The biogeochemical properties of an extensive bloom (∼250,000 km2) of the coccolithophore, Emiliania huxleyi, in the north east Atlantic Ocean were investigated in June 1991. Satellite (NOAA‐AVHRR) imagery showed that the bloom was centered initially at 60°–63°N by 13°–28°W and lasted approximately 3 weeks. Spatial variations in satellite‐measured reflectance were well correlated with surface measurements of the beam attenuation coefficient, levels of particulate inorganic carbon, and coccolith density. Rates of both photosynthesis and calcification were typically relatively low within the coccolithophore‐rich waters, suggesting the population was in a late stage of development at the time of the field observations. Levels of dimethyl sulphide (DMS) in surface waters were high compared to average ocean values, with the greatest concentrations in localized areas characterized by relatively high rates of photosynthesis, calcification, and grazing by microzooplankton. The estimated spatially averaged flux of DMS to the atmosphere was 1122 nmol m−2 h−1, somewhat greater than that determined for the same region in June‐July 1987. Coccolith production (1 × 106 tonnes calcite‐C) had a significant impact on the state of the CO2 system, causing relative increases of up to 50 μatm in surface pCO2 in association with alkalinity and water temperature changes. Gradients in pCO2 were as great as 100 μatm over horizontal distances of 20–40 km. The environmental implications of these findings are discussed in relation to the spatial and temporal distributions of E. huxleyi.

Oligotrophic subtropical gyres are the largest oceanic ecosystems, covering >40% of the Earth's surface. Unicellular cyanobacteria and the smallest algae (plastidic protists) dominate CO 2 fixation in these ecosystems, competing for dissolved inorganic nutrients. Here we present direct evidence from the surface mixed layer of the subtropical gyres and adjacent equatorial and temperate regions of the Atlantic Ocean, collected on three Atlantic Meridional Transect cruises on consecutive years, that bacterioplankton are fed on by plastidic and aplastidic protists at comparable rates. Rates of bacterivory were similar in the light and dark. Furthermore, because of their higher abundance, it is the plastidic protists, rather than the aplastidic forms, that control bacterivory in these waters. These findings change our basic understanding of food web function in the open ocean, because plastidic protists should now be considered as the main bacterivores as well as the main CO 2 fixers in the oligotrophic gyres.

Direct evidence that marine cyanobacteria take up organic nitrogen compounds in situ at high rates is reported. About 33% of the total bacterioplankton turnover of amino acids, determined with a representative [ 35 S]methionine precursor and flow sorting, can be assigned to Prochlorococcus spp. and 3% can be assigned to Synechococcus spp. in the oligotrophic and mesotrophic parts of the Arabian Sea, respectively. This finding may provide a mechanism for Prochlorococcus' competitive dominance over both strictly autotrophic algae and other bacteria in oligotrophic regions sustained by nutrient remineralization via a microbial loop.Oxygenic phototrophic cyanobacteria (6,11,28) have been shown to dominate the tropical and subtropical regions of the world's oceans, thereby changing our conception of oceanic ecosystems. The cyanobacterial genus Prochlorococcus dominates phytoplankton in the central oceanic gyres, while Synechococcus spp. can become very abundant in nutrient-rich tropical regions (20,24). Why these organisms dominate such waters remains to be understood. To explain the ecological success of cyanobacteria in these waters, we propose the hypothesis that in these oceanic ecosystems, where nutrient regeneration within a microbial loop (1, 2) predominates over nitrate production, cyanobacteria can use not only the inorganic nutrients NH 4 ϩ , NO 3 Ϫ , and NO 2 Ϫ (16, 21) but also organic compounds containing reduced nitrogen.Although cyanobacteria are unable to incorporate some organic compounds, e.g., thymidine (10), Synechococcus axenic cultures utilize urea (7, 21) and amino acids (5, 19, 23) at a low rate and even show aminopeptidase activity (18). However, Synechococcus photoheterotrophy has always been considered ecologically unimportant. Current knowledge about Prochloroccocus nutrient assimilation (24) is limited, partly because of the existence of only one axenic Prochloroccocus culture (26). Interestingly, this strain (PCC9511) was isolated in the presence of an amino acid, methionine. It was reported that Prochlorococcus cultures take up NH 4 ϩ but that they cannot utilize NO 3 Ϫ or NO 2 Ϫ ; they can also use urea but not other sources of organic nitrogen (21, 26), although amino acids were not tested as a sole nitrogen source.Genomic data from the draft annotations of the MIT9313 and MED4 strains of Prochlorococcus spp. show that they possess several transporter systems for amino acids, as shown on the Department of Energy Joint Genome Institute website (http://www.jgi.doe.gov); however, a further analysis is necessary for the prediction of possible utilization pathways. The genome of the MED4 strain lacks the nitrate and nitrite reductase genes, a further proof that this strain needs to rely on reduced nitrogen compounds, e.g., NH 4 ϩ , amino acids, etc. In working with laboratory cultures of cyanobacteria, it is very challenging to simulate oceanic oligotrophic conditions and the results of laboratory nutrient addition experiments can be extrapolated only with considerable caution. On the othe...

The algal osmolyte, dimethylsulphoniopropionate (DMSP), is abundant in the surface oceans and is the major precursor of dimethyl sulphide (DMS), a gas involved in global climate regulation. Here, we report results from an in situ Lagrangian study that suggests a link between the microbially driven fluxes of dissolved DMSP (DMSPd) and specific members of the bacterioplankton community in a North Sea coccolithophore bloom. The bacterial population in the bloom was dominated by a single species related to the genus Roseobacter, which accounted for 24% of the bacterioplankton numbers and up to 50% of the biomass. The abundance of the Roseobacter cells showed significant paired correlation with DMSPd consumption and bacterioplankton production, whereas abundances of other bacteria did not. Consumed DMSPd (28 nM day(-1)) contributed 95% of the sulphur and up to 15% of the carbon demand of the total bacterial populations, suggesting the importance of DMSP as a substrate for the Roseobacter-dominated bacterioplankton. In dominating DMSPd flux, the Roseobacter species may exert a major control on DMS production. DMSPd turnover rate was 10 times that of DMS (2.7 nM day(-1)), indicating that DMSPd was probably the major source of DMS, but that most of the DMSPd was metabolized without DMS production. Our study suggests that single species of bacterioplankton may at times be important in metabolizing DMSP and regulating the generation of DMS in the sea.

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