Acclimation to hypoxia in Chlamydomonas reinhardtii: can biophotolysis be the major trigger for long‐term H₂ production?

• Published in: The New phytologist 2014-12, Vol.204 (4), p.890-900
• Main Authors: Scoma, Alberto, Durante, Lorenzo, Bertin, Lorenzo, Fava, Fabio
• Format: Article
• Language: English
• Subjects: Acclimation; Acclimatization; Acetates; Acetic acid; Anaerobic conditions; Anaerobiosis; Cell Hypoxia; Cell Respiration; Chlamydomonas reinhardtii; Chlamydomonas reinhardtii - drug effects; Chlamydomonas reinhardtii - metabolism; Chlamydomonas reinhardtii - physiology; Chlorophyll; Chlorophyll - metabolism; Chlorophylls; D1 protein; Depletion; Diuron; Diuron - pharmacology; Electron sources; electrons; Fermentation; green microalgae; H2 production; hydrogen; Hydrogen - metabolism; Hydrogen based energy; Hydrogen evolution; Hydrogen production; hydrogenase; Hydrogenase - metabolism; Hypoxia; Light; microalgae; Nutrient deficiencies; Oxidoreductions; Phosphates; Photolysis; Photosynthesis; photosynthesis : respiration ratio; Photosystem II; photosystem II (PSII); Photosystem II Protein Complex - antagonists & inhibitors; Photosystem II Protein Complex - metabolism; proteins; Redox properties; redox state; Respiration; Starches; Starvation; Sulfur; Sulphur
• ISSN: 0028-646X; 1469-8137
• DOI: 10.1111/nph.12964

In anaerobiosis, the microalga Chlamydomonas reinhardtii is able to produce H₂ gas. Electrons mainly derive from mobilization of internal reserves or from water through biophotolysis. However, the exact mechanisms triggering this process are still unclear. Our hypothesis was that, once a proper redox state has been achieved, H₂ production is eventually observed. To avoid nutrient depletion, which would result in enhanced fermentative pathways, we aimed to induce long‐lasting H₂ production solely through a photosynthesis : respiration equilibrium. Thus, growing cells were incubated in Tris Acetate Phosphate (TAP) medium under low light and high chlorophyll content. After a 250‐h acclimation phase, a 350‐h H₂ production phase was observed. The light‐to‐H₂ conversion efficiency was comparable to that given in some reports operating under sulphur starvation. Electron sources were found to be water, through biophotolysis, and proteins, particularly through photofermentation. Nonetheless, a substantial contribution from acetate could not be ruled out. In addition, photosystem II (PSII) inhibition by 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU) showed that it actively contributed to maintaining a redox balance during cell acclimation. In appropriate conditions, PSII may represent the major source of reducing power to feed the H₂ evolution process, by inducing and maintaining an ideal excess of reducing power.