Because maintenance of non-scalable cellular components-membranes and chromosomes-requires an increasing fraction of energy as cell size decreases, miniaturization comes at a considerable energetic cost for a phytoplanktonic cell. Consequently, if eukaryotes can use their superior energetic resources to acquire nutrients with more or even similar efficiency compared with prokaryotes, larger unicellular eukaryotes should be able to achieve higher growth rates than smaller cyanobacteria. Here, to test this hypothesis, we directly compare the intrinsic growth rates of phototrophic prokaryotes and eukaryotes from the equatorial to temperate South Atlantic using an original flow cytometric (CO2)-C-14-tracer approach. At the ocean basin scale, cyanobacteria double their biomass twice as frequently as the picoeukaryotes indicating that the prokaryotes are faster growing CO2 fixers, better adapted to phototrophic living in the oligotrophic open ocean-the most extensive biome on Earth.
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