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Articles

Sulfolipid Metabolism in Chlorella ¹

Department of Marine Biology, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California

When S-deficient cells of Chlorella cllipsoidea were incubated in radio-sulfate in light or in aerobic darkness for 1 hour, equal amounts of radioactivity were found in sulfolipid and glutathione but none was detected in sulfoquinovosyl glycerol which is one of the major S-compounds in this alga. No assimilation of radiosulfate was observed under anaerobic darkness.

To elucidate the function of sulfolipid in algal cells uniformly ³⁵S-labeled Chlorella cells were transferred to S-deficient culture medium or unlabeled normal culture medium and the changes of radioactivity in sulfolipid and the related compounds were followed. A) On incubating ³⁵S-labeled algal cells in S-deficient medium under photosynthetic conditions, the amounts of radioactivity in sulfate, sulfoquinovosyl glycerol and sulfolipid decreased rapidly. B) When ³⁵S-labeled cells were cultured photoautotrophically in unlabeled medium, no decrease of radioactivity was observed in sulfoquinovosyl glycerol and sulfolipid. C) A decrease of ³⁵S-sulfolipid and an increase of ³⁵S-sulfoquinovosyl glycerol were observed when the uniformly ³⁵S-labeled algal cells were illuminated in CO₂-free air.

When S-deficient Chlorella cells were incubated in ³⁵S-sulfolipid under photosynthetic conditions, significant radioactivity was found in the insoluble fraction of the cells. A similar result was observed when normal Chlorella cells were incubated in ¹⁴C-sulfolipid and CO₂-free air.

It is inferred from these observations that sulfolipid is a reservoir of sulfur and carbon compounds.

In order to ascertain if the sulfolipid is involved in the mechanism of photosynthetic oxygen evolution, the rate of photosynthesis was measured during the incubation of ³⁵S-labeled cells in a S-deficient medium. Parallelism was not observed between the rate of photosynthetic activity and the decrease of sulfolipid.

¹ This work was supported by The National Science Foundation.