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Cobalt (Co) is a scarce but essential micronutrient for marine plankton in the Southern Ocean and coastal Antarctic seas where dissolved cobalt (dCo) concentrations can be extremely low. This study presents total dCo and labile dCo distributions measured via shipboard voltammetry in the Amundsen Sea, Ross Sea and Terra Nova Bay during the CICLOPS (Cobalamin and Iron Co-Limitation of Phytoplankton Species) expedition. A significantly smaller dCo inventory was observed during the 2017/2018 CICLOPS expedition compared to two 2005/2006 expeditions to the Ross Sea conducted over a decade earlier. The dCo inventory loss (~10–20 pM) was present in both the surface and deep ocean and was attributed to the loss of labile dCo, resulting in the near-complete complexation of dCo by strong ligands in the photic zone. A changing dCo inventory in Antarctic coastal seas could be driven by the alleviation of iron (Fe) limitation in coastal areas where the flux of Fe-rich sediments from melting ice shelves and deep sediment resuspension may have shifted the region towards vitamin B12 and/or zinc (Zn) limitation, both of which are likely to increase the demand for Co among marine plankton. High demand for Zn by phytoplankton can result in increased Co and cadmium (Cd) uptake because these metals often share the same metal uptake transporters. This study compared the magnitudes and ratios of Zn, Cd and Co uptake (ρ) across upper ocean profiles and observed order of magnitude uptake trends (ρZn > ρCd > ρCo) that paralleled the trace metal concentrations in seawater. High rates of Co and Zn uptake were observed throughout the region, and the speciation of available Co and Zn appeared to influence trends in dissolved metal : phosphate stoichiometry and uptake rates over depth. Multi-year loss of the dCo inventory throughout the water column may be explained by an increase in Co uptake into particulate organic matter (POM) and subsequent increased flux of Co into sediments via sinking and burial. This perturbation of the Southern Ocean Co biogeochemical cycle could signal changes in the nutrient limitation regimes, phytoplankton bloom composition, and carbon sequestration sink of the Southern Ocean.

Scarce dissolved surface ocean concentrations of the essential algal micronutrient zinc suggest that Zn may influence the growth of phytoplankton such as diatoms, which are major contributors to marine primary productivity. However, the specific mechanisms by which diatoms acclimate to Zn deficiency are poorly understood. Using global proteomic analysis, we identified two proteins (ZCRP-A/B, Zn/Co Responsive Protein A/B) among four diatom species that became abundant under Zn/Co limitation. Characterization using reverse genetic techniques and homology data suggests putative Zn/Co chaperone and membrane-bound transport complex component roles for ZCRP-A (a COG0523 domain protein) and ZCRP-B, respectively. Metaproteomic detection of ZCRPs along a Pacific Ocean transect revealed increased abundances at the surface (

Despite very low concentrations of cobalt in marine waters, cyanobacteria in the genus Prochlorococcus retain the genetic machinery for the synthesis and use of cobalt-bearing cofactors (cobalamins) in their genomes. We explore cobalt metabolism in a Prochlorococcus isolate from the equatorial Pacific Ocean (strain MIT9215) through a series of growth experiments under iron- and cobalt-limiting conditions. Metal uptake rates, quantitative proteomic measurements of cobalamin-dependent enzymes, and theoretical calculations all indicate that Prochlorococcus MIT9215 can sustain growth with less than 50 cobalt atoms per cell, ∼100-fold lower than minimum iron requirements for these cells (∼5,100 atoms per cell). Quantitative descriptions of Prochlorococcus cobalt limitation are used to interpret the cobalt distribution in the equatorial Pacific Ocean, where surface concentrations are among the lowest measured globally but Prochlorococcus biomass is high. A low minimum cobalt quota ensures that other nutrients, notably iron, will be exhausted before cobalt can be fully depleted, helping to explain the persistence of cobalt-dependent metabolism in marine cyanobacteria.

Despite very low concentrations of cobalt in marine waters, cyanobacteria in the genus Prochlorococcus retain the genetic machinery for the synthesis and use of cobalt-bearing cofactors (cobalamins) in their genomes. We explore cobalt metabolism in a Prochlorococcus isolate from the equatorial Pacific Ocean (strain MIT9215) through a series of growth experiments under iron- and cobalt-limiting conditions. Metal uptake rates, quantitative proteomic measurements of cobalamin-dependent enzymes, and theoretical calculations all indicate that Prochlorococcus MIT9215 can sustain growth with less than 50 cobalt atoms per cell, ∼100-fold lower than minimum iron requirements for these cells (∼5,100 atoms per cell). Quantitative descriptions of Prochlorococcus cobalt limitation are used to interpret the cobalt distribution in the equatorial Pacific Ocean, where surface concentrations are among the lowest measured globally but Prochlorococcus biomass is high. A low minimum cobalt quota ensures that other nutrients, notably iron, will be exhausted before cobalt can be fully depleted, helping to explain the persistence of cobalt-dependent metabolism in marine cyanobacteria.

Phaeocystis antarctica is an important phytoplankter of the Ross Sea where it dominates the early season bloom after sea ice retreat and is a major contributor to carbon export. The factors that influence Phaeocystis colony formation and the resultant Ross Sea bloom initiation have been of great scientific interest, yet there is little known about the underlying mechanisms responsible for these phenomena. Here, we present laboratory and field studies on Phaeocystis antarctica grown under multiple iron conditions using a coupled proteomic and transcriptomic approach. P. antarctica had a lower iron limitation threshold than a Ross Sea diatom Chaetoceros sp., and at increased iron nutrition (> 120 pM Fe') a shift from flagellate cells to a majority of colonial cells in P. antarctica was observed, implying a role for iron as a trigger for colony formation. Proteome analysis revealed an extensive and coordinated shift in proteome structure linked to iron availability and life cycle transitions with 327 and 436 proteins measured as significantly different between low and high iron in strains 1871 and 1374, respectively. The enzymes flavodoxin and plastocyanin that can functionally replace iron metalloenzymes were observed at low iron treatments consistent with cellular iron-sparing strategies, with plastocyanin having a larger dynamic range. The numerous isoforms of the putative iron-starvation-induced protein (ISIP) group (ISIP2A and ISIP3) had abundance patterns coinciding with that of either low or high iron (and coincident flagellate or the colonial cell types in strain 1871), implying that there may be specific iron acquisition systems for each life cycle type. The proteome analysis also revealed numerous structural proteins associated with each cell type: within flagellate cells actin and tubulin from flagella and haptonema structures as well as a suite of calcium-binding proteins with EF domains were observed. In the colony-dominated samples a variety of structural proteins were observed that are also often found in multicellular organisms including spondins, lectins, fibrillins, and glycoproteins with von Willebrand domains. A large number of proteins of unknown function were identified that became abundant at either high or low iron availability. These results were compared to the first metaproteomic analysis of a Ross Sea Phaeocystis bloom to connect the mechanistic information to the in situ ecology and biogeochemistry. Proteins associated with both flagellate and colonial cells were observed in the bloom sample consistent with the need for both cell types within a growing bloom. Bacterial iron storage and B12 biosynthesis proteins were also observed consistent with chemical synergies within the colony microbiome to cope with the biogeochemical conditions. Together these responses reveal a complex, highly coordinated effort by P. antarctica to regulate its phenotype at the molecular level in response to iron and provide a window into the biology, ecology, and biogeochemistry of this group.

