Cyclic Photobiological Algal H2-production

Previous work from our laboratory demonstrated the feasibility of using sulfur-deprived algal cultures to photoproduce H2 continuously for 3-4 days (Melis et al., 2000; Ghirardi et al., 2000a). The production of H2 depends mostly on the photosynthetic H2O-oxidation activity of the cultures (Ghirardi et al., 2000 a and b; Kosourov et al., 2002), and requires concomitant operation of O2-consumption pathways (Ghirardi et al., 2000a; Kosourov et al., 2002; Antal et al., submitted). Our research has focused on the elucidation of the pathways involved in H2production under sulfur-deprivation and on approaches to improve the H2 production yield of the process. This year’s results show that (a) H2-production can be optimized by controlling the initial pH of the medium and (b) different maximum amounts of sulfate can be re-added to the system at different points of the sulfur-deprivation process without decreasing H2 production. Moreover, we present evidence from in situ fluorescence measurements, that H2-production during sulfurdeprivation provides the means to re-activate residual photosynthetic electron transport capacity, which is rapidly down-regulated at the establishment of anaerobiosis. These measurements can be used to determine the transition of the cultures to anaerobic conditions and thus serve as a means to monitor the physiological state of the culture from outside the photobioreactor vessel. Finally, we show that different pathways for photosyntheticallygenerated O2 consumption are prevalent during different phases of the process. We conclude that the rates of H2-production in the system are limited by the rates of electron transport from the H2O-oxidizing complex to the hydrogenase under anaerobic conditions, and that these rates can be manipulated, to a certain extent, by changes in pH and addition of sulfate to the medium.