Function of the chloroplastic NAD(P)H dehydrogenase Nda2 for H₂ photoproduction in sulphur-deprived Chlamydomonas reinhardtii.

The relative contributions of the PSII-dependent and Nda2-dependent pathways for H₂ photoproduction were investigated in the green microalga Chlamydomonas reinhardtii after suphur-deprivation. For this purpose, H₂ gas production was compared for wild-type and Nda2-deficient cells with or without DCMU (a PSII-inhibitor) in the same experimental conditions. Nda2-deficiency caused a 30% decrease of the maximal H₂ photoevolution rate observed shortly after the establishment of anoxia, and an acceleration of the decline of H₂ photoevolution rate with time. DCMU addition to Nda2-deficient cells completely inhibited H₂ photoproduction, showing that the PSII-independent H₂ photoproduction relies on the presence of Nda2, which feeds the photosynthetic electron transport chain with electrons derived from oxidative catabolism. Nda2-protein abundance increased as a result of sulphur deprivation and further during the H₂ photoproduction process, resulting in high rates of non-photochemical plastoquinone reduction in control cells. Nda2-deficiency had no significant effect on photosynthetic and respiratory capacities in sulphur-deprived cells, but caused changes in the cell energetic status (ATP and NADPH/NADP+ ratio). The rapid decline of H₂ photoevolution rate with time in Nda2-deficient cells revealed a more pronounced inhibition of H₂ photoproduction by accumulated H₂ in the absence of non-photochemical plastoquinone reduction. Nda2 is therefore important for linking H₂ photoproduction with catabolism of storage carbon compounds, and seems also involved in regulating the redox poise of the photosynthetic electron transport chain during H₂ photoproduction.