Thaumarchaea are among the most abundant microbial groups in the ocean, but controls on their abundance and the distribution and metabolic potential of different subpopulations are poorly constrained. Here, two ecotypes of ammonia-oxidizing thaumarchaea were quantified using ammonia monooxygenase (amoA) genes across the equatorial Pacific Ocean. The shallow, or water column “A” (WCA), ecotype was the most abundant ecotype at the depths of maximum nitrification rates, and its abundance correlated with other biogeochemical indicators of remineralization such as NO3 : Si and total Hg. Metagenomes contained thaumarchaeal genes encoding for the catalytic subunit of the urease enzyme (ureC) at all depths, suggesting that members of both WCA and the deep, water column “B” (WCB) ecotypes may contain ureC. Coupled urea hydrolysis-ammonia oxidation rates were similar to ammonia oxidation rates alone, suggesting that urea could be an important source of ammonia for mesopelagic ammonia oxidizers. Potential inducement of metal limitation of both ammonia oxidation and urea hydrolysis was demonstrated via additions of a strong metal chelator. The water column inventory of WCA was correlated with the depth-integrated abundance of WCB, with both likely controlled by the flux of sinking particulate organic matter, providing strong evidence of vertical connectivity between the ecotypes. Further, depth-integrated amoA gene abundance and nitrification rates were correlated with particulate organic nitrogen flux measured by contemporaneously deployed sediment traps. Together, the results refine our understanding of the controls on thaumarchaeal distributions in the ocean, and provide new insights on the relationship between material flux and microbial communities in the mesopelagic.

As the annual expanse of Arctic summer ice‐cover steadily decreases, concomitant biogeochemical and ecological changes in this region are likely to occur. Because the Central Arctic Ocean is often nutrient and light limited, it is essential to understand how environmental changes will affect productivity, phytoplankton species composition, and ensuing changes in biogeochemistry in the region. During the transition from late summer to early autumn, water column sampling of various biogeochemical parameters was conducted along an ice‐floe drift station near the North Pole. Our results show that as the upper water column stratification weakened during the late summer–early autumn transition, nutrient concentrations, particulate dimethylsulfoniopropionate (DMSPp) levels, photosynthetic efficiency, and biological productivity, as estimated by ΔO2/Ar ratios, all decreased. Chemotaxonomic (CHEMTAX) analysis of phytoplankton pigments revealed a taxonomically diverse picoautotrophic community, with chlorophyll (Chl) c3‐containing flagellates and the prasinophyte, Pyramimonas spp., as the most abundant groups, comprising ~ 30% and 20% of the total Chl a (TChl a) biomass, respectively. In contrast to previous studies, the picoprasinophyte, Micromonas spp., represented only 5% to 10% of the TChl a biomass. Of the nine taxonomic groups identified, DMSPp was most closely associated with Pyramimonas spp., a Chl b‐containing species not usually considered a high DMSP producer. As the extent and duration of open, ice‐free waters in the Central Arctic Ocean progressively increases, we suggest that enhanced light transmission could potentially expand the ecological niche of Pyramimonas spp. in the region.

Analysis of vitamin B12 in sea water is laborious, time consuming, and often requires storage of relatively large-volume water samples. Alleviating these major limitations will increase the throughput of samples and, as a consequence, improve our understanding of the distribution and role of vitamin B12 in the oceans. Previous studies have indicated that target analyte recovery is negatively affected at flow rates exceeding 1 mL min−1 using home-made C18 Solid Phase Extraction (SPE) cartridges. In this study, the effect of flow rate on recovery of vitamin B12 was tested across a range of flow rates between 1 and 37 mL min−1 using a commercial SPE cartridge containing surface-modified styrene divinylbenzene. Recovery of vitamin B12 at flow rates up to the maximum rate tested did not statistically differ from 1 mL min 1. A second study was conducted to determine whether storage of the SPE cartridges at −20°C had a negative impact on vitamin B12 recovery. Recovery of vitamin B12 from SPE cartridges stored up to 13 days did not differ from unfrozen SPE cartridges. These data suggest that rapid extraction and cold storage of vitamin B12 on commercial SPE cartridges does not negatively affect recovery and offers an economical alternative to field studies.

We report the distribution of cobalt (Co) in the Ross Sea polynya during austral summer 2005–2006 and the following austral spring 2006. The vertical distribution of total dissolved Co (dCo) was similar to soluble reactive phosphate (PO43−), with dCo and PO43− showing a significant correlation throughout the water column (r2 = 0.87, 164 samples). A strong seasonal signal for dCo was observed, with most spring samples having concentrations ranging from ~45–85 pM, whereas summer dCo values were depleted below these levels by biological activity. Surface transect data from the summer cruise revealed concentrations at the low range of this seasonal variability (~30 pM dCo), with concentrations as low as 20 pM observed in some regions where PO43− was depleted to ~0.1 μM. Both complexed Co, defined as the fraction of dCo bound by strong organic ligands, and labile Co, defined as the fraction of dCo not bound by these ligands, were typically observed in significant concentrations throughout the water column. This contrasts the depletion of labile Co observed in the euphotic zone of other ocean regions, suggesting a much higher bioavailability for Co in the Ross Sea. An ecological stoichiometry of 37.6 μmol Co:mol−1 PO43− calculated from dissolved concentrations was similar to values observed in the subarctic Pacific, but approximately tenfold lower than values in the Eastern Tropical Pacific and Equatorial Atlantic. The ecological stoichiometries for dissolved Co and Zn suggest a greater overall use of Zn relative to Co in the shallow waters of the Ross Sea, with a Co:PO43−/Zn:PO43− ratio of 1:17. Comparison of these observed stoichiometries with values estimated in culture studies suggests that Zn is a key micronutrient that likely influences phytoplankton diversity in the Ross Sea. In contrast, the observed ecological stoichiometries for Co were below values necessary for the growth of eukaryotic phytoplankton in laboratory culture experiments conducted in the absence of added zinc, implying the need for significant Zn nutrition in the Zn-Co cambialistic enzymes. The lack of an obvious kink in the dissolved Co:PO43− relationship was in contrast to Zn:PO43− and Cd:PO43− kinks previously observed in the Ross Sea. An excess uptake mechanism for kink formation is proposed as a major driver of Cd:PO43− kinks, where Zn and Cd uptake in excess of that needed for optimal growth occurs at the base of the euphotic zone, and no clear Co kink occurs because its abundances are too low for excess uptake. An unusual characteristic of Co geochemistry in the Ross Sea is an apparent lack of Co scavenging processes, as inferred from the absence of dCo removal below the euphotic zone. We hypothesize that this vertical distribution reflects a low rate of Co scavenging by Mn oxidizing bacteria, perhaps due to Mn scarcity, relative to the timescale of the annual deep winter mixing in the Ross Sea. Thus Co exhibits nutrient-like behavior in the Ross Sea, in contrast to its hybrid-type behavior in other ocean regions, with implications for the possibility of increased marine Co inventories and utility as a paleooceanographic proxy.

