Simplified illustration of the physiological pathways used for H
production in Chlamydomonas (Adapted from The Chlamydomonas Sourcebook ). The two photoproduction pathways involving PSII and PSI under the light period are showed in black. Electrons excited to higher energy (low potential) by PSI are able to reduce ferredoxin (FDX), the physiological electron donor to hydrogenase. Both the PSII-dependent and NPQR-dependent (red) pathways require reduction of the PQ pool and PSI activity. In the case of the PSII-independent pathway (reactions in red), reducing power formed by the catabolism of organic substrates is used for reduction of the PQ pool. During dark fermentation the oxidation of pyruvate catalysed by PFR (green reactions) is used to reduce ferredoxin and putatively mediates the observed production of H2 in the dark. White reactions show the parallel main fermentative products from pyruvate, competing with H2. Dashed arrows show linear and cyclic electron flow.