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Articles

Utilization of Exogenous Inorganic Carbon Species in Photosynthesis by Chlorella pyrenoidosa¹

Department of Biology, Queen's University, Kingston, Ontario K7L 3N6 Canada

The nature of the inorganic carbon utilized during photosynthesis by Chlorella pyrenoidosa was investigated using three experimental techniques (open gas analysis system with "artificial leaf" or "aqueous" chambers and O₂ electrode system) to measure carbon assimilation. Photosynthesis was studied as a function of pH and CO₂ concentration. The CO₂ concentration was inadequate to meet the requirements of photosynthesis only when HCO₃^– was added at high pH. Under all other conditions, the low and constant K_m (CO₂), in contrast to the highly variable K_m (HCO₃^–), suggested that CO₂ was the major species utilized.

Higher rates of photosynthesis were observed under limiting CO₂ conditions above pH 7.5 but rates of hydration of CO₂ were not consistent with the view that the stimulation in photosynthesis was supported by HCO₃^–. In the same pH region lower rates of photosynthesis were observed under saturating CO₂ conditions. These conflicting changes seemed not to be related to pH but to some as yet undetermined effect of bicarbonate. No support was obtained for the view that the quantum efficiency was different under conditions where CO₂ was assimilated compared to conditions where CO₂ and HCO₃^– could be assimilated, although at saturating CO₂, lower maximal rates of photosynthesis were observed in the latter conditions.