Mechanistic insights into pH-dependent H(2) photoproduction in bisulfite-treated Chlamydomonas cells

BACKGROUND: Bisulfite addition is an important H(2) photoproduction strategy that removes O(2) and activates hydrogenase. The pH values of cell cultures can change the ratio of bisulfite to sulfite, which may affect H(2) photoproduction. However, little is known regarding the pH effect of bisulfite addition on H(2) photoproduction and relevant underlying mechanism. RESULTS: Here, changes in H(2) photoproduction with different initial extracellular pH values showed a parabolic distribution and a pH of 8.0 is an optimal value for H(2) photoproduction in Chlamydomonas reinhardtii cells treated with bisulfite. Compared to the growth pH (pH 7.3), increased photoproduction of H(2) at this optimal pH was primarily caused by a relatively high residual activity of photosystem II (PSII), which provides a relatively plentiful source of electrons for H(2) photoproduction. Such increased H(2) photoproduction was most likely a result of decreased the ratio of bisulfite to sulfite, consistent with the result that the toxicity of bisulfite on PSII was much more than that of sulfite. This possibility was corroborated by the result that treatment with a combination of 7 mM bisulfite and 6 mM sulfite further enhanced H(2) photoproduction compared with 13 mM bisulfite alone. CONCLUSIONS: Collectively, our findings provide novel mechanistic insights into pH-dependent H(2) photoproduction in C. reinhardtii cells treated with bisulfite, and demonstrate that sulfite addition is another important strategy for H(2) photoproduction, just like bisulfite addition.