Sustained hydrogen photoproduction by Chlamydomonas reinhardtii: Effects of culture parameters

Abstract

The green alga, Chlamydomonas reinhardtii, is capable of sustained H 2 photoproduction when grown under sulfur‐deprived conditions. This phenomenon is a result of the partial deactivation of photosynthetic O 2‐evolution activity in response to sulfur deprivation. At these reduced rates of water‐oxidation, oxidative respiration under continuous illumination can establish an anaerobic environment in the culture. After 10–15 hours of anaerobiosis, sulfur‐deprived algal cells induce a reversible hydrogenase and start to evolve H 2 gas in the light. Using a computer‐monitored photobioreactor system, we investigated the behavior of sulfur‐deprived algae and found that: (1) the cultures transition through five consecutive phases: an aerobic phase, an O 2‐consumption phase, an anaerobic phase, a H 2‐production phase and a termination phase; (2) synchronization of cell division during pre‐growth with 14:10 h light:dark cycles leads to earlier establishment of anaerobiosis in the cultures and to earlier onset of the H 2‐production phase; (3) re‐addition of small quantities of sulfate (12.5–50 μ M MgSO 4, final concentration) to either synchronized or unsynchronized cell suspensions results in an initial increase in culture density, a higher initial specific rate of H 2 production, an increase in the length of the H 2‐production phase, and an increase in the total amount of H 2 produced; and (4) increases in the culture optical density in the presence of 50 μ M sulfate result in a decrease in the initial specific rates of H 2 production and in an earlier start of the H 2‐production phase with unsynchronized cells. We suggest that the effects of sulfur re‐addition on H 2 production, up to an optimal concentration, are due to an increase in the residual water‐oxidation activity of the algal cells. We also demonstrate that, in principle, cells synchronized by growth under light:dark cycles can be used in an outdoor H 2‐production system without loss of efficiency compared to cultures that up until now have been pre‐grown under continuous light conditions. © 2002 Wiley Periodicals, Inc. Biotechnol Bioeng 78: 731–740, 2002.