Journal Article<i>Crocosphaera</i> as a Major Consumer of Fixed NitrogenMasuda, Takako; Inomura, Keisuke; Kodama, Taketoshi; Shiozaki, Takuhei; Kitajima, Satoshi; Armin, Gabrielle; Matsui, Takato; Suzuki, Koji; Takeda, Shigenobu; Sato, Mitsuhide; Prášil, Ondřej; Furuya, Ken; Clare, Matthew; Alleman, AlexanderGralnick, Jeffrey A.ABSTRACT Crocosphaera watsonii (hereafter referred to as Crocosphaera) is a key nitrogen (N) fixer in the ocean, but its ability to consume combined-N sources is still unclear. Using in situ microcosm incubations with an ecological model, we show that Crocosphaera has high competitive capability both under low and moderately high combined-N concentrations. In field incubations, Crocosphaera 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]^−1 d^−1 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 Crocosphaera in combined-N limiting, oligotrophic oceans heightening its potential significance in its ecosystem and in biogeochemical cycling.2022-06-3010340605Microbiology Spectrum2165-0497https://doi.org/10.1128/spectrum.02177-211655221National Science Foundation