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

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Abstract
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.