Microbiological communities in two acidic mine pit lakes in the Iberian pyrite belt (IPB), Spain

The microbiology and geochemistry of two pit lakes at former metal mines (Cueva de la Mora and Guadiana) located in the Iberian Pyrite Belt in Spain were investigated. Both lakes are meromictic, with more acidic and oxidized mixolimnion zones overlying anoxic monimolimnion zones, and transitional chemoclines with characteristic sharp pH and redox potential gradients. Stratification in the pit lakes was reflected in the size and diversity of the microbial communities in the different zones, with the chemocline of Cueva de la Mora pit lake and the hypolimnion (the lower layer of the mixolimnion) in the Guadiana pit lake containing the most complex and abundant microbial communities. Acidophiles that oxidize and reduce both iron and sulfur co-exist within the chemocline/hypolimnion zones, implying that biogeochemical cycling of these two elements is most intense within these layers of the pit lakes. Novel species of bacteria were detected using molecular techniques and, in some cases, isolated and partially characterized. The latter included a novel acidophilic iron-reducing gammaproteobacterium ("Acidibacter ferrireducens").