbohydrate-active enzymes. The seasonal dynamics suggest a temporal decoupling in the prokaryotic requirements of Fe and organic carbon during the spring phytoplankton bloom and a concerted access to these resources after the summer bloom. Taxonomic assignments revealed differences in the prokaryotic groups harbouring genes of a given Fe-related category and pronounced seasonal successions were observed. Using MAGs we could decipher the respective Fe- and organic substrate-related genes of individual taxa assigned to abundant groups. The ecological strategies related to Fe-acquisition provide insights on how this element could shape microbial community composition with potential implications on organic matter transformations in the Southern Ocean.     

Microbial Community Response during the Iron Fertilization Experiment LOHAFEX

Iron fertilization experiments in high-nutrient, low-chlorophyll areas are known to induce phytoplankton blooms. However, little is known about the response of the microbial community upon iron fertilization. As part of the LOHAFEX experiment in the southern Atlantic Ocean, Bacteria and Archaea were monitored within and outside an induced bloom, dominated by Phaeocystis-like nanoplankton, during the 38 days of the experiment. The microbial production increased 1.6-fold (thymidine uptake) and 2.1-fold (leucine uptake), while total cell numbers increased only slightly over the course of the experiment. 454 tag pyrosequencing of partial 16S rRNA genes and catalyzed reporter deposition fluorescence in situ hybridization (CARD FISH) showed that the composition and abundance of the bacterial and archaeal community in the iron-fertilized water body were remarkably constant without development of typical bloom-related succession patterns. Members of groups usually found in phytoplankton blooms, such as Roseobacter and Gammaproteobacteria, showed no response or only a minor response to the bloom. However, sequence numbers and total cell numbers of the SAR11 and SAR86 clades increased slightly but significantly toward the end of the experiment. It seems that although microbial productivity was enhanced within the fertilized area, a succession-like response of the microbial community upon the algal bloom was averted by highly effective grazing. Only small-celled members like the SAR11 and SAR86 clades could possibly escape the grazing pressure, explaining a net increase of those clades in numbers.     

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 world's 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°S) to the UK (46.4°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 ～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 (～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.     

Blooms of Single Bacterial Species in a Coastal Lagoon of the Southwestern Atlantic Ocean

We investigated seasonal differences in community structure and activity (leucine incorporation) of the planktonic bacterial assemblage in the freshwater and brackish-water zones of a shallow coastal lagoon of the southwestern Atlantic Ocean. Alphaproteobacteria formed the dominant microbial group in both zones throughout the sampling period. After an intrusion of marine water, members of the SAR11 lineage became abundant in the brackish-water zone. These bacteria were apparently distributed over the lagoon during the following months until they constituted almost 30% of all prokaryotic cells at both sampling sites. At the first sampling date (March 2003) a single alphaproteobacterial species unrelated to SAR11, Sphingomonas echinoides, dominated the microbial assemblages in both zones of the lagoon concomitantly with a bloom of filamentous cyanobacteria. Pronounced maxima of leucine incorporation were observed once in each zone of the lagoon. In the freshwater zone, this highly active microbial assemblage was a mix of the typical bacteria lineages expected in aquatic systems. By contrast, a single bacterial genotype with >99% similarity to the facultative pathogen gammaproteobacterial species Stenotrophomonas maltophilia formed >90% of the bacterial assemblage (>10⁷ cell ml⁻¹) in the brackish-water zone at the time point of highest bacterial leucine incorporation. Moreover, these bacteria were equally dominant, albeit less active, in the freshwater zone. Thus, the pelagic zone of the studied lagoon harbored repeated short-term blooms of single bacterial species. This finding may have consequences for environmental protection.     