The direct respiration of sinking organic matter by attached bacteria is often invoked as the dominant sink for settling particles in the mesopelagic ocean. However, other processes, such as enzymatic solubilization and mechanical disaggregation, also contribute to particle flux attenuation by transferring organic matter to the water column. Here we use observations from the North Atlantic Ocean, coupled to sensitivity analyses of a simple model, to assess the relative importance of particle-attached microbial respiration compared to the other processes that can degrade sinking particles. The observed carbon fluxes, bacterial production rates, and respiration by water column and particle-attached microbial communities each spanned more than an order of magnitude. Rates of substrate-specific respiration on sinking particle material ranged from 0.007 ± 0.003 to 0.173 ± 0.105 day−1. A comparison of these substrate-specific respiration rates with model results suggested sinking particle material was transferred to the water column by various biological and mechanical processes nearly 3.5 times as fast as it was directly respired. This finding, coupled with strong metabolic demand imposed by measurements of water column respiration (729.3 ± 266.0 mg C m−2 d−1, on average, over the 50 to 150 m depth interval), suggested a large fraction of the organic matter evolved from sinking particles ultimately met its fate through subsequent remineralization in the water column. At three sites, we also measured very low bacterial growth efficiencies and large discrepancies between depth-integrated mesopelagic respiration and carbon inputs.

Phaeocystis antarctica is an important phytoplankter of the Ross Sea where it dominates the early season bloom after sea ice retreat and is a major contributor to carbon export. The factors that influence Phaeocystis colony formation and the resultant Ross Sea bloom initiation have been of great scientific interest, yet there is little known about the underlying mechanisms responsible for these phenomena. Here, we present laboratory and field studies on Phaeocystis antarctica grown under multiple iron conditions using a coupled proteomic and transcriptomic approach. P. antarctica had a lower iron limitation threshold than a Ross Sea diatom Chaetoceros sp., and at increased iron nutrition (> 120 pM Fe') a shift from flagellate cells to a majority of colonial cells in P. antarctica was observed, implying a role for iron as a trigger for colony formation. Proteome analysis revealed an extensive and coordinated shift in proteome structure linked to iron availability and life cycle transitions with 327 and 436 proteins significantly different between low and high iron in strains 1871 and 1374, respectively. The enzymes flavodoxin and plastocyanin that can functionally replace iron metalloenzymes were observed at low iron treatments consistent with cellular iron sparing strategies, with plastocyanin being more dynamic in range. The numerous isoforms of the putative iron-starvation induced protein ISIP group (ISIP2A and ISIP3) had abundance patterns coincided with that of either low or high iron (and coincident flagellate or the colonial cell types in strain 1871), implying that there may be specific iron acquisition systems for each life cycle type. The proteome analysis also revealed numerous structural proteins associated with each cell type: within flagellate cells actin and tubulin from flagella and haptonema structures as well as a suite of calcium-binding proteins with EF domains were observed. In the colony-dominated samples a variety of structural proteins were observed that are also often found in multicellular organisms including spondins, lectins, fibrillins, and glycoproteins with von Willebrand domains. A large number of proteins of unknown function were identified that became abundant at either high and low iron availability. These results were compared to the first metaproteomic analysis of a Ross Sea Phaeocystis bloom to connect the mechanistic information to the in situ ecology and biogeochemistry. Proteins associated with both flagellate and colonial cells were observed in the bloom sample consistent with the need for both cell types within a growing bloom. Bacterial iron storage and B12 biosynthesis proteins were also observed consistent with chemical synergies within the colony microbiome to cope with the biogeochemical conditions. Together these responses reveal a complex, highly coordinated effort by P. antarctica to regulate its phenotype at the molecular level in response to iron and provide a window into the biology, ecology, and biogeochemistry of this group.

Marine phytoplankton play critical roles in the biogeochemistry of open and coastal oceans. However, the impact that individual species have on an ecosystem-wide scale can strongly depend on the production of cellular compounds, especially those that are climatically active such as dimethylsulfide (DMS). Herein, we use sorting flow cytometry to separate a distinct phytoplankton population from four samples taken along the Patagonian shelf near the Falkland Islands. Morphological, genetic, and biochemical analyses demonstrated that three of the sorted samples were dominated by a bloom of the dinoflagellate Prorocentrum minimum. Cellular quotas of the DMS-precursor dimethylsulfoniopropionate (DMSP) ranged from 1.23–4.11 pg cell−1 in the same population at different sampling stations. Causes of this variability may be due to different growth stages of the P. minimum bloom or changes in other environmental variables. Overall, in situ intracellular DMSP concentrations were lower than what would be expected based on previous, culture-based measurements. We demonstrate the difficulties inherent in sorting individual phytoplankton species from natural samples in order to determine in situ species-specific cellular quotas of important biogeochemical compounds.

SummaryPhytoplankton blooms are ephemeral events of exceptionally high primary productivity that regulate the flux of carbon across marine food webs [1–3]. Quantification of bloom turnover [4] is limited by a fundamental difficulty to decouple between physical and biological processes as observed by ocean color satellite data. This limitation hinders the quantification of bloom demise and its regulation by biological processes [5, 6], which has important consequences on the efficiency of the biological pump of carbon to the deep ocean [7–9].Here, we address this challenge and quantify algal blooms’ turnover using a combination of satellite and in situ data, which allows identification of a relatively stable oceanic patch that is subject to little mixing with its surroundings. Using a newly developed multisatellite Lagrangian diagnostic, we decouple the contributions of physical and biological processes, allowing quantification of a complete life cycle of a mesoscale (∼10–100 km) bloom of coccolithophores in the North Atlantic, from exponential growth to its rapid demise. We estimate the amount of organic carbon produced during the bloom to be in the order of 24,000 tons, of which two-thirds were turned over within 1 week. Complimentary in situ measurements of the same patch area revealed high levels of specific viruses infecting coccolithophore cells, therefore pointing at the importance of viral infection as a possible mortality agent. Application of the newly developed satellite-based approaches opens the way for large-scale quantification of the impact of diverse environmental stresses on the fate of phytoplankton blooms and derived carbon in the ocean.

