A Comparison of Hydrogen Photoproduction by Sulfur-Deprived Chlamydomonas reinhardtii Under Different Growth Conditions

Continuous photoproduction of H₂ by the green alga, Chlamydomonas reinhardtii, is observed after incubating the cultures for about a day in the absence of sulfate and in the presence of acetate. Sulfur deprivation causes the partial and reversible inactivation of photosynthetic O₂ evolution in algae, resulting in the light-induced establishment of anaerobic conditions in sealed photobioreactors, expression of two [FeFe]-hydrogenases in the cells, and H₂ photoproduction for several days. We have previously demonstrated that sulfur-deprived algal cultures can produce H₂ gas in the absence of acetate, when appropriate experimental protocols were used (Tsygankov, A.A., Kosourov, S.N., Tolstygina, I.V., Ghirardi, M.L., Seibert, M., 2006. Hydrogen production by sulfur-deprived Chlamydomonas reinhardtii under photoautotrophic conditions. Int. J. Hydrogen Energy 31, 1574-1584). We now report the use of an automated photobioreactor system to compare the effects of photoautotrophic, photoheterotrophic and photomixotrophic growth conditions on the kinetic parameters associated with the adaptation of the algal cells to sulfur deprivation and H₂ photoproduction. This was done under the experimental conditions outlined in the above reference, including controlled pH. From this comparison we show that both acetate and CO₂ are required for the most rapid inactivation of photosystem II and the highest yield of H₂ gas production. Although, the presence of acetate in the system is not critical for the process, H₂ photoproduction under photoautotrophic conditions can be increased by optimizing the conditions for high starch accumulation. These results suggest ways of engineering algae to improve H₂ production, which in turn may have a positive impact on the economics of applied systems for H₂ production.