Blooms of single bacterial species in a coastal lagoon of the southwestern Atlantic Ocean

We investigated seasonal differences in community structure and activity (leucine incorporation) of the planktonic bacterial assemblage in the freshwater and brackish-water zones of a shallow coastal lagoon of the southwestern Atlantic Ocean. Alphaproteobacteria formed the dominant microbial group in both zones throughout the sampling period. After an intrusion of marine water, members of the SAR11 lineage became abundant in the brackish-water zone. These bacteria were apparently distributed over the lagoon during the following months until they constituted almost 30% of all prokaryotic cells at both sampling sites. At the first sampling date (March 2003) a single alphaproteobacterial species unrelated to SAR11, Sphingomonas echinoides, dominated the microbial assemblages in both zones of the lagoon concomitantly with a bloom of filamentous cyanobacteria. Pronounced maxima of leucine incorporation were observed once in each zone of the lagoon. In the freshwater zone, this highly active microbial assemblage was a mix of the typical bacteria lineages expected in aquatic systems. By contrast, a single bacterial genotype with >99% similarity to the facultative pathogen gammaproteobacterial species Stenotrophomonas maltophilia formed >90% of the bacterial assemblage (>10(7) cell ml(-1)) in the brackish-water zone at the time point of highest bacterial leucine incorporation. Moreover, these bacteria were equally dominant, albeit less active, in the freshwater zone. Thus, the pelagic zone of the studied lagoon harbored repeated short-term blooms of single bacterial species. This finding may have consequences for environmental protection.     

Stable Composition of the Nano- and Picoplankton Community during the Ocean Iron Fertilization Experiment LOHAFEX

The iron fertilization experiment LOHAFEX was conducted in a cold-core eddy in the Southern Atlantic Ocean during austral summer. Within a few days after fertilization, a phytoplankton bloom developed dominated by nano- and picoplankton groups. Unlike previously reported for other iron fertilization experiments, a diatom bloom was prevented by iron and silicate co-limitation. We used 18S rRNA gene tag pyrosequencing to investigate the diversity of these morphologically similar cell types within the nano- and picoplankton and microscopically enumerated dominant clades after catalyzed reported deposition fluorescence in situ hybridization (CARD-FISH) with specific oligonucleotide probes. In addition to Phaeocystis, members of Syndiniales group II, clade 10–11, and the Micromonas clades ABC and E made up a major fraction of the tag sequences of the nano- and picoplankton community within the fertilized patch. However, the same clades were also dominant before the bloom and outside the fertilized patch. Furthermore, only little changes in diversity could be observed over the course of the experiment. These results were corroborated by CARD-FISH analysis which confirmed the presence of a stable nano- and picoplankton community dominated by Phaeocystis and Micromonas during the entire course of the experiment. Interestingly, although Syndiniales dominated the tag sequences, they could hardly be detected by CARD-FISH, possibly due to the intracellular parasitic life style of this clade. The remarkable stability of the nano- and picoplankton community points to a tight coupling of the different trophic levels within the microbial food web during LOHAFEX.     

Diatoms shape the biogeography of heterotrophic prokaryotes in early spring in the Southern Ocean

The interplay among microorganisms profoundly impacts biogeochemical cycles in the ocean. Culture-based work has illustrated the diversity of diatom–prokaryote interactions, but the question of whether these associations can affect the spatial distribution of microbial communities is open. Here, we investigated the relationship between assemblages of diatoms and of heterotrophic prokaryotes in surface waters of the Indian sector of the Southern Ocean in early spring. The community composition of diatoms and that of total and active prokaryotes were different among the major ocean zones investigated. We found significant relationships between compositional changes of diatoms and of prokaryotes. In contrast, spatial changes in the prokaryotic community composition were not related to geographic distance and to environmental parameters when the effect of diatoms was accounted for. Diatoms explained 30% of the variance in both the total and the active prokaryotic community composition in early spring in the Southern Ocean. Using co-occurrence analyses, we identified a large number of highly significant correlations between abundant diatom species and prokaryotic taxa. Our results show that key diatom species of the Southern Ocean are each associated with a distinct prokaryotic community, suggesting that diatom assemblages contribute to shaping the habitat type for heterotrophic prokaryotes.     

