Ammonium uptake and dinitrogen fixation by the unicellular nanocyanobacterium Crocosphaera watsonii in nitrogen‐limited continuous cultures

Abstract

Ammonium uptake and nitrogen (N) fixation of the unicellular nanocyanobacterium Crocosphaera watsonii isolated from the western subtropical North Pacific were determined in N‐limited continuous cultures. Six steady‐state growth rates ranging from 0.10 to 0.35 d−1, corresponding to 20–75% of the maximum growth rate, were established under saturating light. Unlike other larger diazotrophs, nitrogen fixation of C. watsonii was not inhibited by ambient ammonium ranging from < 3 to 59 nmol L−1, and nitrogen fixation did not vary consistently with dilution rate and ranged from 4.4 to 12.9 fmol N cell−1 d−1, with the highest rates at intermediate dilution rates. In contrast, ammonium uptake increased significantly with increasing dilution rates over the range of 10 to 80 fmol N cell−1 d−1 and contributed 65–95% to the daily cellular N requirement. The dissolved organic nitrogen (DON) excretion increased with increasing dilution rate; however, only a small portion of assimilated nitrogen was excreted as DON. In contrast, in ammonium‐free medium, where N assimilation occurred only by dinitrogen (N2) fixation, 60% of the fixed N was excreted. Interestingly, ammonium enrichment did not increase the growth rate of C. watsonii, but cellular contents of N, phosphorus, and chlorophyll a significantly increased for most dilution rates compared with cells grown in ammonium‐free medium. C. watsonii was capable of fixing N2 while taking up ammonium at environmentally relevant low concentrations of < 3 nmol L−1, and N2 fixation was independent of nanomolar concentrations. Therefore, C. watsonii can compete with nondiazotrophic phytoplankton for ammonium in oligotrophic subtropical gyres.