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

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 (N-2) 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 N-2 while taking up ammonium at environmentally relevant low concentrations of < 3 nmol L--1,L- and N-2 fixation was independent of nanomolar NH4+ concentrations. Therefore, C. watsonii can compete with nondiazotrophic phytoplankton for ammonium in oligotrophic subtropical gyres.