Role of moderate thermophilic bacteria volcanic rock alteration

Biogeochemical weathering of continental volcanic rocks has been poorly documented whereas associated processes might present several original applications. Here, core samples from the weathering front of a volcanic geological formation were incubated in contact with geothermal water in actual temperature conditions (59°C), in presence or absence of the associated microbial communities, with or without addition of EDTA. The microbial processes were stimulated by organic matter supply and low O2 availability was imposed. The presence of bacteria strongly influenced the water geochemistry, in particular trace metals concentrations (Fe, Mn, Co, Ni) were clearly lower in biotic conditions. In contrast, As concentration was higher in presence of bacteria. Microbial communities were influenced by the presence of rock, and by the addition of EDTA. They included four main metabolic groups, whose decreasing order of importance was the following: fermentative bacteria > S-reducing bacteria > FeIII-reducers > aerobic heterotrophs. Results suggest that thermophilic microbial communities inhabiting hydrothermal water could contribute to the leaching of trace elements from volcanic rocks and to their immobilization in secondary minerals such as metallic sulfides.