Photoinhibition in Spirulina platensis: Response of Photosynthesis and HCO3- Uptake Capability to CO2 Depleted Conditions

The apparent photosynthetic affinity-for inorganic carbon inSpirulina is severely reduced by exposing the cells to CO₂-depletedconditions in the light. The extent of reduction depends onthe 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 presenceof catalase. The amount of O₂ released following the additionof catalase to the cell suspension also depends on the O₂ andCO₂ concentrations. The addition of H₂O₂ either to the cellsuspension or by a treatment with methyl viologen caused a severeinhibition of the capability of the cells to accumulate inorganiccarbon internally. It is suggested that the reduction of thephotosynthetic apparent affinity to C_Inorg upon subjecting thecells to photoinhibitory conditions is caused by an alterationof their ability to accumulate C_Inorg due to the accumulationof H₂O₂.