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.

Increased Lipid Accumulation in the Chlamydomonas reinhardtii sta7-10 Starchless Isoamylase Mutant and Increased Carbohydrate Synthesis in Complemented Strains

“…6 cells was observed, values that are consistent with those recently recorded by Chochois et al (5). As reported previously (8,34,39,40), both sta6 and sta7-10 mutant cells contained severely attenuated levels of starch and essentially no starch-derived glucose was detected in the sta6 mutant, while sta7-10 cells contained 1.…”

“…The other major carbon resource observed during nitrogen stress in C. reinhardtii is the formation of lipid bodies (5,26,34,46,48). Since the blockage of starch synthesis in the starchless mutants creates the potential for diverting metabolic precursors into lipid biosynthetic pathways, we investigated whether lipids were differentially accumulated in the starchless mutants by quantifying lipid-derived fatty acid methyl esters (FAMEs) using GC-FID.…”

“…The unicellular green alga Chlamydomonas reinhardtii has emerged as a model organism for studying algal physiology, photosynthesis, metabolism, nutrient stress, and the synthesis of bioenergy carriers (12,15,19,24,32). During acclimation to nitrogen deprivation, C. reinhardtii cells accumulate significant quantities of starch and form lipid bodies (4,5,8,26,28,30,34,43,46,48). Despite the significance of these products in algal physiology and in biofuels applications, the metabolic, enzymatic, and regulatory mechanisms controlling the partitioning of metabolites into these distinct carbon stores in algae are poorly understood.…”

Rewiring hydrogenase-dependent redox circuits in cyanobacteria

“…4B and 5). Although the presence of at least two distinct (i.e., light-dependent and light-independent) pathways for electron transfer to hydrogenases have been well documented (9,36,49), to our knowledge there have been no previous reports of genetic means to enhance the rate of one pathway relative to the other. Rewiring the flow of reducing equivalents by such a strategy may not only be useful for reducing the cost of bioreactor design (i.e., by reducing the need for illumination, and associated light penetration and heat distribution issues), but also open the possibility of importing parallel redox pathways that are partially insulated from host redox machinery (Fig.…”

“…In light-dependant reactions, electrons derived from carbohydrate metabolism are donated to the plastoquinone pool and are subsequently reenergized through Photosystem I (PSI) before being transferred to hydrogenase through petFclass plant-type ferredoxins (18,21,33). In dark fermentation of starch to hydrogen, electrons from the breakdown of glycogen may be directly donated to the hydrogenase, or may be transferred through intermediate electron carriers, such as NAD(P) H or ferredoxin (35,36). In HydA-expressing Synechococcus, hydrogen production was greatly reduced in the dark or in the presence of the plastoquinone inhibitor dibromothymoquinone (DBMIB) (Fig.…”