Survival mechanisms of microorganisms occurring in acid mine drainage: sulfur, iron, carbon, and nitrogen metabolic pathways

Despite its economic importance, mining usually generates intense environmental degradation. The excavation process carried out in mining activities exposes minerals to atmospheric oxygen and water, conditioning a series of biogeochemical processes that can lead to the production of acid mine drainage (AMD). AMD has low pH and high concentrations of sulphates and heavy metals, creating environments with extreme conditions for life. These environments are usually inhabited by microorganisms able to acquire energy from iron and sulfur, using limited sources of carbon and nitrogen. In addition, these microorganisms need mechanisms to resist to extremely low pH and high concentration of heavy metals that can be toxic and lethal to the cellular structure. Acid stress tolerance involves active mechanisms to maintain intracellular pH at adequate levels despite low external values, and adaptive processes against acid stress allowing microorganisms to operate metabolically at low pH. The set of these adaptations give microorganisms the possibility of surviving in AMD environments and, consequently, represent potential for bioremediation and other biotechnological applications like biomining and search for biomolecules for industrial processes. The purpose of this review was to compile the metabolic and adaptive mechanisms involved in the survival of microorganisms occurring in AMD environments, focusing on how they utilize sulfur, iron, carbon and nitrogen metabolic pathways.