LIGHT/DARK PERIODICITY IN NITROGEN ASSIMILATION OF THE MARINE PHYTOPLANKTERS SKELETONEMA COSTATUM AND COCCOLITHUS HUXLEYI IN N‐LIMITED CHEMOSTAT CULTURE

Abstract

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. Copyright © 1971, Wiley Blackwell. All rights reserved