Interaction and assembly processes of abundant and rare microbial communities during a diatom bloom process

Diatom blooms can significantly influence the dynamics of microbial communities, yet little is known about the interaction and assembly mechanisms of abundant and rare taxa during bloom process. Here, using 16S rRNA gene amplicon sequencing, we investigated the co-occurrence patterns and assembly processes of abundant and rare microbial communities during an early spring diatom bloom in Xiangshan bay. Our results showed that α-diversity indices in the rare subcommunity (RS) were significantly higher than those in the abundant and common subcommunities. β-Diversity of the RS was the highest among three subcommunities, and the variation of β-diversity in the three subcommunities was mainly induced by species turnover, which was also the highest in the RS. The assembly of microbial communities was mainly driven by the neutral processes, but the roles of neutral processes might differ in each subcommunity. Co-occurrence network analysis revealed that abundant and common operational taxonomic units were more often located in central positions within the network. Most of the modules in the network were specific to a particular bloom stage, owing to the succession of Skeletonema costatum. Overall, these findings expand current understanding of the microbial interaction and assembly mechanisms in marine environment suffering harmful algal bloom disturbance.     

Increased sea ice melt as a driver of enhanced Arctic phytoplankton blooming

Phytoplankton primary production in the Arctic Ocean has been increasing over the last two decades. In 2019, a record spring bloom occurred in Fram Strait, characterized by a peak in chlorophyll that was reached weeks earlier than in other years and was larger than any previously recorded May bloom. Here, we consider the conditions that led to this event and examine drivers of spring phytoplankton blooms in Fram Strait using in situ, remote sensing, and data assimilation methods. From samples collected during the May 2019 bloom, we observe a direct relationship between sea ice meltwater in the upper water column and chlorophyll a pigment concentrations. We place the 2019 spring dynamics in context of the past 20 years, a period marked by rapid change in climatic conditions. Our findings suggest that increased advection of sea ice into the region and warmer surface temperatures led to a rise in meltwater input and stronger near-surface stratification. Over this time period, we identify large-scale spatial correlations in Fram Strait between increased chlorophyll a concentrations and increased freshwater flux from sea ice melt.     

Seasonality of North Atlantic phytoplankton from space: impact of environmental forcing on a changing phenology (1998–2012)

Seasonal pulses of phytoplankton drive seasonal cycles of carbon fixation and particle sedimentation, and might condition recruitment success in many exploited species. Taking advantage of long‐term series of remotely sensed chlorophyll a (1998–2012), we analyzed changes in phytoplankton seasonality in the North Atlantic Ocean. Phytoplankton phenology was analyzed based on a probabilistic characterization of bloom incidence. This approach allowed us to detect changes in the prevalence of different seasonal cycles and, at the same time, to estimate bloom timing and magnitude taking into account uncertainty in bloom detection. Deviations between different sensors stressed the importance of a prolonged overlap between successive missions to ensure a correct assessment of phenological changes, as well as the advantage of semi‐analytical chlorophyll algorithms over empirical ones to reduce biases. Earlier and more intense blooms were detected in the subpolar Atlantic, while advanced blooms of less magnitude were common in the Subtropical gyre. In the temperate North Atlantic, spring blooms advanced their timing and decreased in magnitude, whereas fall blooms delayed and increased their intensity. At the same time, the prevalence of locations with a single autumn/winter bloom or with a bimodal seasonal cycle increased, in consonance with a poleward expansion of subtropical conditions. Changes in bloom timing and magnitude presented a clear signature of environmental factors, especially wind forcing, although changes on incident photosynthetically active radiation and sea surface temperature were also important depending on latitude. Trends in bloom magnitude matched changes in mean chlorophyll a during the study period, suggesting that seasonal peaks drive long‐term trends in chlorophyll a concentration. Our results link changes in North Atlantic climate with recent trends in the phenology of phytoplankton, suggesting an intensification of these impacts in the near future.     