This study focuses on the potential explanations for a Phaeocystis antarctica summer bloom occurred in stratified waters of Terra Nova Bay (TNB) - which is part of the Antarctic Special Protected Area (n.161) in the Ross Sea - trough a multi-parameter correlative approach. Many previous studies have highlighted that water column stratification typically favors diatom dominance compared to the colonial haptophyte P. antarctica, in the Ross Sea, and this correlation has often been used to explain the historic dominance of diatoms in TNB. To explore the spatial and temporal progression of P. antarctica bloom in coastal waters, four stations were sampled three times each between December 31, 2009 and January 13, 2010. Taxonomic and pigment composition of phytoplankton communities, macro-nutrient concentrations and various different indices, all indicated the relative dominance of P. antarctica. Cell abundances revealed that P. antarctica contributed 79% of total cell counts in the upper 25 m and 93% in the lower photic zone. Similarly, a strong correlation was observed between Chl-a and the Hex:Fuco pigment ratio, corroborating the microscopic analyses. Recent studies have shown that iron can trigger colonial P. antarctica blooms. Based on the Hex:Chl-c3 proxy for iron limitation in P. antarctica, we hypothesize that anomalously higher iron fluxes were responsible for the unusual bloom of colonial P. antarctica observed in TNB.

Nuclear magnetic resonance (NMR) spectroscopy has been used to obtain metabolic profiles of the polar diatom Fragilariopsis cylindrus, leading to the identification of a novel metabolite in this organism. Initial results from an ongoing metabolomics study have led to the discovery of isethionic acid (2-hydroxyethanesulfonic acid, CAS: 107-36-8) as a major metabolite in F. cylindrus. This compound is being produced by the organism under normal culture conditions. This finding is the first report of a diatom producing isethionic acid. In addition to isethionic acid, four other metabolites, dimethylsulfoniopropionate (DMSP), betaine, homarine, and proline were present and may serve as osmoprotectants in F. cylindrus. NMR-based metabolite profiles of F. cylindrus were obtained along a growth curve of the organism. The relative concentration levels of the five metabolites were monitored over a growth period of F. cylindrus from 18 to 25 days. All showed an increase in relative concentration with time, except for proline, which began to decrease after day 21.

We present full-depth zonal sections of total dissolved cobalt, iron, manganese, and labile cobalt from the South Atlantic Ocean. A basin-scale plume from the African coast appeared to be a major source of dissolved metals to this region, with high cobalt concentrations in the oxygen minimum zone of the Angola Dome and extending 2500 km into the subtropical gyre. Metal concentrations were elevated along the coastal shelf, likely due to reductive dissolution and resuspension of particulate matter. Linear relationships between cobalt, N2O, and O2, as well as low surface aluminum supported a coastal rather than atmospheric cobalt source. Lateral advection coupled with upwelling, biological uptake, and remineralization delivered these metals to the basin, as evident in two zonal transects with distinct physical processes that exhibited different metal distributions. Scavenging rates within the coastal plume differed for the three metals; iron was removed fastest, manganese removal was 2.5 times slower, and cobalt scavenging could not be discerned from water mass mixing. Because scavenging, biological utilization, and export constantly deplete the oceanic inventories of these three hybrid-type metals, point sources of the scale observed here likely serve as vital drivers of their oceanic cycles. Manganese concentrations were elevated in surface waters across the basin, likely due to coupled redox processes acting to concentrate the dissolved species there. These observations of basin-scale hybrid metal plumes combined with the recent projections of expanding oxygen minimum zones suggest a potential mechanism for effects on ocean primary production and nitrogen fixation via increases in trace metal source inputs.

Dinoflagellate algae of the genus Symbiodinium occur as endosymbionts in a variety of hosts including coral. The response of Symbiodinium spp. to environmental changes could dictate survival of their hosts and the ecological success of coral reef ecosystems. Oxidative stress has been linked to a breakdown in this symbiotic relationship, known as bleaching. Increased temperature is one of the primary environmental changes linked to this phenomenon. Preliminary studies have established high concentrations of the sulfur compound dimethylsulfoniopropionate (DMSP) in Symbiodinium spp., with increased temperature. To examine the potential use of DMSP as an antioxidant, a 5 day incubation experiment was conducted at two temperatures with the algae S. microadriaticum (CCMP1633) isolated from the cnidarian host Aiptasia pulchella. An HPLC assay for the activity of the enzyme B12-dependent methionine synthase was modified and used to determine the link between de novo production of methionine, a precursor to DMSP, and temperature induced oxidative stress. DMSP concentrations per cell increased approximately 38 % in the 33 °C treatment cultures over 120 h. However, these cells also increased more than 2-fold in biovolume (127 ± 43 %), and SYTO-BC stain indicated increased DNA content (approximately 4-fold), suggesting arrested cell division. Normalization of DMSP to biovolume revealed that the concentrations actually decreased approximately 49 % after 2 days in cultures exposed to elevated temperature (33 °C), but were not significantly different from the control treatment at 120 h (27 °C). Concomitant changes in the 33 °C treatment relative to the control (after 120 h) resulted in an approximately 8-fold increase in reactive oxygen species, a 37 % (±7 %) decrease in photosynthetic efficiency of photosystem II, and a 5-fold increase in xanthophyll cycling. Methionine synthase activity (MSA) correlated to the decrease in DMSP concentration (R 2 = 0.778), with decreasing activity at the high temperature. Given this decrease in MSA, the increase in DMSP per cell may be due to DMSP production utilizing methionine from protein turnover, and not de novo synthesis via MSA. The findings of this study provide insight into the responses of algal symbionts to environmental changes, shed light on the potential use of DMSP and other known photo-protective mechanisms such as xanthophyll cycling under temperature induced oxidative stress, and support the suspected cessation of cell division under these conditions. This information could be crucial to understanding cellular responses to environmental changes and the ability of these organisms to survive under elevated sea surface temperatures projected for the near future.

Dimethylsulfoniopropionate (DMSP) plays important roles in oceanic carbon and sulfur cycling and may significantly impact climate. It is a biomolecule synthesized from the methionine (Met) pathway and proposed to serve various physiological functions to aid in environmental stress adaptation through its compatible solute, cryoprotectant, and antioxidant properties. Yet, the enzymes and mechanisms regulating DMSP production are poorly understood. This study utilized a proteomics approach to investigate protein changes associated with salinity-induced DMSP increases in the model sea-ice diatom Fragilariopsis cylindrus (CCMP 1102). We hypothesized proteins associated with the Met-DMSP biosynthesis pathway would increase in relative abundance when challenged with elevated salinity. To test this hypothesis axenic log-phase cultures initially grown at a salinity of 35 were gradually shifted to a final salinity of 70 over a 24-h period. Intracellular DMSP was measured and two-dimensional gel electrophoresis was used to identify protein changes at 48 h after the shift. Intracellular DMSP increased by approximately 85% in the hypersaline cultures. One-third of the proteins increased under high salinity were associated with amino acid pathways. Three protein isoforms of S-adenosylhomo-cysteine hydrolase, which synthesizes a Met precursor, increased 1.8- to 2.1-fold, two isoforms of S-adenosyl Met synthetase increased 1.9- to 2.5-fold, and S-adenosyl Met methyltransferase increased by 2.8-fold, suggesting active methyl cycle proteins are recruited in the synthesis of DMSP. Proteins from the four enzyme classes of the proposed algal Met transaminase DMSP pathway were among the elevated proteins, supporting our hypothesis and providing candidate genes for future characterization studies.

