Limiting steps of hydrogen production in Chlamydomonas reinhardtii and Synechocystis PCC 6803 as analysed by light-induced gas exchange transients

In the cyanobacterium Synechocystis PCC 6803 and in the microalga Chlamydomonas reinhardtii, transient hydrogen photo-production is observed when cells are exposed to light in anoxia. We measured changes in H2, O2, and CO2 concentrations using time-resolved mass spectrometry in wild-type and mutant strains of Chlamydomonas and Synechocystis. In both organisms, non-photochemical reduction of the plastoquinone pool is shown to contribute to the initial H2 photo-production. This pathway, which does not produce O2, exhibits a low rate in normal conditions. From the effect of the uncoupler FCCP, we conclude that PS II-independent H2 production in Chlamydomonas is limited by the trans-thylakoidal proton gradient. In Synechocystis, from the study of a mutant deficient in the NDH-1 complex (M55), we conclude that PS II-independent H2 production is limited by recycling of NAD(P)H through the NDH-1 complex. Based on these conclusions, we propose strategies for optimising H2 photo-production in these organisms.