Journal of Plankton Research, Vol 21, 1337-1353, Copyright © 1999 by Oxford University Press
Rates of NH4+ uptake, intracellular transformation and dissolved organic nitrogen release in two clones of marine Synechococcus spp
D Bronk
Department of Marine Sciences, School of Marine Programs, University of Georgia, Athens, GA 30602-3636, USA
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.