Laboratory culture experiments were used to investigate the growth rate of colonial Phaeocystis antarctica as a function of irradiance and dissolved iron concentration. The experiments were conducted with a P. antarctica strain isolated from the southern Ross Sea, Antarctica, and made use of natural, low-iron (

14 pages,11 figures The major source of reduced sulfur in the remote marine atmosphere is the biogenic compound dimethylsulfide (DMS), which is ubiquitous in the world’s oceans and released through food web interactions. Relevant fluxes and concentrations of DMS, its phytoplankton-produced precursor, dimethylsulfoniopropionate (DMSP) and related parameters were measured during an intensive Lagrangian field study in two mesoscale eddies in the Sargasso Sea during July–August 2004, a period characterized by high mixed-layer DMS and low chlorophyll—the so-called ‘DMS summer paradox’. We used a 1-D vertically variable DMS production model forced with output from a 1-D vertical mixing model to evaluate the extent to which the simulated vertical structure in DMS and DMSP was consistent with changes expected from field-determined rate measurements of individual processes, such as photolysis, microbial DMS and dissolved DMSP turnover, and air–sea gas exchange. Model numerical experiments and related parametric sensitivity analyses suggested that the vertical structure of the DMS profile in the upper 60 m was determined mainly by the interplay of the two depth variable processes—vertical mixing and photolysis—and less by biological consumption of DMS. A key finding from the model calibration was the need to increase the DMS(P) algal exudation rate constant, which includes the effects of cell rupture due to grazing and cell lysis, to significantly higher values than previously used in other regions. This was consistent with the small algal cell size and therefore high surface area-to-volume ratio of the dominant DMSP-producing group—the picoeukaryotes. We gratefully acknowledge the financial assistance provided through NSF Biocomplexity funding (OPP-0083078) and an Australian Research Council Discovery Grant. We are grateful to the comments by D.J. Kieber. We recognize the participation and help of K. Bailey, J. Bisgrove, B. Blomquist, I. Forn, H. Harada, B. Huebert, D. Jones, L. Maroney, A. Neely, S. Riseman, C. Smith, J. Stefels, K. Tinklepaugh, M. Vila-Costa, G. Westby, H. Zemmelink and the R/V Seward Johnson crew. DiTullio et al., 2001; Simo ́ and Dachs, 2002; Simon and Azam, 1989; Zemmelink et al., 2005

The goal of this study was to examine the relationships between phytoplankton and heterotrophic bacteria and the effect of light on bacterial growth and respiration in the Northeast Atlantic Ocean in summer. Heterotrophic microbes were a substantial component of the plankton as indicated by the ratio of bacterial biomass to phytoplankton biomass, which varied from 0.15 to 0.83, averaging 0.60. Aerobic anoxygenic phototrophic (AAP) bacteria made up on average 10% of bacterial abundance and 13 % of bacterial biomass. AAP bacterial biomass was on average 2-fold higher than Synechococcus sp. biomass, whereas Prochlorococcus sp. was never more than 1 % of bacterial biomass. The bacterial production to primary production ratio ranged from 0.04 to 0.14 and was on average 0.07. The bacterial growth efficiency (BGE) in light incubations (10%) was 3-fold lower than in the dark (32 %). Consequently, the calculated flux of carbon through bacteria in the light was also about 3-fold lower in the dark, since ratios of bacterial carbon demand (BCD) to primary production inferred from light and dark estimates of BGE were 0.7 and 0.2, respectively. However, BCD and respiration rates were not greater than primary production, suggesting that this region of the North Atlantic was net autotrophic even after the spring bloom. The BGE data and the abundance of photoheterotrophic microbes, such as AAP bacteria, highlight the importance of the effects of light on carbon cycling by bacteria in the Northeast Atlantic Ocean.

Primary production in the Ross Sea, one of the most productive areas in the Southern Ocean, has previously been shown to be seasonally limited by iron. In two of three bottle incubation experiments conducted in the austral summer, significantly higher chlorophyll a (Chl a) concentrations were measured upon the addition of iron and B12, relative to iron additions alone. Initial bacterial abundances were significantly lower in the two experiments that showed phytoplankton stimulation upon addition of B12 and iron relative to the experiment that did not show this stimulation. This is consistent with the hypothesis that the bacteria and archaea in the upper water column are an important source of B12 to marine phytoplankton. The addition of iron alone increased the growth of Phaeocystis antarctica relative to diatoms, whereas in an experiment where iron and B12 stimulated total phytoplankton growth, the diatom Pseudonitzschia subcurvata went from comprising approximately 70% of the phytoplankton community to over 90%. Cobalt additions, with and without iron, did not alter Chl a biomass relative to controls and iron additions alone in the Ross Sea. Iron and vitamin B12 plus iron treatments caused reductions in the DMSP (dimethyl sulfoniopropionate) : Chl a ratio relative to the control and B12 treatments, consistent with the notion of an antioxidant function for DMSP. These results demonstrate the importance of a vitamin to phytoplankton growth and community composition in the marine environment.

Interpretation of photosynthetic pigment data using iterative programs such as CHEMTAX are widely used to examine algal community structure in the surface ocean. The accuracy of such programs relies on understanding the effects of environmental parameters on the pigment composition of taxonomically diverse algal groups. Phaeocystis antarctica is an important contributor to total autotrophic production and the biogeochemical cycling of carbon and sulfur in the Southern Ocean. Here we report the results of a laboratory culture experiment in which we examined the effects of ambient dissolved iron concentration on the pigment composition of colonial P. antarctica, using a new P. antarctica strain isolated from the southern Ross Sea in December 2003. Low-iron ( 3) indicative of Fe stress. We also observed that the ratio of fucoxanthin to 19′-hexanoyloxyfucoxanthin (Fuco:Hex ratio) was highly correlated (r 2 = 0.82) with initial dissolved Fe concentration, with Fuco:Hex ratios

We have carried out field and laboratory experiments to examine the iron requirements of colonial Phaeocystis antarctica in the Ross Sea. In December 2003, we performed an iron/light-manipulation bioassay experiment in the Ross Sea polynya, using an algal assemblage dominated by colonial Phaeocystis antarctica, collected from surface waters with an ambient dissolved Fe concentration of ∼0.4 nM. Results from this experiment suggest that P. antarctica growth rates were enhanced at high irradiance (∼50% of incident surface irradiance) but were unaffected by iron addition, and that elevated irradiance mediated a significant decrease in cellular chlorophyll a content. We also conducted a laboratory iron dose–response bioassay experiment using a unialgal, non-axenic strain of colonial P. antarctica and low-iron (

