Rates of NH4+ uptake, intracellular transformation and dissolved organic nitrogen release in two clones of marine Synechococcus spp

Evidence suggests that phytoplankton can release significant amounts of dissolved organic carbon (DOC), particularly when they are nitrogen (N) deficient. To investigate this phenomenon from the perspective of dissolved organic N (DON), 15N tracer techniques were used to trace the flow of N through two clones of Synechococcus , WH7803 and WH8018, which were grown in batch culture under N-sufficient and N-deficient conditions. At various stages during the experiment, a subsample of the culture was removed and the response to a pulse of 15N-labeled NH4+ was measured with respect to rates of NH4+ uptake, intracellular NH4+ transformation, and release of total and low-molecular-weight (LMW; <10 000 Da) DON. In contrast to what is commonly observed for DOC, high rates of DON release were not observed during N starvation in either clone. The highest rates of DON release, both absolute and as a percentage of gross N uptake, occurred during N-sufficient growth, and release rates decreased by a factor of 4-7 when NH4+ was depleted in the medium.