Effect of ammonium on the regulation of nitrate assimilation in natural phytoplankton populations

Abstract

The effect of ammonium additions (0.5 to 10 μg-at. N · 1−1) on the nitrate reductase activity and on the nitrate uptake of natural marine phytoplankton populations was investigated. Specific nitrate reductase activity was significantly reduced at all ammonium concentrations after 24 h of the addition, but not after 6 h. Total nitrate reductase activity did not decrease even at the highest ammonium concentrations. From this it is concluded that the ammonium concentrations used during this study affected mostly the synthesis of new active enzyme. Nitrate uptake rates were also reduced by the addition of ammonium. In contrast to the nitrate reductase results, maximum inhibition was, however, observed after 6 h rather than after 24 h. The lag in the time of response between the inhibition after the ammonium addition suggests 1. (1) that NH4+ affects nitrate assimilation in two steps, first nitrate uptake and second nitrate reduction; and 2. (2) that the mechanisms controlling these processes are to a certain extent independent of each other