Aerobic and anaerobic microbial consumption of hydrogen in geothermal spring water

[FeFe]-hydrogenase in Yellowstone National Park: evidence for dispersal limitation and phylogenetic niche conservatism

Hydrogen (H₂) has an important role in the anaerobic degradation of organic carbon and is the basis for many syntrophic interactions that commonly occur in microbial communities. Little is known, however, with regard to the biotic and/or abiotic factors that control the distribution and phylogenetic diversity of organisms which produce H₂ in microbial communities. In this study, we examined the [FeFe]-hydrogenase gene (hydA) as a proxy for fermentative bacterial H₂ production along physical and chemical gradients in various geothermal springs in Yellowstone National Park (YNP), WY, USA. The distribution of hydA in YNP geothermal springs was constrained by pH to environments co-inhabited by oxygenic phototrophs and to environments predicted to have low inputs of abiotic H₂. The individual HydA asssemblages from YNP springs were more closely related when compared with randomly assembled communities, which suggests ecological filtering. Model selection approaches revealed that geographic distance was the best explanatory variable to predict the phylogenetic relatedness of HydA communities. This evinces the dispersal limitation imposed by the geothermal spring environment on HydA phylogenetic diversity even at small spatial scales. pH differences between sites is the second highest ranked explanatory variable of HydA phylogenetic relatedness, which suggests that the ecology related to pH imposes strong phylogenetic niche conservatism. Collectively, these results indicate that pH has imposed strong niche conservatism on fermentative bacteria and that, within a narrow pH realm, YNP springs are dispersal limited with respect to fermentative bacterial communities.

Biotechnological intensification of biogas production

The importance of syntrophic relationships among microorganisms participating in biogas formation has been emphasized, and the regulatory role of in situ hydrogen production has been recognized. It was assumed that the availability of hydrogen may be a limiting factor for hydrogenotrophic methanogens. This hypothesis was tested under laboratory and field conditions by adding a mesophilic (Enterobacter cloacae) or thermophilic hydrogen-producing (Caldicellulosyruptor saccharolyticus) strain to natural biogas-producing consortia. The substrates were waste water sludge, dried plant biomass from Jerusalem artichoke, and pig manure. In all cases, a significant intensification of biogas production was observed. The composition of the generated biogas did not noticeably change. In addition to being a good hydrogen producer, C. saccharolyticus has cellulolytic activity; hence, it is particularly suitable when cellulose-containing biomass is fermented. The process was tested in a 5-m(3) thermophilic biogas digester using pig manure slurry as a substrate. Biogas formation increased at least 160-170% upon addition of the hydrogen-producing bacteria as compared to the biogas production of the spontaneously formed microbial consortium. Using the hydrogenase-minus control strain provided evidence that the observed enhancement was due to interspecies hydrogen transfer. The on-going presence of C. saccharolyticus was demonstrated after several months of semicontinuous operation.

The global methane cycle: isotopes and mixing ratios, sources and sinks

A review of the global cycle of methane is presented with emphasis on its isotopic composition. The history of methane mixing ratios, reconstructed from measurements of air trapped in ice-cores is described. The methane record now extends back to 420 kyr ago in the case of the Vostok ice cores from Antarctica. The trends in mixing ratios and in delta13C values are reported for the two Hemispheres. The increase of the atmospheric methane concentration over the past 200 years, and by 1% per year since 1978, reaching 1.7 ppmv in 1990 is underlined. The various methane sources are presented. Indeed the authors describe the methane emissions by bacterial activity under anaerobic conditions in wet environments (wetlands, bogs, tundra, rice paddies), in ruminant stomachs and termite guts, and that originating from fossil carbon sources, such as biomass burning, coal mining, industrial losses, automobile exhaust, sea floor vent, and volcanic emissions. Furthermore, the main sinks of methane in the troposphere, soils or waters via oxidation are also reported, and the corresponding kinetic isotope effects.