Light-dependent redistribution of ions in suspensions of chloroplast thylakoid membranes

Abstract

Ion movements associated with the pH rise that is observed upon illumination of thylakoid suspensions at low pH have been studied by a multiparameter technique. Light-dependent, dark-reversible fluxes of H(+), Cl(-), Na(+), K(+) and divalent cations were monitored, together with simultaneous changes in the optical density of the suspension. Extensive uptake of Cl(-) and efflux of Mg(2+) accompany the apparent inward movement of H(+) in the light. Only minor efflux of K(+) is seen and Na(+) appears immobile. The Cl(-) and Mg(2+) fluxes together compensate for most of the charge transferred as H(+), contributing respectively about 49% and 43% on an equivalent basis. The ratio of Cl(-) influx to Mg(2+) efflux is variable, but usually >1.0. The Mg(2+) flux can be supplanted by (1) K(+) flux, if the K(+)/Mg(2+) activity ratio in the suspension is high, and (2) Ca(2+) flux, if the thylakoids are equilibrated with suspending media containing Ca(2+). The affinity of the divalentcation-binding sites, or carrier mechanism, is greater for Ca(2+) than for Mg(2+). Schemes can be drawn up to account for the observed ion movements on the basis of either a chemical or a chemiosmotic mechanism for energy transduction in chloroplasts. In intact chloroplasts, light-dependent control of Mg(2+) distribution between thylakoid and stroma could serve to regulate enzyme activities in the carbon fixation pathway, and hence photosynthesis.