Phytoplankton community structure in the eastern Equatorial Pacific (EEP) and the Peru upwelling system was determined using ChemTax analysis of pigment data. Photosynthetic efficiencies (F v : F m ), concentrations of macronutrients, iron (Fe), and the redox proteins flavodoxin (Flv) and ferredoxin (Fd) were used to assess the physiological status of the algal communities with respect to Fe. Diel periodicity in F v :F m ratios was observed in Fe-stressed populations of Prochlorococcus and type 4 haptophytes (Phaeocystis) in the EEP with daytime values (0.05-0.30) displaying a distinct noontime minimum. On the Peruvian continental shelf, upwelling of Fe-rich subsurface waters led to massive diatom blooms (reaching chlorophyll a [Chl a] values >45 μg L -1 ) with near-maximal photosynthetic efficiencies (F v : F m >0.6). Assimilation and luxurious uptake of Fe, scavenging processes, and Ek-man transport off the shelf break led to rapid decreases in Fe concentration and dramatic shifts in phytoplankton assemblages. The shelf break region was dominated by cryptophyte populations with relatively high photosynthetic competency (F v :F m = 0.4-0.5). In this region, subnanomolar Fe concentrations limited further diatom growth. As Fe concentrations decreased offshore, haptophyte (coccolithophorids and Phaeocystis) populations contributed approximately 40-80% toward the total Chl a concentration. Haptophyte populations were Fe stressed, as determined by high Flv indices (Flv [Flv + Fd] -1 ) and low photosynthetic efficiencies (F v : F m ∼0.3). The Flv index was very high in regions where Fe concentrations were ≤0.2 nmol L -1 , but it was 0 when Fe levels were >0.4 nmol L -1 . In the Peru upwelling, Fe concentrations between 0.2 and 0.4 nmol L -1 represented a transitional region that marked the threshold switch in community Fd and Flv expression.

The effects of iron on phytoplankton community structure in 'High Nutrient Low Chlorophyll' regions of the ocean have been examined using both shipboard batch cultures (growouts) and open ocean mesoscale fertilization experiments. The addition of iron in these areas frequently results in a shift from communities dominated by small non-siliceous species towards ones dominated by larger diatoms. We used a new shipboard continuous culture experimental design in iron-limited Peru upwelling waters to examine shifts in phytoplankton structure and their bio- geochemical consequences following simulated upwelled iron inputs. By allowing the added iron to pre-equilibrate with natural seawater ligands, we were able to supply iron in realistic chemical spe- cies at rates and concentrations similar to those found in upwelled waters off Peru. The community shifted strongly from cyanobacteria towards diatoms, and the extent of this shift was proportional to the increase in iron supply. Eukaryotic nanophytoplankton were the first to respond to the iron addi- tion, followed by a community dominated by small pennate diatoms by Day 5. These community changes led to increased biogenic silica:particulate organic nitrogen (BSi:PON) and biogenic silica:particulate organic carbon (BSi:POC) production ratios, driven mainly by increases in diatom numbers with increasing iron. Our experiment demonstrated both similarities to and differences with parallel growout experiments and previous mesoscale fertilization experiments, and suggest that the shipboard continuous culture method can be applied to questions that cannot be easily addressed by either of these previous iron addition techniques.

Lake Bonney is a permanently ice-covered lake in the McMurdo Dry Valleys, Antarctica. The lake has two chemically stratified lobes (referred to as the east and west lobes), each with distinct biogenic sulfur profiles. Dimethylsulfoniopropionate (DMSPp) and dimethylsulfoxide (DMSOp) exceeded 32 and 2 nmol L 21 , respectively, in the photic surface waters of the lake. Maximum DMSPp levels occurred in the deep-chlorophyll layer of both lobes, a zone dominated by chrysophytes and chlorophytes, which are thought to be the source of dimethylated sulfur in the deep waters of the lake following sedimentation and biogeochemical processing. Waters beneath the chemoclines of both lobes are cold (,08C), saline (.3 times seawater), suboxic, and devoid of phytoplankton biomass and activity. Dimethylsulfide (DMS) levels (.330 nmol L 21 ) in the deep west lobe are among the highest recorded in a natural aquatic ecosystem. In contrast, saline waters of the deep east lobe contain relatively little DMS (,70 nmol L 21 ) but high DMSOd (270 nmol L 21 ), with the latter being the highest observed in any natural aquatic ecosystem. We argue that the differences in the biogenic sulfur profiles between the deep waters of the two lobes arise principally from subtle differences in the redox conditions found in each lobe.

Studies in high nutrient, low chlorophyll (HNLC) regions have demonstrated that increased Fe availability results in an increase in phytoplankton biomass and changes in community composition. Here we present experiments in which the availability of iron (Fe) was increased or reduced to monitor the response of individual groups of phytoplankton (large eukaryotes, pico- eukaryotes and cyanobacteria) by flow cytometry. Additions (0.5 to 5.0 nM Fe) and reductions in available Fe (through addition of 1 to 10 nM of the fungal siderophore desferrioxamine B) were made to enclosed communities from the South American eastern boundary current off Peru, where ambi- ent dissolved Fe concentrations were

Phytoplankton pigments were measured using HPLC during non-ENSO conditions in mid-summer along a South Pacific transect from 67° S to the equator along 170° W. Highest concen- trations of chlorophyll a (chl a) occurred in the Polar and the Subtropical Fronts (PF and STF, respec- tively) with concentrations exceeding 500 ng l -1 . In the STF, there was a distinct subsurface chl a maximum (SCM) at 40 m, which gradually deepened northward to 120 m in the Subtropical Conver- gence Zone. Northwards, the SCM shoaled to about 30 m in the Equatorial Zone (EZ). Relatively high concentrations of fucoxanthin and 19'-butanoyloxyfucoxanthin occurred in the nutrient-rich waters south of the Subantarctic Front, and CHEMTAX analyses indicated that diatoms, chrysophytes, pelagophytes, and haptophytes dominated the phytoplankton assemblage. Northward of the PF to the STF, where silicate concentrations were

Inputs of low concentrations of new and regenerated forms of nitrogen (≤1 µM) have large impacts on phytoplankton community structure and ocean biogeochemistry in the oligotrophic central gyres. However, current manipulative experimental methods cannot effectively simulate low-level, continuous supplies of nitrate, such as those from upwelling or eddy events, or steady-state inputs of regenerated ammonium and dissolved organic nitrogen from zooplankton grazing. Using a new shipboard continuous culture system based on laboratory chemostat methodology, we compared the effects of a continuous supply of 1 µM nitrate, ammonium, and urea-N at a dilution rate of 0.5 d−1 on algal community composition in the North Atlantic. In the Gulf Stream, continuous inputs of 1 µM nitrate dramatically changed phytoplankton community structure from dominance by cyanobacteria to dominance by diatoms. Equimolar continuous supplies of ammonium resulted in much smaller increases in total phytoplankton biomass, and favored a community co-dominated by diatoms and cyanobacteria, and promoted the growth of pelagophytes. In the Sargasso Sea, continuous 1 µM urea-N inputs greatly increased the biomass and dominance of Synechococcus relative to the initial community and compared with control and 1 µM nitrate additions. The shipboard natural community continuous culture system is uniquely suited for realistically simulating inputs of low levels of limiting nutrients, allowing new types of prognostic enrichment experiments that give novel insights into the processes that control phytoplankton community structure in the ocean.

