Demonstration of Both a Photosynthetic and a Nonphotosynthetic CO(2) Requirement for NH(4) Assimilation in the Green Alga Selenastrum minutum.

Nitrogen-limited and nitrogen-sufficient cell cultures of Selenastrum minutum (Naeg.) Collins (Chlorophyta) were used to investigate the dependence of NH(4) (+) assimilation on exogenous CO(2). N-sufficient cells were only able to assimilate NH(4) (+) maximally in the presence of CO(2) and light. Inhibition of photosynthesis with 3-(3,4-dichlorophenyl)-1,1-dimethylurea, diuron also inhibited NH(4) (+) assimilation. These results indicate that NH(4) (+) assimilation by N-sufficient cells exhibited a strict requirement for photosynthetic CO(2) fixation. N-limited cells assimilated NH(4) (+) both in the dark and in the light in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, diuron, indicating that photosynthetic CO(2) fixation was not required for NH(4) (+) assimilation. Using CO(2) removal techniques reported previously in the literature, we were unable to demonstrate CO(2)-dependent NH(4) (+) assimilation in N-limited cells. However, employing more stringent CO(2) removal techniques we were able to show a CO(2) dependence of NH(4) (+) assimilation in both the light and dark, which was independent of photosynthesis. The results indicate two independent CO(2) requirements for NH(4) (+) assimilation. The first is as a substrate for photosynthetic CO(2) fixation, whereas the second is a nonphoto-synthetic requirement, presumably as a substrate for the anaplerotic reaction catalyzed by phosphoenolpyruvate carboxylase.