Storage of light-driven transthylakoid proton motive force as an electric field (Δψ) under steady-state conditions in intact cells of Chlamydomonasreinhardtii

Proton motive force (pmf) is physiologically stored as either a Delta pH or a membrane potential (Delta psi) across bacterial and mitochondrial energetic membranes. In the case of chloroplasts, previous work (Cruz et al. 2001, Biochemistry 40: 1226 - 1237) indicates that Delta psi is a significant fraction of pmf, in vivo, and in vitro as long as the activities of counterions are relatively low. Kinetic analysis of light-induced changes in the electrochromic shift (ECS) in intact leaves was consistent with these observations. In this work, we took advantage of the spectroscopic properties of the green alga, Chlamydomonas reinhardtii, to demonstrate that light-driven Delta psi was stored in vivo over the hours time scale. Analysis of the light-induced ECS kinetics suggested that the steady-state Delta psi in 400 mu mol photons m(-2) s(-1) red light was between 20 and 90 mV and that this represented about 60% of the light-induced increase in pmf. By extrapolation, it was surmised that about half of total (basal and light-induced) pmf is held as Delta psi. It is hypothesized that Delta psi is stabilized either by maintaining low chloroplast ionic strength or by active membrane ion transporters. In addition to the strong implications for regulation of photosynthesis by the xanthophyll cycle, these results imply that pmf partitioning is important across a wide range of species.