title: Hydrogen production in Chlamydomonas: photosystem II-dependent and -independent pathways differ in their requirement for starch metabolism.
abstract: Under sulfur deprivation conditions, the green alga Chlamydomonas reinhardtii produces hydrogen in the light in a sustainable manner thanks to the contribution of two pathways, direct and indirect. In the direct pathway, photosystem II (PSII) supplies electrons to hydrogenase through the photosynthetic electron transport chain, while in the indirect pathway, hydrogen is produced in the absence of PSII through a photosystem I-dependent process. Starch metabolism has been proposed to contribute to both pathways by feeding respiration and maintaining anoxia during the direct pathway and by supplying reductants to the plastoquinone pool during the indirect pathway. At variance with this scheme, we report that a mutant lacking starch (defective for sta6) produces similar hydrogen amounts as the parental strain in conditions of sulfur deprivation. However, when PSII is inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea, conditions where hydrogen is produced by the indirect pathway, hydrogen production is strongly reduced in the starch-deficient mutant. We conclude that starch breakdown contributes to the indirect pathway by feeding electrons to the plastoquinone pool but is dispensable for operation of the direct pathway that prevails in the absence of DCMU. While hydrogenase induction was strongly impaired in the starch-deficient mutant under dark anaerobic conditions, wild-type-like induction was observed in the light. Because this light-driven hydrogenase induction is DCMU insensitive and strongly inhibited by carbonyl cyanide-p-trifluoromethoxyphenylhydrazone or 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, we conclude that this process is regulated by the proton gradient generated by cyclic electron flow around PSI.
authors.affiliation: Commissariat à l'Energie Atomique, Direction des Sciences du Vivant, Institut de Biologie Environnementale et de Biotechnologie, Laboratoire de Bioénergétique et Biotechnologie des Bactéries et Microalgues, Commissariat à l'Energie Atomique Cadarache, F-13108 Saint-Paul-lez-Durance, France.
authors.forename: Vincent
authors.initials: V
authors.lastname: Chochois
authors.forename: David
authors.initials: D
authors.lastname: Dauvillée
authors.forename: Audrey
authors.initials: A
authors.lastname: Beyly
authors.forename: Dimitri
authors.initials: D
authors.lastname: Tolleter
authors.forename: Stéphan
authors.initials: S
authors.lastname: Cuiné
authors.forename: Hélène
authors.initials: H
authors.lastname: Timpano
authors.forename: Steven
authors.initials: S
authors.lastname: Ball
authors.forename: Laurent
authors.initials: L
authors.lastname: Cournac
authors.forename: Gilles
authors.initials: G
authors.lastname: Peltier
mesh_terms: D000085:Acetates / Q000378:metabolism; D000693:Anaerobiosis; D000818:Animals; D002696:Chlamydomonas / Q000166:cytology / Q000201:enzymology / Q000378:metabolism*; D003903:Deuterium / Q000378:metabolism; D005816:Genetic Complementation Test; D006859:Hydrogen / Q000378:metabolism*; D006864:Hydrogenase / Q000378:metabolism; D042541:Intracellular Space / Q000378:metabolism; D009154:Mutation / Q000235:genetics; D045332:Photosystem II Protein Complex / Q000378:metabolism*; D013213:Starch / Q000378:metabolism*; D013455:Sulfur / Q000172:deficiency
chemical_list: D000085:Acetates; D045332:Photosystem II Protein Complex; D013455:Sulfur; D006859:Hydrogen; D013213:Starch; D003903:Deuterium; D006864:Hydrogenase
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