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, 1827 (10), 1183-90

Acetate in Mixotrophic Growth Medium Affects Photosystem II in Chlamydomonas Reinhardtii and Protects Against Photoinhibition


Acetate in Mixotrophic Growth Medium Affects Photosystem II in Chlamydomonas Reinhardtii and Protects Against Photoinhibition

Thomas Roach et al. Biochim Biophys Acta.


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 (1)O2 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 (1)O2 originating from charge recombination events in photosystem II, which are governed by the midpoint potentials (Em) of the quinone electron acceptors. Thermoluminescence indicated that the Em of the primary quinone acceptor (QA/QA(-)) of mixotrophic cells was stabilised while the Em of the secondary quinone acceptor (QB/QB(-)) was destabilised, therefore favouring direct non-radiative charge recombination events that do not lead to (1)O2 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 QA and QB affecting photosystem II charge recombination events and photoinhibition.

Keywords: 2,2,6,6-tetramethyl-4-piperidone; 2,6-dichloro-1,4-benzoquinone; 3-(3,4-dichlorophenyl)-1,1-dimethylurea; Acetate; Chl; Chlamydomonas reinhardtii; DCBQ; DCMU; E(m); Electron paramagnetic resonance spectroscopy; Fm; Fv; HSM; LHC; P(680); PS; Pheo; Photoinhibition; Q(A); Q(B); S(2/3); Singlet oxygen; TAP; TEMPD; TL; Thermoluminescence; Tris–acetate–phosphate; Y(z); chlorophyll; high salt medium; light harvesting complex; maximum chlorophyll fluorescence signal; midpoint redox potential; oxidation states of the oxygen evolving complex of PSII; pheophytin, primary electron acceptor in PSII; photosystem; primary electron donor in PSII; redox active tyrosine residue of PSII; the primary quinone acceptor of PSII; the secondary quinone acceptor of PSII; thermoluminescence, T(m) TL emission maximum; variable chlorophyll fluorescence signal.

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