Dissolved organic nitrogen and carbon release by a marine unicellular diazotrophic cyanobacterium

Dinitrogen (N 2 ) fixation rates may be underestimated when recently fixed N 2 is released as dissolved organic nitrogen (DON). DON release (DONr) is substantial in the filamentous cyanobacterium Trichodesmium but has never been reported in unicellular diazotrophic cyanobacteria. We used axenic cultures of the marine unicellular diazotroph Cyanothece sp. Miami BG 043511 to measure dissolved organic matter release under N 2 -fixing conditions. DONr was measured as the transfer of 15 N 2 from the culture medium to the extracellular DON pool. On average, the DON released represented ~1% of the total N 2 fixed. The average release of dissolved organic carbon, as determined by 14 C, represented ~2% of the total carbon fixed. These results suggest that cultured populations of unicellular diazotrophs do not release much dissolved organic matter, but it cannot be excluded that DONr is important in the field when grazers and bacteria are present, or when the organism is exposed to environmental stresses such as turbulence, excess light, temperature changes, or nutrient limitation.