Mechanistic insights into pH-dependent H₂ photoproduction in bisulfite-treated Chlamydomonas cells

Bisulfite addition is an important H₂ photoproduction strategy that removes O₂ and activates hydrogenase. The pH values of cell cultures can change the ratio of bisulfite to sulfite, which may affect H₂ photoproduction. However, little is known regarding the pH effect of bisulfite addition on H₂ photoproduction and relevant underlying mechanism.

Here, changes in H₂ photoproduction with different initial extracellular pH values showed a parabolic distribution and a pH of 8.0 is an optimal value for H₂ photoproduction in Chlamydomonas reinhardtii cells treated with bisulfite. Compared to the growth pH (pH 7.3), increased photoproduction of H₂ 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 H₂ photoproduction. Such increased H₂ 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 H₂ photoproduction compared with 13 mM bisulfite alone.

Collectively, our findings provide novel mechanistic insights into pH-dependent H₂ photoproduction in C. reinhardtii cells treated with bisulfite, and demonstrate that sulfite addition is another important strategy for H₂ photoproduction, just like bisulfite addition.