We have identified hpm91, a Chlamydomonas mutant lacking Proton Gradient Regulation5 (PGR5) capable of producing hydrogen (H2 ) for 25 days with more than 30-fold yield increase compared to wild type. Thus, hpm91 displays a higher capacity of H2 production than a previously characterized pgr5 mutant. Physiological and biochemical characterization of hpm91 reveal that the prolonged H2 production is due to enhanced stability of PSII, which correlates with increased reactive oxygen species (ROS) scavenging capacity during sulfur deprivation. This anti-ROS response appears to protect the photosynthetic electron transport chain from photo-oxidative damage and thereby ensures electron supply to the hydrogenase.