Renewable H-2 photoproduction by Chlamydomonas reinhardtii offers a desirable bio-system for solar fuels. However, its large-scale application is hindered mainly due to lack of ideal strains. We previously isolated a mutant hpm91 which lacks PGR5 and sustains H-2 photoproduction for 25 days. To understand the photosynthetic basis for this remarkable phenotype, we hereby investigated its photochemical characteristics during sulfur-deprived H-2 photoproduction using in vivo chlorophyll fluorescence spectroscopy. Compared to wild type, effective quantum yield of PSII and PSI of hpm91 increased upto 78.9% and 147.6%, respectively. Electron transport rate of each photosystem is closely correlated with the increase of quantum yield, suggesting overall enhanced photochemistry of hpm91 under such condition. Moreover, ATP synthase activity decays slower and remains higher in this mutant. These are in vivo evidence demonstrating increased photosynthetic efficiency of hpm91 promotes its H2 photoproduction. Together with its competent photoheterotrophic growth in a larger photobioreactor, we propose that hpm91 is a valuable strain for re-engineering Chlamydomonas towards improving light energy efficiency in a large-scale system. (C) 2019 Institute of Botany, Chinese Academy of Sciences. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.