Factors determining periphytic algae succession in a tropical hypereutrophic reservoir

The objectives of this study were to evaluate the effect of cyanobacterial blooms on periphyton algal succession and to identify the factors determining community dynamics in a tropical hypereutrophic reservoir. A variety of factors affecting periphytic community structure were identified at two different sites with domestic sewage and spring water inflows and two climatic periods. Microscope glass slides were used to assess periphyton growth. Sampling was carried out at short regular intervals (3–5 days) over 30 days. Climatic periods were limnologically distinct. The rainy period was characterized by an intense cyanobacterial bloom and the dry period by a less intense bloom. Periphyton biomass and growth tended to increase with colonization time during the dry period. Cyanobacteria and Bacillariophyceae were the most representative groups in the rainy period whereas Bacillariophyceae was dominant in the dry period. Community species had successional patterns in both climatic periods. The successional trajectory for sites 1 (domestic sewage) and 2 (spring water) was different in the dry period but similar in the rainy period. We concluded that the community structure over 30 days of colonization under hypereutrophic conditions was primarily determined by seasonal scale (bloom intensity), followed by successional scale (autogenic), and, finally, by the local scale (spring water and sewage inflow). Positive periphyton biological response (higher biomass and algal growth, dominance of diatoms, Cyanobacteria reduction) during small variations of bloom intensity may indicate rapid re-establishment of the community during recovery of the ecosystem.     

Seasonal growth potential of rare lake water bacteria suggest their disproportional contribution to carbon fluxes

We studied the seasonal growth potential of opportunistic bacterial populations in Lake Zurich (Switzerland) by a series of grazer‐free dilution culture assays. Pronounced shifts in the composition of the bacterial assemblages were observed within one doubling of total cell numbers, from initially abundant Actinobacteria to other fast‐growing microbial lineages. Small populations with growth potentials far above community average were detected throughout the year with striking seasonal differences in their respective taxonomic affiliations. Members of Cytophaga‐Flavobacteria (CF) were disproportionally proliferating only during phytoplankton blooms in spring and summer, while Beta‐ and Gammaproteobacteria showed superior growth at all other occasions. Growth rates of Alphaproteobacteria and esp. Sphingomonadaceae were significantly correlated to water temperatures and were far above community average in summer. Within the genus Flavobacterium, two species‐like populations showed a tendency for fast growth in most experiments, while four others were exclusively proliferating either during a spring or during a summer phytoplankton bloom. Their high growth potentials but low in situ abundances hint at a tight control by bacterivorous grazers and at a consequently accelerated carbon flux to higher trophic levels.     

ALGAL ASSEMBLAGES IN ANTARCTIC PACK ICE AND IN ICE-EDGE PLANKTON1

A significant amount of the primary production in the Southern Ocean and other ice-covered oceans takes place in localized ice edge plankton blooms. The dynamics of these blooms appear to be closely related to seasonal melting of sea ice. Algal cells released from the ice are a possible source of ice edge planktonic assemblages, but evidence for this “seeding” has been equivocal. We compared algal assemblages in ice and water in the Weddell Sea during the austral spring of 1983 at a receding ice edge with a well-developed ice edge bloom. The high degree of similarity between ice and water column assemblages, the spatial and temporal patterns in the distribution and abundances of species, and preliminary evidence for the viability and growth of ice-associated species provide evidence for seeding from sea ice of some species in Antarctica.     

Diversity and biomass dynamics of unicellular marine fungi during a spring phytoplankton bloom

