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

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...

Siderophores are chelates produced by bacteria as part of a highly specific iron uptake mechanism. They are thought to be important in the bacterial acquisition of iron in seawater and to influence iron biogeochemistry in the ocean. We have identified and quantified two types of siderophores in seawater samples collected from the Atlantic Ocean. These siderophores were identified as hydroxamate siderophores, both ferrioxamine species representative of the more soluble marine siderophores characterized to date. Ferrioxamine G was widely distributed in surface waters throughout the Atlantic Ocean, while ferrioxamine E had a more varied distribution. Total concentrations of the two siderophores were between 3 and 20 pM in the euphotic zone. If these compounds are fully complexed in seawater, they represent approximately 0.2-4.6% of the <0.2 microm iron pool. Our data confirm that siderophore-mediated iron acquisition is important for marine heterotrophic bacteria and indicate that siderophores play an important role in the oceanic biogeochemical cycling of iron.

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