<i>Crocosphaera</i>as a major consumer of fixed nitrogen despite its capability of nitrogen fixation

<jats:title>Abstract</jats:title><jats:p><jats:italic>Crocosphaera watsonii</jats:italic>(hereafter<jats:italic>Crocosphaera</jats:italic>) is a key nitrogen (N) fixer in the ocean, but its ability to consume combined N sources is still unclear. Using<jats:italic>in situ</jats:italic>microcosm incubations with an ecological model, we show that<jats:italic>Crocosphaera</jats:italic>has high competitive capability both under low and moderately high combined N concentrations. In field incubations,<jats:italic>Crocosphaera</jats:italic>accounted for the highest consumption of ammonium and nitrate, followed by pico-eukaryotes. The model analysis shows that cells have a high ammonium uptake rate (∼7 mol N (mol N)<jats:sup>-1</jats:sup>d<jats:sup>-1</jats:sup>at the maximum), which allows them to compete against pico-eukaryotes and non-diazotrophic cyanobacteria when combined N is sufficiently available. Even when combined N is depleted, their capability of nitrogen fixation allows higher growth rates compared to potential competitors. These results suggest the high fitness of<jats:italic>Crocosphaera</jats:italic>in combined N limiting, oligotrophic oceans, and thus heightens its potential significance in its ecosystem and in biogeochemical cycling.</jats:p>