The Stimulatory Effect of Light upon Growth and CO2 Fixation in Blastocladiella. I. The S. K. I. Cycle

Abstract

The aquatic fungus Blastocladiella emersonii and a carotenoid-bearing mutant derived therefrom grow more rapidly in the light than in the dark. The accelerated growth of the fungus was attributed to a direct effect of illumination. Experiments with labeled and unlabeled bicarbonate showed that illumination induced a large increase in CO2 fixation over dark fixation levels, and concomitantly a large increase in the labeled suc-cinate and a decrease in the labeled ketoglutarate pools in the organism. Also, both labeled and unlabeled glucose were consumed more rapidly in the light than in the dark. Cell-free preparations of the fungus mediated an enzymatic, TPN-dependent oxidation of isocitrate which was inhibited by bicarbonate and light. These same preparations mediated an enzymatic oxidation of reduced TPN which was accelerated by ketoglutarate and bicarbonate; simultaneously, ketoglutarate was carboxylated. These latter reactions were accelerated by light. The mechanism of the effect of illumination was interpreted in terms of a light-stimulated, cyclic process, the S. K. I. (succinate-ketoglutarate-isocitrate) cycle. This involved carboxylation of ketoglutarate, via isocitric dehydrogenase, and conversion of isocitrate to succinate and oxalate, and the further carboxylation of succinate to yield ketoglutarate once again.