Response of marine phytoplankton to nitrogen deficiency: Decreased nitrate uptake vs enhanced ammonium uptake

The uptake of nitrate and ammonium was measured separately in uni-algal, nitrogen-deficient cultures of four species of marine phytoplankton. Nitrogen-deficient phytoplankton took up ammonium at initial rates which greatly exceeded those measured for nitrogen-sufficient phytoplankton. However, nitrate uptake by nitrogendeficient cultures was generally much slower than either nitrate or ammonium uptake by nitrogen-sufficient cultures or ammonium uptake by nitrogen-deficient cultures. Considerable species differences were observed in the degree to which nitrogen deficiency increased ammonium uptake or decreased nitrate uptake. Loss of ability to take up nitrate, but enhanced ability to take up ammonium, as a result of nitrogen deficiency may be an adaptation to the different mechanisms by which nitrate and ammonium are supplied to the euphotic zone. In areas with an intermittent supply of nitrogen, changes in the ability of some species to take up nitrogen as a result of nitrogen starvation will influence species composition and complicate interpretations of measurements of nitrogen uptake.Contribution no. 1249 from the Department of Oceanography, University of Washington, and contribution no. 82006 from the Bigelow Laboratory for Ocean Sciences