Uptake rate of nitrate and ammonium was studied as a function of nitrate or ammonium concentration with cultures of 16 species of marine phytoplankton.Half-saturation constants (the concentration supporting an uptake rate one-half the maximum rate) were computed as a measure of the ability of a species to use low levels of nitrate and ammonium.

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The filtering rates of Calanus helgolundicus were measured on synchronously growing populations of the diatom Ditylum brightwellii, by following changes in cell numbers and also by measuring the ingestion of l"C labeled cells at selected intervals during 2-day periods.

SUMMARY Skeletonema costatum and Coccolithus huxleyi were grown in nitrogen‐limited chemostat cultures with illumination provided in light/dark cycles. S. costatum assimilated nitrate and ammonium primarily during the day and less so at night. Conversely, the concentration of nitrate and ammonium in the culture medium varied periodically, increasing at night and decreasing in the light. C. huxleyi assimilated both N sources at a rate sufficient to keep them at very low levels both day and night. However, the activity of N‐assimilating enzymes, measured in cell‐free extracts, were higher in the light than in the dark periods, implying light/dark differences in the capacity to assimilate nitrogen. Such periodicity in the rate of uptake and enzymatic activity appears to complicate the mathematical expression of nutrient‐limited growth of phytoplankton exposed to natural light/dark cycles. Three aspects of dial periodicity in N assimilation have been observed in natural phytoplankton communities in the sea: (1) in assimilation rate, (2) in activity of enzymes of N‐assimilation, and (3) in the ammonium concentration of the seawater. The cultures also showed periodicity in these parameters and appear to be useful model systems for study.

SUMMARY Apparent Km 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.