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Fig. 1 | Biotechnology for Biofuels

Fig. 1

From: Water-splitting-based, sustainable and efficient H2 production in green algae as achieved by substrate limitation of the Calvin–Benson–Bassham cycle

Fig. 1

H2 production in Chlamydomonas reinhardtii. a Schematic presentation of the photosynthetic electron transport chain in green algae. Solar energy is captured by the light-harvesting complexes (LHC) of photosystem II and I (PSII and PSI). Electrons extracted from water by the oxygen-evolving complex (OEC) of PSII are transferred to the photosynthetic electron transport chain via the plastoquinone (PQ)-pool, the cytochrome b6f complex (cyt b6f), plastocyanin (PC), PSI and ferredoxin (Fd). From Fd, electrons can be transferred by the ferredoxin-NADP+ oxidoreductase (FNR) to NADP+ or to the hydrogenase (HydA; for clarity, oxygen-dependent alternative pathways are not shown). H+ accumulated in the thylakoid lumen are used for ATP production via ATP synthase. The ATP and NADPH generated during primary photosynthetic processes are consumed for CO2 fixation in the Calvin–Benson–Bassham (CBB) cycle, which produces sugars and ultimately starch. When cultures are grown in the presence of acetate, glycerate 3-phosphate (GP) may also feed the CBB cycle. Hydrogenases are expressed under anoxic conditions; upon illumination, significant H2 production may occur. The water-oxidation dependent pathway of H2 production is denoted with red line. Depending on the conditions, starch degradation may also contribute to H2 production either via the NAD dehydrogenase (NDH) complex, PSII-independent pathway, or via pyruvate-Fd-oxidoreductase (PFR). b H2 production in CC124 Chlamydomonas cultures (50 µg chl (a + b)/ml, at 320 µmol photons/m2/s) as determined in the headspaces of sealed cultures using gas chromatography during 24 h in acetate-containing (TAP, HSA) and acetate-free media (TP, HS) following dark anaerobic incubation (4 h darkness with 3 × 10 min N2 flushing). In the right Y axis, the percentage of H2 in the headspaces of the cultures are shown. Time point 0 indicates the time when the cultures were transferred to the light. c Net O2 production under the same conditions as in b. d Daily H2 production in sulphur-containing HS medium of cultures subjected to dark anaerobic incubation and in cultures transferred to sulphur-free acetate-containing (TAP-S) or acetate-free (TP-S) media. All the alga cultures (HS, TAP-S, TP-S) were illuminated continuously and flushed every 24 h with N2 after determining the gas concentrations in the headspaces of the sealed bottles to avoid excessive H2 accumulation (cf. [7]) and overpressure in the headspace. e Net O2 production under the same conditions as in d. Mean values (± SEM) are each based on 6 biological replicates. Statistical significance levels are presented relative to the Chlamydomonas culture subjected to dark anaerobic incubation in HS medium as *p < 0.05, **p < 0.01, ***p < 0.001

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