Nitrate reductase activity as a function of in situ nitrate uptake and environmental factors of euphotic zone profiles

Abstract

The activity of nitrate reductase in euphotic zone profiles from several oceanic areas has been compared with in situ nitrate uptake rates estimated by the 15N technique. There were marked variations in the ratio of uptake to reduction and these have been related to the available light intensity, nitrate concentration, and the absolute value of nitrate uptake. It is shown that uptakereduction ratios greater than one are not due to dephasing between the two processes and that accumulation of nitrate inside the cells cannot account for the discrepancy. Nitrate uptake depends primarily on the external nitrate concentration, while nitrate reductase activity seems to be controlled by the intracellular nitrate level. The importance of those results as regards the significance of the nitrate reductase assay as an index of nitrate assimilation by marine phytoplankton is discussed.