The interaction between ammonium and nitrate uptake in phytoplankton

Abstract

A thorough review of field studies shows that the reduction of nitrate uptake rate in the presence of ammonium is rarely so severe, and that it is a highly variable phenomenon. To simplify quantification of the interaction between nitrate and ammonium uptake, it is proposed that it be divided into an indirect interaction, preference, and a direct effect, inhibition. In order to determine preference and inhibition it is necessary to measure uptake of each inorganic nitrogen source alone and in the presence of increasing concentrations of the other nitrogen source. Preference for ammonium uptake is manifested primarily in a higher V sub(max) and lower K sub(s) for ammonium uptake than for nitrate uptake and is accentuated by low light and low nitrogen availability. However, although ammonium is the preferred nitrogen source for uptake,