Fig. 1.

Photosynthetic electron transport pathways that support the production of NADPH and hydrogen. (A) In LEF, light-activated PSII extracts electrons from water and transfers them in a Z-scheme pattern to plastoquinone (PQ), then, through Cytochrome b₆f, (Cytb₆f) to PC, which then feeds electrons to light-oxidized PSI. PSI-derived electrons are used to reduce Fd, which then transfers reductant to either FNR, for NADPH production, or to HydA, for hydrogen production. During aerobic LEF, hydrogen production is absent due to (i) oxygen formation by PSII and (ii) NADPH-dependent carbon dioxide assimilation mediated by FNR. In the figure, FNR is shown associated with the thylakoid membrane, as proposed by previous studies (see text) and also confirmed here. LEF can support hydrogen production only during anaerobic growth, when oxygen is either removed by argon purging or consumed by respiration (see text). (B) During CEF, electrons are cycled around PSI, which supports generation of a proton gradient. Electrons originating from Fd are either transferred to Cyb6f or to FNR to produce NADPH, which is consumed by NADPH dehydrognease to reduce the PQ pool. Presumably HydA is unable to compete (or competes weakly) with CEF mechanism(s) during anaerobic growth.