Microbial communities have important functions during spring phytoplankton blooms, regulating bloom dynamics and processing organic matter. Despite extensive research into such processes, an in-depth assessment of the fungal component is missing, especially for the smaller size fractions. We investigated the dynamics of unicellular mycoplankton during a spring phytoplankton bloom in the North Sea by 18S rRNA gene tag sequencing and a modified CARD-FISH protocol. Visualization and enumeration of dominant taxa revealed unique cell count patterns that varied considerably over short time scales. The Rozellomycota sensu lato (s.l.) reached a maximum of 10⁵ cells L⁻¹, being comparable to freshwater counts. The abundance of Dikarya surpassed previous values by two orders of magnitude (10⁵ cells L⁻¹) and the corresponding biomass (maximum of 8.9 mg C m⁻³) was comparable to one reported for filamentous fungi with assigned ecological importance. Our results show that unicellular fungi are an abundant and, based on high cellular ribosome content and fast dynamics, active part of coastal microbial communities. The known ecology of the visualized taxa and the observed dynamics suggest the existence of different ecological niches that link primary and secondary food chains, highlighting the importance of unicellular fungi in food web structures and carbon transfer.     

Effects of a Prorocentrum minimum bloom on light availability for and potential impacts on submersed aquatic vegetation in upper Chesapeake Bay

Extraordinary spring blooms of the dinoflagellate Prorocentrum minimum have been a recurring feature of upper Chesapeake Bay for many years. Though not thought to be toxic in Chesapeake Bay, these blooms produce extraordinarily high concentrations of chlorophyll, thereby increasing light attenuation. A particularly large event occurred in the spring of 2000. Here, we assess the impact of the spring 2000 P. minimum bloom on habitat quality for submerged aquatic vegetation (SAV) in the mesohaline region of Chesapeake Bay and its tributaries. We determined the light absorption and scattering spectrum of P. minimum on a per cell basis by analyzing inherent optical properties of natural samples from the Rhode River, Maryland, which were overwhelmingly dominated by P. minimum. Using these per cell properties, we constructed a model of light penetration incorporating observed cell counts of P. minimum to predict the impact of the bloom on other tributaries and main stem locations that experienced the bloom. Model estimates of diffuse attenuation coefficients agreed well with the limited measurements that were available. Impacts of the mahogany tide on diffuse attenuation coefficient ranged from negligible (10–30% increase above the seasonal median in the Patapsco and Magothy rivers), to a greater than six-fold increase (Potomac River). Attenuation coefficients in tributaries to the north and south of the bloom region either decreased or were unchanged relative to seasonal medians. Segments with SAV losses in 2000 were mostly the same as those that experienced the P. minimum bloom. Segments north and south of the bloom area mostly had SAV increases in 2000. Though all of the segments that experienced a decline in SAV area after the spring 2000 bloom showed an increase in 2002, the 2000 setback interrupted what otherwise has been a slow recovery in mid-Bay SAV, demonstrating the adverse impact of P. minimum blooms on SAV populations in Chesapeake Bay.     

Reproductive response of the copepod Rhincalanus gigas to an iron-induced phytoplankton bloom in the Southern Ocean

The reproductive response of Rhincalanus gigas to the build up of a phytoplankton bloom in the Southern Ocean was studied during the European iron fertilization experiment (EIFEX). Egg production experiments were conducted over a period of approximately 5 weeks during development of a diatom dominated bloom. R. gigas showed a clear response to increasing chlorophyll a concentrations and the total egg production of the R. gigas population was highest just after the peak of the bloom at day 29 after fertilization. The average peak production was 50 eggs female⁻¹ day⁻¹. The percentage of egg producing females increased from about 0 to 90% during the course of the experiment. Accordingly, the maturation of the gonads reflected the positive response towards enhanced chlorophyll a concentrations. The fast reproductive response indicate that R. gigas was food limited during the period of this study in the Antarctic Polar Front region (APF).     

