Effect of calcium ion on Hydrilla verticillata thylakoid membrane O2 evolution

Calcium ion-dependent reactivation of O2 evolution activity has been investigated in Hydrilla verticillata thylakoid preparations. Washing the thylakoids in calcium-free buffer or calcium-free buffer containing 1.5 M NaCl or 1.5 M NaCl plus 20% methanol, reversibly inhibited O2 evolution activity. The activity was restored on addition of calcium as calcium chloride and partially by strontium chloride. Immobilization of thylakoids with glutaraldehyde (GA) arrested the loss in O2 evolution activity caused by calcium-free high salt washing. However, calcium sensitivity was discernible in GA immobilized thylakoids subjected to calcium-free high salt washing. Since glutaraldehyde checks the loss of extrinsic thylakoid polypeptides due to washing, it is assumed that the calcium ion has regulatory functions in the photosynthetic electron transport, besides its interaction with thylakoid proteins.