as a Major Consumer of Fixed Nitrogen.

Crocosphaera watsonii (hereafter referred to as ) is a key nitrogen (N) fixer in the ocean, but its ability to consume combined-N sources is still unclear. Using microcosm incubations with an ecological model, we show that has high competitive capability both under low and moderately high combined-N concentrations. In field incubations, accounted for the highest consumption of ammonium and nitrate, followed by picoeukaryotes. The model analysis shows that cells have a high ammonium uptake rate (~7 mol N [mol N] d at the maximum), which allows them to compete against picoeukaryotes and nondiazotrophic 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 in combined-N limiting, oligotrophic oceans heightening its potential significance in its ecosystem and in biogeochemical cycling. Crocosphaera watsonii is as a key nitrogen (N) supplier in marine ecosystems, and it has been estimated to contribute up to half of oceanic N fixation. Conversely, a recent study reported that can assimilate combined N and proposed that unicellular diazotrophs can be competitors with non-N fixing phytoplankton for combined N. Despite its importance in nitrogen cycling, the methods by which compete are not currently fully understood. Here, we present a new role of as a combined-N consumer: a competitor against nondiazotrophic phytoplankton for combined N. In this study, we combined microcosm experiments and an ecosystem model to quantitatively evaluate the combined-N consumption by and other non-N fixing phytoplankton. Our results suggest the high fitness of in combined-N limiting, oligotrophic oceans and, thus, heightens its potential significance in its ecosystem and in biogeochemical cycling.