Chloride dependence of photosystem II operation in the hypercarbonate-requiring cyanobacterium Limnospira maxima.

The role of chloride on the donor side of Photosystem II (PSII) is considered to be associated with proton removal. These protons are generated at the water oxidizing complex (WOC) during oxidation of water and transferred through specific “water channels” to the lumen. While structural studies have confirmed the presence of chloride at consistent sites near the WOC, the complete mechanism by which chloride ions influence PSII function and stability is not fully resolved. This work utilizes the hypercarbonate-requiring filamentous cyanobacterium Limnospira maxima to investigate these roles through bromide substitution, probing the interdependencies between halide ions and proton transfer in an unusually bicarbonate-rich environment. Bicarbonate has been posited to be a participant in the proton removal process, but also plays roles on the acceptor side of PSII. Low temperature (77 K) spectrofluorometry suggests that halide substitution results in reduced exciton transfer from the phycobilisome to the PSII reaction center. Chlorophyll fast repetition rate (FRR) fluorometry further reveals diminished PSII quantum efficiency under bromide conditions compared to the native chloride containing system. While the rates of electron transfer from QA− to QB were found to be faster in bromide substituted cells, P700 and plastocyanin (PC) redox kinetics reveal a shift in the baseline redox balance of the photosynthetic electron transport chain. Overall, these findings demonstrate that bromide substitution influences the efficiency of proton removal from the WOC despite the elevated bicarbonate concentration in Limnospira maxima, which in turn modulates the distribution of electrons across the photosynthetic apparatus.