Dissimilatory reduction of sulfate and zero-valent sulfur at low pH and its significance for bioremediation and metal recovery

Redox transformations of sulfur, involving dissimilatory and assimilatory oxidation and reduction reactions, occurs in water bodies and terrestrial environments worldwide, leading to dynamic cycling of this element throughout the biosphere. In cases where zero-valent (elemental) sulfur, sulfate and other oxidized forms are used as electron acceptor in (primarily) anaerobic microbial metabolisms, the end product is hydrogen sulfide (HS⁻ or H₂S, dependent on pH). While neutrophilic and alkalophilic sulfidogenic prokaryotes have been known for many decades, acid-tolerant and acidophilic strains and species have been isolated and characterized only in the past twenty or so years, even though evidence for sulfide generation on these environments was previously well documented. This review outlines the background and current status of the biodiversity and metabolisms of sulfate- and sulfur-reducing prokaryotes that are metabolically active in low pH environments, and describes the developing technologies in which they are being used to remediate acidic waste waters (which are often metal-contaminated) and to recover metal resources.