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

SUMMARY

The aquatic fungus Blastocladiella emersonii, and a carotenoid-bearing mutant derived therefrom, grew more rapidly in the light than in the dark. The accelerated growth of B. emersonii was attributed to a direct effect of illumination not mediated by carotenoid. Experiments with labeled and unlabeled bicarbonate revealed that illumination induced a large increase in CO₂ fixation over dark fixation levels, and concomitantly a large increase in the labeled succinate and a decrease in the labeled ketoglutarate pools in the organism. Similarly, both labeled and unlabeled glucose were consumed more rapidly in the light than in the dark.

Cell-free preparations of B. emersonii 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 reactions were further accelerated by light.

The mechanism of the effect of illumination was tentatively interpreted in terms of a light-stimulated, cyclic process, the S. K. I. cycle. This involved carboxylation of ketoglutarate, via isocitric dehydrogenase and perhaps citritase, to succinate and oxalate, and the further carboxylation of some of the succinate to yield ketoglutarate once again.