Acetate in mixotrophic growth medium affects photosystem II in Chlamydomonas reinhardtii and protects against photoinhibition

Chlamydomonas reinhardtii is a photoautotrophic green alga, which can be grown mixotrophically in acetate-supplemented media (Tris-acetate-phosphate). We show that acetate has a direct effect on photosystem II (PSII). As a consequence, Tris-acetate-phosphate-grown mixotrophic C. reinhardtii cultures are less susceptible to photoinhibition than photoautotrophic cultures when subjected to high light. Spin-trapping electron paramagnetic resonance spectroscopy showed that thylakoids from mixotrophic C. reinhardtii produced less O-1(2) than those from photoautotrophic cultures. The same was observed in vivo by measuring DanePy oxalate fluorescence quenching. Photoinhibition can be induced by the production of O-1(2) originating from charge recombination events in photosystem II, which are governed by the midpoint potentials (E-m) of the quinone electron acceptors. Thermoluminescence indicated that the E-m of the primary quinone acceptor (Q(A)/Q(A)(-)) of mixotrophic cells was stabilised while the E-m of the secondary quinone acceptor (Q(B)/Q(B)(-)) was destabilised, therefore favouring direct non-radiative charge recombination events that do not lead to O-1(2) production. Acetate treatment of photosystem II-enriched membrane fragments from spinach led to the same thermoluminescence shifts as observed in C. reinhardtii, showing that acetate exhibits a direct effect on photosystem II independent from the metabolic state of a cell. A change in the environment of the non-heme iron of acetate-treated photosystem II particles was detected by low temperature electron paramagnetic resonance spectroscopy. We hypothesise that acetate replaces the bicarbonate associated to the non-heme iron and changes the environment of Q(A) and Q(B) affecting photosystem II charge recombination events and photoinhibition. (C) 2013 Elsevier B.V. All rights reserved.