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Chapter 22 : Photosynthetic Water-Splitting for Hydrogen Production
Category: Applied and Industrial Microbiology
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This chapter emphasizes photobiological, H2 producing organisms and processes that are able to link photosynthetic water oxidation (reductant-generation) directly to [FeFe]-hydrogenase-catalyzed H2 production function. The biological catalysts involved in H2 metabolism are either nitrogenases or hydrogenases. Interestingly, the [NiFe], [FeFe], and FeS-cluster free types of hydrogenases are almost completely segregated within specific groups of organisms, suggesting convergent evolution. Two distinct H2 photoproduction pathways have been described in green algae, and there is evidence for a third, light-independent, fermentative H2 pathway coupled to starch degradation. A section summarizes the genetics, expression, maturation, structure, and modeling aspects of [FeFe]-hydrogenases, which catalyze H2 production in green algae. The hydrogenase structural genes that have been cloned and sequenced from species of Chlamydomonas, Chlorella, and Scenedesmus are homologues of the [FeFe]-hydrogenases from bacterial organisms. A majority of the [FeFe]-hydrogenase genes and proteins so far isolated exhibit complex structures that are organized into modular domains. Experimental investigations on the molecular engineering of O2 accessibility in [FeFe]-hydrogenase are currently under way. A better understanding of anaerobic metabolism in Chlamydomonas and metabolic fluxes associated with diurnal periods of light and dark will facilitate the development of physiological models able to predict metabolic fluxes under various environmental conditions. Photosynthesis and H2 production in unicellular green algae can in principle operate with a nearly 100% absorbed photon utilization efficiency. The rate of electron transport in the thylakoid membrane of photosynthesis is of importance for defining yield and efficiency of the overall process.
Approximate areas of the country required to displace all gasoline used in the United States using different technologies: algal H2 produced from water at a future 10% solar efficiency (1), corn grain ethanol at current production yields (2), and cellulosic ethanol from switchgrass at an estimated optimal yield (3). For comparison, box 4 represents the total area of the 2006 corn crop.
Enzymes that depend on electron transfer from reduced Fd in C. reinhardtii. The broad arrow from Fd to FNR represents the preferential flux of electrons under normal physiological conditions. Electrons are transported to Fd directly from PSI. This figure was provided by A. Dubini.