Hydrogen photoproduction under continuous illumination by sulfur-deprived, synchronous Chlamydomonas reinhardtii cultures

Unsynchronized Chlamydomonas reinhardtii cells subsequently deprived of sulfur produce H2 under continuous illumination in the laboratory for 3–4 days. However, cultures grown outdoors will be exposed to day-and-night cycles that may synchronize their growth and cell division. While it is clear that only insignificant amounts of H2 can be produced by sulfur-deprived cells during the night period, little work has been done to examine the effects of the light/dark cycles preceding sulfur deprivation on subsequent H2 photoproduction. We show that (a) C. reinhardtii cells exhibit synchronized growth and cell division in the presence of acetate, (b) cells with the highest specific rates of H2 photoproduction also have the highest rates of biomass accumulation, and (c) the highest rates of starch and protein degradation coincide with the highest rates of formate and acetate accumulation, but not with H2 photoproduction. This work shows that it is possible to maximize the production of H2 by sulfur-depriving synchronized cultures at about 4 h after the beginning of the light period.