Half-saturation constants for nitrate reductase and nitrate translocation in marine phytoplankton

Abstract

Half-saturation constants for the NO3− dependence of nitrate reductase in natural assemblages of marine phytoplankton ranged from 42 to 201 μM and averaged 82 μM. These values are too large to lend credence to a passive diffusion mechanism of NO3− translocation across the plasmalemma. On the contrary, they point to the necessity of a transport mechanism.