Screening of anaerobic activities in sediments of an acidic environment: Tinto River.

The Tinto River (Huelva, Spain) is a natural acidic rock drainage environment produced by the bio-oxidation of metallic sulfides from the Iberian Pyritic Belt. A geomicrobiological model of the different microbial cycles operating in the sediments was recently developed through molecular biological methods, suggesting the presence of iron reducers, methanogens, nitrate reducers and hydrogen producers. In this study, we used a combination of molecular biological methods and targeted enrichment incubations to validate this model and prove the existence of those potential anaerobic activities in the acidic sediments of Tinto River. Methanogenic, sulfate-reducing, denitrifying and hydrogen-producing enrichments were all positive at pH between 5 and 7. Methanogenic enrichments revealed the presence of methanogenic archaea belonging to the genera Methanosarcina and Methanobrevibacter. Enrichments for sulfate-reducing microorganisms were dominated by Desulfotomaculum spp. Denitrifying enrichments showed a broad diversity of bacteria belonging to the genera Paenibacillus, Bacillus, Sedimentibacter, Lysinibacillus, Delftia, Alcaligenes, Clostridium and Desulfitobacterium. Hydrogen-producing enrichments were dominated by Clostridium spp. These enrichments confirm the presence of anaerobic activities in the acidic sediments of the Tinto River that are normally assumed to take place exclusively at neutral pH.