Light‐dependent Decline of Nh4+ Assimilation Under Co2 Deprivation in N‐limited Chlorella Sorokiniana (Chlorophyceae)1

Cells of Chlorella sorokiniana Shihira & Krauss 211/8k, grown in a chemostat under conditions of nitrate limitation, assimilated ammonium in light (307 ± 12 μmol · mL packed cell volume (pcv)−1· h−1) and darkness (305 ± 30 μmol · mL pcv−1· h−1) at similar rates when supplied with air containing 5% CO2. Ammonium assimilation in cell suspensions flushed with pure nitrogen to establish anaerobic conditions was almost totally prevented both in darkness and in light. Cell suspensions flushed with CO2−‐free air and grown in the dark assimilated ammonium at a rate of 314 ± 23 μmol · mL pcv−1· h−1. Light assimilation was 128 ± 10 μmol · mL pcv−1· h−1, a rate which was only 41% of that in darkness. Resupply of ammonium to N‐limited chemostat cells resulted in a 3.5‐fold stimulation of dark respiration and a suppression of photosynthetic O2 evolution. We hypothesize that the partial decline of ammonium assimilation in Chlorella 211/8k in the light under conditions of limited CO2 supply is due to the fact that the photosynthetic carboxylation reaction competes with the dark carboxylation reaction for the available CO2, and that the production of C4 and C5 precursors foramina acid biosynthesis will be slower than in the dark. Alternatively we suggest that an increase in the concentration of photogenerated compound(s) not used for CO2 assimilation attains an intracellular level suitable to trigger a control mechanism operating at the level of starch break‐down or glycolysis.