Photosynthetic Response to Alkaline pH in Anabaena variabilis

The rate of O₂ evolution and alkalization of the medium in low CO_2⁻ grown Anabaena variabilis was observed as affected by the pH in the medium. Both rates are severely inhibited by pH values higher than 9.5, but the latter is more sensitive to this treatment. This finding, as well as the lag observed in alkalization of the medium, but not in O₂ evolution, following the addition of HCO₃⁻ indicates that the transport of HCO₃ and OH⁻ (or H⁺) are not compulsorily coupled. The inhibition of photosynthesis by strongly alkaline pH is attributed to an alteration of the internal pH and, hence, the rate of carboxylation. This conclusion is supported by data showing that the rate of O₂ evolution is affected by pH more strongly at saturating [HCO₃⁻] than at limiting [HCO₃⁻]. Also, the rate of O₂ evolution at saturating light intensity is affected by pH more strongly than is the initial slope of the curve against light intensity or the rate of dark respiration.