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Redirection of metabolism for biological hydrogen production.

ABSTRACT: A major route for hydrogen production by purple photosynthetic bacteria is biological nitrogen fixation. Nitrogenases reduce atmospheric nitrogen to ammonia with the concomitant obligate production of molecular hydrogen. However, hydrogen production in the context of nitrogen fixation is a rather inefficient process because about 75% of the reductant consumed by the nitrogenase is used to generate ammonia. In this study we describe a selection strategy to isolate strains of purple photosynthetic bacteria in which hydrogen production is necessary for growth and independent of nitrogen fixation. We obtained four mutant strains of the photosynthetic bacterium Rhodopseudomonas palustris that produce hydrogen constitutively, even in the presence of ammonium, a condition where wild-type cells do not accumulate detectable amounts of hydrogen. Some of these strains produced up to five times more hydrogen than did wild-type cells growing under nitrogen-fixing conditions. Transcriptome analyses of the hydrogen-producing mutant strains revealed that in addition to the nitrogenase genes, 18 other genes are potentially required to produce hydrogen. The mutations that caused constitutive hydrogen production mapped to four different sites in the NifA transcriptional regulator in the four different strains. The strategy presented here can be applied to the large number of diverse species of anoxygenic photosynthetic bacteria that are known to exist in nature to identify strains for which there are fitness incentives to produce hydrogen.

SUBMITTER: Rey FE

PROVIDER: S-EPMC1828789 | biostudies-literature | 2007 Mar

REPOSITORIES: biostudies-literature

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Redirection of metabolism for biological hydrogen production.

Rey Federico E FE Heiniger Erin K EK Harwood Caroline S CS

Applied and environmental microbiology 20070112 5

A major route for hydrogen production by purple photosynthetic bacteria is biological nitrogen fixation. Nitrogenases reduce atmospheric nitrogen to ammonia with the concomitant obligate production of molecular hydrogen. However, hydrogen production in the context of nitrogen fixation is a rather inefficient process because about 75% of the reductant consumed by the nitrogenase is used to generate ammonia. In this study we describe a selection strategy to isolate strains of purple photosynthetic ...[more]

PMID: 17220249

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Project description:Metabolic flexibility in aerobic methane oxidising bacteria (methanotrophs) enhances cell growth and survival in instances where resources are variable or limiting. Examples include the production of intracellular compounds (such as glycogen or polyhydroxyalkanoates) in response to unbalanced growth conditions and the use of some energy substrates, besides methane, when available. Indeed, recent studies show that verrucomicrobial methanotrophs can grow mixotrophically through oxidation of hydrogen and methane gases via respiratory membrane-bound group 1d [NiFe] hydrogenases and methane monooxygenases respectively. Hydrogen metabolism is particularly important for adaptation to methane and oxygen limitation, suggesting this metabolic flexibility may confer growth and survival advantages. In this work, we provide evidence that, in adopting a mixotrophic growth strategy, the thermoacidophilic methanotroph, Methylacidiphilum sp. RTK17.1 changes its growth rate, biomass yields and the production of intracellular glycogen reservoirs. Under nitrogen-fixing conditions, removal of hydrogen from the feed-gas resulted in a 14 % reduction in observed growth rates and a 144% increase in cellular glycogen content. Concomitant with increases in glycogen content, the total protein content of biomass decreased following the removal of hydrogen. Transcriptome analysis of Methylacidiphilum sp. RTK17.1 revealed a 3.5-fold upregulation of the Group 1d [NiFe] hydrogenase in response to oxygen limitation and a 4-fold upregulation of nitrogenase encoding genes (nifHDKENX) in response to nitrogen limitation. Genes associated with glycogen synthesis and degradation were expressed constitutively and did not display evidence of transcriptional regulation. Collectively these data further challenge the belief that hydrogen metabolism in methanotrophic bacteria is primarily associated with energy conservation during nitrogen fixation and suggests its utilisation provides a competitive growth advantage within hypoxic habitats.

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Redirection of metabolism for biological hydrogen production.

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Redirection of metabolism for biological hydrogen production.

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