We investigated phytoplankton Fe limitation using shipboard incubation experiments in the high-nutrient South American eastern boundary current regime. Low ambient Fe concentrations (;0.1 nM) in water collected from the Humboldt and Peru Currents were supplemented with a range of added Fe levels up to 2.5 nM. Phytoplankton chlorophyll a, photosystem II photosynthetic efficiency, and nitrate and phosphate drawdown increased in proportion to the amount of Fe added. The Humboldt Current algal community after Fe additions included colonial and flagellated Phaeocystis globosa and large pennate diatoms, whereas the Peru Upwelling assemblage was dominated by coccolithophorids and small pennate diatoms. Apparent half-saturation constants for growth of the two communities were 0.17 nM Fe (Humboldt Current) and 0.26 nM Fe (Peru Upwelling). Net molar dissolved Si(OH)4 : NO drawdown ratios were low in both experiments (;0.2‐0.7), but net particulate silica to nitrogen production 2 3 ratios were higher. Fe limitation decreased net NO : PO utilization ratios in the Humboldt Current incubation to 2 32 34 well below Redfield values. Phytoplankton community sinking rates were decreased by Fe additions in the Peru Upwelling, suggesting potential Fe effects on carbon export. Our results confirm that primary producers in the Peru Upwelling/Humboldt Current system can be limited by Fe, but the biological and biogeochemical consequences of Fe limitation differ from Fe-limited California coastal upwelling waters that have been previously studied.

We report the results of a field incubation experiment demonstrating a substantial shift in the taxonomic composition of Equatorial Pacific phytoplankton assemblages exposed to CO2 levels of 150 and 750 ppm (dissolved CO2 ~3 to 25 µM). By the end of the experiment, the phytoplankton community in all samples was dominated by diatoms and Phaeocystis sp. However, the relative abun- dance of these phytoplankton taxa differed significantly between CO2 treatments. Taxonomic pig- ment analysis and direct microscopic examination of samples revealed that the abundance of diatoms decreased by ~50% at low CO2 relative to high CO2, while the abundance of Phaeocystis increased by ~60% at low CO2. This CO2-dependent shift was associated with a significant change in nutrient utilization, with higher ratios of nitrate:silicate (N:Si) and nitrate:phosphate (N:P) consumption by phytoplankton in the low CO2 treatment. Despite the significant changes in taxonomic composition and nutrient consumption ratios, total biomass and primary productivity did not differ significantly between the CO2 treatments. Our results suggest that CO2 concentrations could potentially influence competition among marine phytoplankton taxa and affect oceanic nutrient cycling.

Proteomics has great potential for studies of marine microbial biogeochemistry, yet high microbial diversity in many locales presents us with unique challenges. We addressed this challenge with a targeted metaproteomics workflow for NtcA and P-II, two nitrogen regulatory proteins, and demonstrated its application for cyanobacterial taxa within microbial samples from the Central Pacific Ocean. Using METATRYP, an open-source Python toolkit, we examined the number of shared (redundant) tryptic peptides in representative marine microbes, with the number of tryptic peptides shared between different species typically being 1% or less. The related cyanobacteria Prochlorococcus and Synechococcus shared an average of 4.8 ± 1.9% of their tryptic peptides, while shared intraspecies peptides were higher, 13 ± 15% shared peptides between 12 Prochlorococcus genomes. An NtcA peptide was found to target multiple cyanobacteria species, whereas a P-II peptide showed specificity to the high-light Prochlorococcus ecotype. Distributions of NtcA and P-II in the Central Pacific Ocean were similar except at the Equator likely due to differential nitrogen stress responses between Prochlorococcus and Synechococcus. The number of unique tryptic peptides coded for within three combined oceanic microbial metagenomes was estimated to be ∼4 × 10(7) , 1000-fold larger than an individual microbial proteome and 27-fold larger than the human proteome, yet still 20 orders of magnitude lower than the peptide diversity possible in all protein space, implying that peptide mapping algorithms should be able to withstand the added level of complexity in metaproteomic samples.

Shipboard bottle incubation experiments were performed to investigate the effects of iron, light, and silicate on algal production of particulate dimethylsulfoniopropionate (DMSPp) in the Subantarctic Zone (SAZ) south of Tasmania during March 1998. Iron enrichment resulted in threefold to ninefold increases in DMSPp concentrations relative to control treatments, following 7 and 8-day incubation experiments. Additions of Fe and Si preferentially stimulated the growth of lightly-silicified pennate diatoms and siliceous haptophytes, respectively, to which we attribute the increased DMSPp production in the incubation bottles. Both of these algal groups were previously believed to be low DMSPp producers; however, our experimental data suggest that addition of iron and silicate to the low-silicate low-iron waters of the SAZ will result in increased production of DMSPp by lightly silicified diatoms and siliceous haptophytes, respectively. Increased irradiance enhanced DMSPp production in iron-amended treatments with both low (0.5 nM) and high (5 nM) concentrations of added iron. However, the role of light in stimulating DMSPp production was apparently of secondary importance compared to the effects of iron addition. The combination of high irradiance and high iron enrichment produced the highest DMSPp production in the experiments, suggesting that iron and light may have a synergistic effect in limiting algal DMSPp production in subantarctic waters.

The influence of irradiance and iron (Fe) supply on phytoplankton processes was investigated, north (47S, 142E) and south (54S, 142E) of the Subantarctic Front in austral autumn (March 1998). At both sites, resident cells exhibited nutrient stress (Fv/ Fm 0.3). Shipboard perturbation experiments examined two light (mean in situ and elevated) and two Fe (nominally 0.5 and 3 nM) treatments under silicic acid-replete conditions. Mean in situ light levels (derived from incident irradiances, mixed layer depths (MLDs), wind stress, and a published vertical mixing model) differed at the two sites, 25% of incident irradiance I 0 at 47S and 9% I 0 at 54S because of MLDs of 40 (47S) and 90 m (54S), when these stations were occupied. The greater MLD at 54S is reflected by tenfold higher cellular chlorophyll a levels in the resident phytoplankton. In the 47S experiment, chlorophyll a levels increased to 1 gL 1 only in the high-Fe treatments, regardless of irradiance levels, suggesting Fe limitation. This trend was also noted for cell abundances, silica production, and carbon fixation rates. In contrast, in the 54S experiment there were increases in chlorophyll a (to 2 gL 1 ), cell abundances, silica production, and carbon fixation only in the high-light treatments to which Fe had been added, suggesting that Fe and irradiance limit algal growth rates. Irradiance by altering algal Fe quotas is a key determinant of algal growth rate at 54S (when silicic acid levels are nonlimiting); however, because of the integral nature of Fe/light colimitation and the restricted nature of the current data set, it was not possible to ascertain the relative contributions of Fe and irradiance to the control of phytoplankton growth. On the basis of a climatology of summer mean MLD for subantarctic (SA) waters south of Australia the 47 and 54S sites appear to represent minimum and maximum MLDs, where Fe and Fe/ irradiance, respectively, may limit/colimit algal growth. The implications for changes in the factors limiting algal growth with season in SA waters are discussed.

