Methanogenesis in the sediments of Rio Tinto, an extreme acidic river.

Río Tinto (Iberian Pyritic Belt, SW Spain) is well known for its low pH (mean pH 2.3), high redox potential (> +400 mV) and high concentration of heavy metals. In this work we describe and analyse the presence of methanogenic archaea in the extreme acidic and oxidizing environment of the Tinto basin. Methane formation was measured in microcosms inoculated with sediments from the Rio Tinto basin. Methanol, formate, volatile fatty acids and lactate stimulated the production of methane. Methane formation was associated with a decrease of redox potential and an increase in pH. Cores showed characteristic well-defined black bands in which a high acetate concentration was measured among the otherwise reddish-brown sediments with low acetate concentration. Methanosaeta concilii was detected in the black bands. In enrichment cultures, M. concilii (enriched with a complex substrate mixture), Methanobacterium bryantii (enriched with H(2)) and Methanosarcina barkeri (enriched with methanol) were identified. Our results suggest that methanogens thrive in micro-niches with mildly acidic and reducing conditions within Rio Tinto sediments, which are, in contrast, immersed in an otherwise extremely acidic and oxidizing environment.