The resulting anaerobic conditions induce re-synthesis of NAC2 protein and result in increased stability of D2; under these conditions, O2 evolution is gradually

restored and H2 production is inhibited. The alternate expression and repression of the NAC2 gene, without the need for removing copper see more from the medium serves the same purpose as the sulfur-deprivation process described in “O2 sensitivity of hydrogenases” section, allowing the operation of a 2-phase system where carbon reserves accumulate during the oxygenic phases and subsequently support the respiratory activity needed to achieve anaerobiosis during the second phase. The authors report the production of 20 μmol H2/liter during one cycle, which corresponds to a maximal rate of 1 mmol H2 mol−1 Chl s−1. Increased O2 consumption/sequestration Anaerobiosis can be achieved either by decreasing O2 evolution or increasing respiration, i.e., by manipulating the photosynthesis/respiration selleck compound ratio (P/R ratio) and bringing it below 1. The apr1 mutant, which exhibited an attenuated P/R ratio (Ruhle et al. 2008), was shown to become anaerobic in the light, mimicking the physiological status of sulfur-deprived cells. In this

strain, starch is degraded under non-stress conditions and the reducing equivalents are transferred by the NAD(P)H plastoquinone-oxidoreductase (NPQR, also called NDA2) to the plastoquinone pool (PQ) (Mus et al. 2005), keeping it reduced. As a consequence, CEF and photophosphorylation still occur, although PSII activity is substantially downregulated. The apr1 mutant becomes anaerobic under photoheterotrophic, sulfur-replete conditions and induces hydrogenase synthesis in the light. However, it does

not produce hydrogen, contrary to expectations. In the past, it has been shown that hydrogen production in anaerobically adapted algae is highest when the carbon dioxide concentrations are low (Cinco et al. 1993), due to competition between hydrogenase and FNR for photosynthetic reductant. Photoreduced O-methylated flavonoid FDX transfers electrons mainly to FNR, which then supplies NADPH to the Calvin–Benson Cycle. Thus, to disrupt the effect of the Calvin Benson cycle activity on hydrogen metabolism, glycolaldehyde (GA) was added to the apr1 culture. GA disrupts the Calvin–Benson cycle activity by inhibiting the phosphoribulokinase, which catalyzes the ATP-dependent phosphorylation of ribulose-5-phosphate to ribulose-1,5-bisphosphate. Consequently, it was observed that the in vivo hydrogen production rate of apr1 cell samples was twice the rate determined in WT sulfur-deprived cells, thus confirming the usefulness of the low P/R ratio concept (Ruhle et al. 2008). Another approach to induce anaerobiosis is by introducing O2 sequesters into the chloroplast.