Rapid incorporation of 13N03 by NH4- limited phytoplankton

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Abstract
Nitrate reductase, the enzyme which catalyzes the reduction of nitrate to nitrite, is repressed by ammonium and induced by the presence of nitrate. In oligotrophic oceans, inorganic nitrogen concentrations are low and it is believed that phytoplankton primarily use regenerated ammonium. Under these conditions, nitrate reductase activity should be reduced. We investigated the metabolism of nitrate in ammonium-limited chemostats of marine phytoplankton using the short-lived radiotracer 13N. We found that nitrate is rapidly taken up, reduced and incorporated into protein by ammonium-limited phytoplankton due to the constitutive activity of nitrate reductase. These results provide a mechanism for the utilization of episodic pulses of nitrate, which have been suggested to be responsible for a significant fraction of nitrate-based production in the open ocean.