Photoinhibition in Spirulina platensis: Response of photosynthesis and HCO^-₃ uptake capability to CO₂ depleted conditions

The apparent photosynthetic affinity-for inorganic carbon in Spirulina is severely reduced by exposing the cells to CO₂-depleted conditions in the light. The extent of reduction depends on the length of exposure, the light intensity, and the O₂ concentration. The photosynthetic dependence on inorganic carbon (C_Inorg) concentration, however, is similar at 3% O₂ and at 25% O₂ but in the presence of catalase. The amount of O₂ released following the addition of catalase to the cell suspension also depends on the O₂ and CO₂ concentrations. The addition of H₂O₂ either to the cell suspension or by a treatment with methyl viologen caused a severe inhibition of the capability of the cells to accumulate inorganic carbon internally. It is suggested that the reduction of the photosynthetic apparent affinity to C_Inorg upon subjecting the cells to photoinhibitory conditions is caused by an alteration of their ability to accumulate C_Inorg due to the accumulation of H₂O₂.