Characterization of the water oxidizing complex of photosystem II of the Chl d-containing cyanobacterium Acaryochloris marina via its reactivity towards endogenous electron donors and acceptors

Abstract

Acaroychloris (A.) marina is a unique oxygen evolving organism that contains a large amount of chlorophyll d (Chl d) and only very few Chl a molecules. This feature raises questions on the nature of the photoactive pigment, which supports light-induced oxidative water splitting in Photosystem II (PS II). In this study, flash-induced oxygen evolution patterns (FIOPs) were measured to address the question whether the S_i state transition probabilities and/or the redox-potentials of the water oxidizing complex (WOC) in its different S_i states are altered in A. marina cells compared to that of spinach thylakoids. The analysis of the obtained data within the framework of different versions of the Kok model reveals that in light activated A. marina cells the miss probability is similar compared to spinach thylakoids. This finding indicates that the redox-potentials and kinetics within the WOC, of the reaction center (P680) and of Y_Z are virtually the same for both organisms. This conclusion is strongly supported by lifetime measurements of the S₂ and S₃ states. Virtually identical time constants were obtained for the slow phase of deactivation. Kinetic differences in the fast phase of S₂ and S₃ decay between A. marina cells and spinach thylakoids reflect a shift of the E_m of Y_D/Y^ox_D to lower values in the former compared to the latter organisms, as revealed by the observation of an opposite change in the kinetics of S₀ oxidation to S₁ by Y^ox_D. A slightly increased double hit probability in A. marina cells is indicative of a faster Q_A⁻ to Q_B electron transfer in these cells compared to spinach thylakoids.