EVIDENCE FOR METHANOGENIC/METHANOTROPHIC HYDROGEN METABOLISM IN A TECTONICALLY ACTIVE THERMAL AQUIFER, LIDY HOT SPRINGS, ID

Hydrogen concentrations, in geothermal (60°C) spring waters at Lidy Hot Springs, ID, were observed to vary between 1 and 20 nM over several years. A series of studies using a variety of molecular techniques, have demonstrated that the microbial composition of these thermal waters was greater than 90% Archaea. Q-PCR assays also demonstrated the presence of low numbers (less than 2 % of the microbial population) of sulfate-reducing bacteria in the Lidy waters. Phylogenetic analyses indicated that the archaeal sequences were most closely related to known methanogens. However, quantitative polymerase chain reaction (Q-PCR) analyses targeting the methyl coenzyme M reductase gene (mcrA), that is considered to be ubiquitous in and unique to all known methanogens, were negative. Highly enriched values for the δ13C (δ13C ~ -10 per mil) and for δD (δD ~ -130 per mil) of dissolved methane were consistent with the influence of microbial methanotrophic activity on the geochemistry of the Lidy waters.

These observations suggest the following hypothesis: Hydrogen is delivered to the organic-poor, andesitic rhyolite aquifer system by piezioelectrolytic splitting of water during sporadic earthquake events. When the resultant increase in dissolved hydrogen concentrations exceeds the physiologic threshold for indigenous methanogens, methanogenic activity is induced. As dissolved hydrogen concentrations decrease in between tectonic events, the activity of the indigenous microbial community may shift from methanogenesis to active anaerobic methane oxidation. An analogous, anaerobic-methane-oxidizing, microbial consortium, consisting of Archaea and sulfate-reducers, has been reported in deep-sea thermal systems. Thus, the Lidy geothermal aquifer system may support an unusual microbial community that is dominated by Archaea and characterized by cycles of high hydrogen (favoring active methanogenesis) and low hydrogen (favoring active methanotrophy).