Sustained Hydrogen Photoproduction by Chlamydomonas reinhardtii - Effects of Culture Parameters

Abstract

The green alga, Chlamydomonas reinhardtii, is capable of sustained H2 photoproduction when grown under sulfur-deprived conditions. This ability is the result of partial inactivation of the photosynthetic O2-evolution activity in response to sulfur deprivation, which results in the establishment of an anaerobic environment in the culture due to oxidative respiration under continuous illumination. After 10-15 hours of anaerobiosis, sulfur-deprived algal cells induce the reversible hydrogenase and start evolving H2 in the light. Using a computer-monitored photobioreactor system, we investigated the behavior of sulfur-deprived algal cultures and found that (a) they transition through the following four consecutive phases: an aerobic phase, an O2-consumption phase, an anaerobic phase and a H2-production phase; (b) synchronization of cell division by light-and-dark cycles leads to earlier establishment of anaerobiosis in the cultures and to earlier onset of the H2-production phase; (c) re-addition of small quantities of sulfate (12.5 ¿ 50 m M MgSO4, final concentration) to the synchronized or unsynchronized cell suspensions results in an initial increase in culture density, a higher specific initial rate of H2 production, an increase in the length of the H2-production phase, and an increase in the total H2 output; and (d) increases in the optical culture density in the presence of 50 mM sulfate result in decreases in the initial specific rates of H2 production but in early starts of the H2-production phase. We discuss some of the biochemical reasons why sulfate re-addition affects H2 production.