Function of the chloroplastic NAD(P)H dehydrogenase Nda2 for H(2) photoproduction in sulphur-deprived Chlamydomonas reinhardtii.

The relative contributions of the PSII-dependent and Nda2-dependent pathways for H(2) photoproduction were investigated in the green microalga Chlamydomonas reinhardtii after suphur-deprivation. For this purpose, H(2) 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 H(2) photoevolution rate observed shortly after the establishment of anoxia, and an acceleration of the decline of H(2) photoevolution rate with time. DCMU addition to Nda2-deficient cells completely inhibited H(2) photoproduction, showing that the PSII-independent H(2) 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 H(2) 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 H(2) photoevolution rate with time in Nda2-deficient cells revealed a more pronounced inhibition of H(2) photoproduction by accumulated H(2) in the absence of non-photochemical plastoquinone reduction. Nda2 is therefore important for linking H(2) photoproduction with catabolism of storage carbon compounds, and seems also involved in regulating the redox poise of the photosynthetic electron transport chain during H(2) photoproduction.