Sensitivity of N-2 fixation to combined nitrogen forms (NO3- and NH4+) in two strains of the marine diazotroph Crocosphaera watsonii (Cyanobacteria)

Despite the increasing recognition of the biogeochemical importance of marine unicellular diazotrophic cyanobacteria, the environmental factors controlling their distribution and activity in the ocean are still very poorly understood. In particular, very few studies have considered the effect of combined nitrogen (N) on unicellular diazotroph activity, although their presence has been reported in surface waters exhibiting micromolar concentrations of dissolved in organic nitrogen (DIN). Here, we studied the effect of nitrate (NO3-) and ammonium (NH4+) additions (0.2 to 10 mu mol l(-1)) on N-2 fixation and N uptake in 2 strains of Crocosphaera watsonii (WH8501 and WH0003) maintained in batch cultures in an N-free medium (YBC II) and a 12 h light: 12 h dark cycle. Compared with the untreated control, increasing concentrations of NH4+ to 10 mu mol l(-1) rapidly inhibited night-integrated N-2 fixation by up to 36 and 83% for WH8501 and WH0003, respectively. In contrast, night-integrated N-2 fixation was not significantly inhibited at any NO3- concentration. These results contrast with data obtained on Trichodesmium spp. (Cyanobacteria) in which night-integrated N-2 fixation is inhibited up to 50% at the same NO3- concentrations. This study indicates that C. watsonii is able to take up combined nitrogen while keeping its ability to fix dinitrogen at high rates.