The Photometabolism of Acetate byChlorella pyrenoidosa

Abstract

Chlorella pyrenoidosa can utilize sodium acetate as a carbon source for growth in the light. Growth proceeds under aerobic conditions both in the presence and in the absence of carbon dioxide, but under anaerobic conditions only in its presence. The assimilation of acetate does not result from oxidation to carbon dioxide followed by photosynthetic fixation because the products of ¹⁴C-acetate assimilation are different from the products of ¹⁴CO₂ fixation in the presence of unlabelled acetate. In aerobic conditions 10-⁶ M DCMU induces a pattern of acetate assimilation in the light similar to that in the dark. Thus, in the presence of DCMU in the light, less acetate carbon is incorporated into cells, particularly into lipids, polysaccharide, and protein, and more is released as carbon dioxide than in its absence. The effect of 4 × 10^-3 M MFA on acetate assimilation in the presence of 10^-6 M DCMU is the same in light and dark. Acetate assimilation is unaffected by desaspidine and sodium bisulphite. The mean generation time of C. pyrenoidosa growing on acetate in the light under aerobic conditions is 20 hours. When 10^-5 M DCMU is added the mean generation time is 60 hours, the same as that for Chlorella growing on acetate in the dark. The activity of the enzymes of the glyoxylate cycle, isocitrate lyase (E.C.4.1.3.1.) and malate synthetase (E.C.4.1.3.2.) is repressed in the light, but activity of both enzymes increases markedly when DCMU is added.