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Appl. Environ. Microbiol. doi:10.1128/AEM.00852-08
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

The Relative Importance of H₂ and H₂S as Energy Sources for Primary Production in Geothermal Springs

Thermal Biology Institute, and Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, MT 59717; Department of Geology, University of Vermont, Burlington, VT 05405; Department of Biology, University of Copenhagen, DK-3000 Helsingør, Denmark

^* To whom correspondence should be addressed. Email: timmcder{at}montana.edu.

	Abstract

Geothermal waters contain numerous potential electron donors capable of supporting chemolithotrophy-based primary production. Thermodynamic predictions of energy yields for specific electron donor and acceptor pairs in such systems are available, although direct assessments of these predictions are rare. This study assessed the relative importance of dissolved H₂ and H₂S as energy sources for support of chemolithotrophic metabolism in an acidic geothermal spring in Yellowstone National Park. H₂S and H₂ concentration gradients were observed in the outflow channel, and vertical H₂S and O₂ gradients were evident within the microbial mat. H₂S levels and microbial consumption rates were approximately three orders of magnitude greater than H₂. Hydrogenobaculum-like organisms dominated the bacterial component of the microbial community and isolates representing three distinct 16S rRNA gene phylotypes (phylotype = 100% identity) were isolated and characterized. Within-phylotype, O₂ requirements varied as did energy source utilization; some isolates could grow only with H₂S, some only H₂, while others could utilize either as an energy source. These metabolic phenotypes were consistent with in situ geochemical conditions measured using aqueous chemical analysis, and in-field measurements using gas chromatography and microelectrodes. Pure culture experiments with an isolate that could utilize H₂S and H₂, and that represented the dominant phylotype (70% of the PCR clones), found that H₂S and H₂ were used simultaneously, without evidence of induction or catabolite repression, and at relative rate differences comparable to those measured in ex situ field assays. Under in situ relevant concentrations, growth of this isolate with H₂S was better than H₂. Major conclusions drawn from this study are that phylogeny may not necessarily be reliable for predicting physiology, and that H₂S can dominate over H₂ as an energy source in terms of availability, apparent in situ consumption rates, and growth-supporting energy.