The effect of sulfur re-addition on H2 photoproduction by sulfur-deprived green algae

Sulfur deprivation of algal cultures selectively and partially inactivates photosystem II (PSII)-catalyzed O-2 evolution, induces anaerobiosis and hydrogenase expression, and results in sustained H-2 photoproduction for several days. We show that re-addition of limiting amounts of sulfate (1-10 mu M final concentration) to the cultures during the H-2-production phase temporarily reactivates PSII photochemical and O-2-evolution activity and re-establishes higher rates of electron transport through the photosynthetic electron transport chain. The reactivation of PSII occurs by de novo D1 protein synthesis, but does not result in the re-establishment of aerobic conditions in the reactor, detectable by dissolved-O-2 sensors. However, concomitant H-2 photoproduction is inhibited, possibly due to excessive intra-cellular levels of photosynthetically-evolved O-2. The partial recovery of electron transport rates correlates with the re-oxidation of the plastoquinone (PQ) pool, as observed by pulse-amplitude modulated (PAM) and fluorescence-induction measurements. These results show that the presence of a more oxidized PQ pool releases some of the down-regulation of electron transport caused by the anaerobic conditions.