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Metabolism and Enzymology

Carbon Oxysulfide Inhibition of the CO₂-Concentrating Process of Unicellular Green Algae ¹

Department of Biochemistry, Michigan State University, East Lansing, Michigan 48824

Carbonyl sulfide (COS), a substrate for carbonic anhydrase, inhibited alkalization of the medium, O₂ evolution, dissolved inorganic carbon accumulation, and photosynthetic CO₂ fixation at pH 7 or higher by five species of unicellular green algae that had been air-adapted for forming a CO₂-concentrating process. This COS inhibition can be attributed to inhibition of external HCO₃^– conversion to CO₂ and OH^– by the carbonic anhydrase component of an active CO₂ pump. At a low pH of 5 to 6, COS stimulated O₂ evolution during photosynthesis by algae with low CO₂ in the media without alkalization of the media. This is attributed to some COS hydrolysis by carbonic anhydrase to CO₂. Although COS had less effect on HCO₃^– accumulation at pH 9 by a HCO₃^– pump in Scenedesmus, COS reduced O₂ evolution probably by inhibiting internal carbonic anhydrases. Because COS is hydrolyzed to CO₂ and H₂S, its inhibition of the CO₂ pump activity and photosynthesis is not accurate, when measured by O₂ evolution, by NaH¹⁴CO₃ accumulation, or by ¹⁴CO₂ fixation.

¹ Supported in part by the Michigan State University Agricultural Experiment Station. Y.S. was supported by the Japan-United States Cooperative Photoconversion and Photosynthesis Research Program.