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Photosynthetic water oxidation at elevated dioxygen partial pressure monitored by time-resolved X-ray absorption measurements

The atmospheric dioxygen (O2) is produced at a tetramanganese complex bound to the proteins of photosystem II (PSII). To investigate product inhibition at elevated oxygen partial pressure (pO2 ranging from 0.2 to 16 bar), we monitored specifically the redox reactions of the Mn complex in its catalytic S-state cycle by rapid-scan and time-resolved X-ray absorption near-edge spectroscopy (XANES) at the Mn K-edge. By using a pressure cell for X-ray measurements after laser-flash excitation of PSII particles, we found a clear pO2 influence on the redox reactions of the Mn complex, with a similar half-effect pressure as determined (2–3 bar). However, XANES spectra and the time courses of the X-ray fluorescence collected with microsecond resolution suggested that the O2 evolution transition itself (S3⇒S0+O2) was not affected. Additional (nonstandard) oxidation of the Mn complex at high pO2 explains our experimental findings more readily. Our results suggest that photosynthesis at ambient conditions is not limited by product inhibition of the O2 formation step.