Inorganic Nitrogen Assimilation of Ditylum Brightwellii, a Marine Plankton Diatom¹,²

SUMMARY Apparent K m values for nitrite reductase, glutamic dehydrogenase, and nitrate reductase are of the order 10 −4 molar for nitrite, ammonia, and nitrate, respectively while half‐saturation constants for the corresponding uptake mechanisms approximate 10 −6 molar. Ammonium and nitrate are accumulated in the vacuolated cells of the diatom (about 10 and 40 mmoles/liter cell volume, respectively) and these intracellular pools serve as substrate for the assimilatory enzymes. Nitrite is either not accumulated or is concentrated, in a very small cellular compartment. Ammonium and nitrate in the external medium exert modifying effects on uptake and assimilatory mechanisms which can be distinguished from effects of the ions accumulated within the cells. Several of these effects are described and fitted into a general scheme of nitrogen assimilation by D. brightwellii .