Subantarctic Southern Ocean surface waters in the austral summer and autumn are characterized by high concentrations of nitrate and phosphate but low concentrations of dissolved iron (Fe, ∼0.05 nM) and silicic acid (Si

We investigated the hydrography, nutrients, and dissolved and particulate carbon pools in the western Pacific sector of the Antarctic Circumpolar Current (ACC) during austral summer 1996 to assess the region's role in the carbon cycle. Low fCO2 values along two transects indicated that much of the study area was a sink for atmospheric CO2. The fCO2 values were lowest near the Polar Front (PF) and the Subtropical Front (STF), concomitant with maxima of chlorophyll a and particulate and dissolved organic carbon. The largest biomass accumulations did not occur at fronts, which had high surface geostrophic velocities (20–51 cm s−1), but in relatively low velocity regions near fronts or in an eddy. Thus vertical motion and horizontal advection associated with fronts may have replenished nutrients in surface waters but also dispersed phytoplankton. Although surface waters north of the PF have been characterized as a “high nutrient-low chlorophyll” region, low silicic acid (Si) concentrations (2–4 μM) may limit production of large diatoms and therefore the potential carbon flux. Low concentrations (4–10 μM Si) at depths of winter mixing constrain the level of Si replenishment to surface waters. It has been suggested that an increase in aeolian iron north of the PF may increase primary productivity and carbon export. Our results, however, indicate that while diatom growth and carbon export may be enhanced, the extent ultimately would be limited by the vertical supply of Si. South of the PF, the primary mechanism by which carbon is exported to deep water appears to be through diatom flux. We suggest that north of the PF, particulate and dissolved carbon may be exported primarily to intermediate depths through subduction and diapycnal mixing associated with Subantarctic Mode Water and Antarctic Intermediate Water formation. These physical-biological interactions and Si dynamics should be included in future biogeochemical models to provide a more accurate prediction of carbon flux.

Dissolved iron and manganese and total dissolvable iron were measured in water column samples collected from the polynya region of the southern Ross Sea during cruises in November–December 1994 (spring 1994) and December 1995 to January 1996 (summer 1995). Iron and manganese addition bottle incubation experiments were also performed during these cruises in order to assess the nutritional sufficiency of ambient iron and manganese concentrations for growth of the phytoplankton community. Generally high dissolved iron concentrations (>0.5 nM) and relatively complex iron and manganese vertical profiles were obtained during the spring 1994 cruise, compared with the summer 1995 data. Dissolved iron concentrations in the upper water column averaged 1.0 nM during spring 1994 and 0.23 nM in summer 1995, excluding two stations where concentrations exceeding 1 nM are attributed to inputs from melting sea ice. The observed differences in the distribution of iron and manganese between spring 1994 and summer 1995 are attributed to seasonal decreases in the up welling of bottom waters and melting of sea ice, which supply these metals into the upper water column, combined with the cumulative removal of iron and manganese from the water column throughout the spring and summer, due to biological uptake, vertical export and scavenging by suspended and sinking particles. Results of the metal addition bottle incubation experiments indicate that ambient dissolved iron concentrations are adequate for phytoplankton growth requirements during the spring and early summer, when algal production is highest and Phaeocystis antarctica dominates the algal community, whereas low dissolved iron concentrations limit algal community growth later in the summer, except in the stratified, iron-enriched waters near melting sea ice, where diatoms are able to bloom. Our observations and the inferred seasonal distributions of P. antarctica and diatoms in these waters suggest that iron availability and vertical mixing (i.e., irradiance) exert the primary controls on phytoplankton growth and community structure in the southern Ross Sea during the spring and summer.

Patterns of nutrient utilization and primary productivity (PP) in late austral spring and early summer in the southwestern Ross Sea were characterized with respect to phytoplankton taxonomic composition, polynya dynamics, and upper ocean hydrography during the 1996–1997 oceanographic program Research on Ocean-Atmosphere Variability and Ecosystem Response in the Ross Sea. Phytoplankton biomass in the upper 150 m of the water column ranged from 40 to 540 mg chlorophyll a (Chl a) m−2, exceeding 200 mg Chl a m−2 everywhere except the extreme northern and eastern boundaries of the Ross Sea polynya. Diatom biomass was greatest in the shallow mixed layers of Terra Nova Bay, while the more deeply mixed waters of the Ross Sea polynya were dominated by Phaeocystis antarctica. Daily production computed from the disappearance of NO3 (1.14 g C m−2 d−1) and total dissolved inorganic carbon (TDIC, 1.29 g C m−2 d−1) is consistent with estimates made from an algorithm forced with satellite measurements of Chl a (1.25 g C m−2 d−1) and from measurements of 14C uptake (1.33 g C m−2 d−1). Phytoplankton PP in the Ross Sea averaged 100 g C m−2 yr−1 during 1996–1997. Despite the early formation of the Terra Nova Bay polynya the diatom bloom there did not reach its peak PP until middle to late January 1997 (most likely because of more intense wind mixing in November), ∼6 weeks after the P. antarctica bloom in the Ross Sea polynya had reached the same stage of development. From 70 to 100% of the C and N deficits in the upper 150 m could be accounted for by particulate organic matter, indicating that there had been little dissolved organic matter production or export of particulate material prior to our cruise. This suggests that early in the season, PP and zooplankton grazing are decoupled in the southwestern Ross Sea. The NO3∶PO4 disappearance ratio in waters dominated by P. antarctica (19.0±0.61) was significantly greater than in waters where diatoms were most common (9.52±0.33), and both were significantly different from the Redfield N∶P ratio of 16. Vertical profiles of TDIC suggest that P. antarctica took up 110% more CO2 per mole of PO4 removed than did diatoms, an important consideration for climate models that estimate C uptake from the removal of PO4.

The Southern Ocean is very important for the potential sequestration of carbon dioxide in the oceans1 and is expected to be vulnerable to changes in carbon export forced by anthropogenic climate warming2. Annual phytoplankton blooms in seasonal ice zones are highly productive and are thought to contribute significantly to pCO2 drawdown in the Southern Ocean. Diatoms are assumed to be the most important phytoplankton class with respect to export production in the Southern Ocean; however, the colonial prymnesiophyte Phaeocystis antarctica regularly forms huge blooms in seasonal ice zones and coastal Antarctic waters3. There is little evidence regarding the fate of carbon produced by P. antarctica in the Southern Ocean, although remineralization in the upper water column has been proposed to be the main pathway in polar waters4,5. Here we present evidence for early and rapid carbon export from P. antarctica blooms to deep water and sediments in the Ross Sea. Carbon sequestration from P. antarctica blooms may influence the carbon cycle in the Southern Ocean, especially if projected climatic changes lead to an alteration in the structure of the phytoplankton community6,7.