Timing and size of blooms of the ctenophore Mnemiopsis leidyi in relation to temperature in Narragansett Bay, RI

The ctenophore Mnemiopsis leidyi is at the northern extreme of its geographic range in Narragansett Bay, an estuary on the northeast coast of the United States. Blooms have typically been observed in late summer and fall according to records from 1950 to 1979. We document an expansion of the seasonal range of this important planktonic predator to include springtime blooms during the 1980s and 1990s. This shift to an earlier seasonal maximum is associated with increasing water temperature in Narragansett Bay. Temperatures in spring have risen, on average, 2 °C from 1950 to 1999 with warm years being associated with the positive phase of the North Atlantic Oscillation. During 1999, M. leidyi appeared earlier in spring and was more abundant than during any previous year for which records are available. Changes in the seasonal pattern and abundance of this predator are likely to have important effects on planktonic ecosystem dynamics of Narragansett Bay. These include reduction of zooplankton abundance in spring followed by increases in size and frequency of summer phytoplankton blooms. Earlier blooms of M. leidyi may also reduce survival of eggs and larvae of fish because, as in 1999, they coincide with the period of peak spawning.     

Bacterial community dynamics during a bloom caused by Akashiwo sanguinea in the Xiamen sea area,China

Phytoplankton blooms are a worldwide ecological problem and one of the major algae that cause phytoplankton blooms is Akashiwo sanguinea. Though much research has addressed the abiotic causes (e.g. growth condition) of A. sanguinea blooms, few studies have examined the dynamics of microbial communities associated with these blooms. In this study, polymerase chain reaction (PCR)-based denaturing gradient gel electrophoresis (DGGE) analysis of 16S rDNA genes was used to document changes in the phylogenetic diversity of microbial communities associated with an A. sanguinea bloom that occurred in the Xiamen sea in May 2010. Surface sea water was sampled once a day within five consecutive days at four sites, and the microbial community composition was determined using DGGE. Sea water concentrations of chlorophyll a, nitrate and phosphate were also measured. The results indicated that the A. sanguinea bloom was probably stimulated by low salinity (26–30‰) and ended probably because inorganic nutrients were consumed and resulted in a N/P ratio unfavorable for this alga. Gammaproteobacteria populations increased significantly during bloom declines and then decreased post-bloom. Divergences in the microbial community composition during different bloom periods were the result of changes in Candidatus, Pelagibacter, Alteromonas, Rhodobacteraceae, Vibrio and Pseudoalteromonas populations. Sediminimonas qiaohouensis was the first bacterium shown to be significantly negatively correlated with A. sanguinea concentration. This study indicated that bacteria may play an important role in A. sanguinea–bloom regulation and provides a deeper insight into bacterial community succession during and after an A. sanguinea–bloom.     

Bacterial Community Structure Associated with a Dimethylsulfoniopropionate-Producing North Atlantic Algal Bloom

The bacteria associated with oceanic algal blooms are acknowledged to play important roles in carbon, nitrogen, and sulfur cycling, yet little information is available on their identities or phylogenetic affiliations. Three culture-independent methods were used to characterize bacteria from a dimethylsulfoniopropionate (DMSP)-producing algal bloom in the North Atlantic. Group-specific 16S rRNA-targeted oligonucleotides, 16S ribosomal DNA (rDNA) clone libraries, and terminal restriction fragment length polymorphism analysis all indicated that the marine Roseobacter lineage was numerically important in the heterotrophic bacterial community, averaging >20% of the 16S rDNA sampled. Two other groups of heterotrophic bacteria, the SAR86 and SAR11 clades, were also shown by the three 16S rRNA-based methods to be abundant in the bloom community. In surface waters, the Roseobacter, SAR86, and SAR11 lineages together accounted for over 50% of the bacterial rDNA and showed little spatial variability in abundance despite variations in the dominant algal species. Depth profiles indicated that Roseobacter phylotype abundance decreased with depth and was positively correlated with chlorophyll a, DMSP, and total organic sulfur (dimethyl sulfide plus DMSP plus dimethyl sulfoxide) concentrations. Based on these data and previous physiological studies of cultured Roseobacter strains, we hypothesize that this lineage plays a role in cycling organic sulfur compounds produced within the bloom. Three other abundant bacterial phylotypes (representing a cyanobacterium and two members of the α Proteobacteria) were primarily associated with chlorophyll-rich surface waters of the bloom (0 to 50 m), while two others (representing Cytophagales and δ Proteobacteria) were primarily found in deeper waters (200 to 500 m).