Calcium and photosynthetic oxygen evolution in cyanobacteria

Calcium activation of oxygen evolution from French-press preparations of Phormidium luridum is largely reversible upon removal of added Ca²⁺. Activation occurs via a first-order binding with a dissociation constant of 2.8 mM. An 8-fold increase in oxygen evolution rate observed upon Ca²⁺ addition is accounted for by a 4-fold increase in the number of active photosynthetic units, and a doubling of turnover rate. While both Ca²⁺ and Mg²⁺ stimulate turnover, unit activation is Ca²⁺ specific. Under optimal conditions, 30% of the units functioning in the intact cell can be recovered in the Ca²⁺-activated preparation.

The Ca²⁺ requirement of P. luridum preparations is not relieved by proton-carrying uncouplers, or by rate-saturating concentrations of the Hill acceptor, ferricyanide. Taken together with the reported stimulation by Ca²⁺ of oxygen evolution in the presence of DCMU (Piccioni, R.G. and Mauzerall, D.C. (1976) Biochim. Biophys. Acta 423, 605–609) these observations strongly suggest a site of Ca²⁺ action within Photosystem II.

The pronounced specificity of the Ca²⁺ requirement appears in preparations of other cyanobacteria (Anabaena flos-aquae and Anacystis nidulans) but not in the eucaryote Chlorella vulgaris. While milder cell-disruption methods bring about some Ca²⁺ dependence in P. luridum, French-press treatment is required for maximal expression of Ca²⁺-specific effects. French-press breakage causes a release of endogenous Ca²⁺ from cells, supporting the view that added Ca²⁺ restores oxygen evolution by satisfying a physiological requirement for the cation.