THE REGULATION OF PHOTOSYNTHETIC ELECTRON-TRANSPORT DURING NUTRIENT DEPRIVATION IN CHLAMYDOMONAS-REINHARDTII

The light-saturated rate of photosynthetic O-2 evolution in Chlamydomo nas reinhardtii declined by approximately 75% on a per-cell basis afte r 4 d of P starvation or 1 d of S starvation. Quantitation of the part ial reactions of photosynthetic electron transport demonstrated that t he light-saturated rate of photosystem (PS) I activity was unaffected by P or S limitation, whereas light-saturated PSII activity was reduce d by more than 50%. This decline in PSII activity correlated with a de cline in both the maximal quantum efficiency of PSII and the accumulat ion of the secondary quinone electron acceptor of PSII nonreducing cen ters (PSII centers capable of performing a charge separation but unabl e to reduce the plastoquinone pool). In addition to a decline in the l ight-saturated rate of O-2 evolution, there was reduced efficiency of excitation energy transfer to the reaction centers of PSII (because of dissipation of absorbed tight energy as heat and because of a transit ion to state 2). These findings establish a common suite of alteration s in photosynthetic electron transport that results in decreased linea r electron flow when C. reinhardtii is limited for either P or S. It w as interesting that the decline in the maximum quantum efficiency of P SII and the accumulation of the secondary quinone electron acceptor of PSII nonreducing centers were regulated specifically during S-limited growth by the Sad gene product, which was previously shown to be crit ical for the acclimation of C. reinhardtii to S limitation (J.P. Davie s, F.H. Yildiz, and A.R. Grossman [1996] EMBO J 15:2150-2159).