growth promoting microorganisms 2. Acid mine drainage 2.1 Implication of microorganisms in formation and remediation of acid mine drainage 2.1.1 Implication of microorganisms in formation of acid mine drainage Many metals naturally occur as sulfide ores (e.g. Pb in galena, Cu in chalcopyrite, Zn in sphalerite). Acid mine drainage is generated when sulfide minerals (especially pyrite or pyrrhotite) come into contact with oxygen and water (Hallberg 2010). Although AMD occurs naturally, mining operations like excavation and milling accelerate the process by increasing surface area of exposure of sulfide minerals to water, oxygen and microorganisms (Simate and Ndlovu, 2014). AMD is often characterized by pH values below 4 and sometimes as low as-3, as measured at the Richmond mine in California (Nordstrom et al., 2000). Such low pH facilitates the mobilization of metals and metalloids, increasing their dispersion into the water (Humphries et al., 2017). AMD represents one of the most problematic pollution of water in industrial and post-industrial areas throughout the world (Hallberg, 2010). Numerous works have been done to characterize the autochthonous microbial communities (Bacteria, Archaea, Eucaryota) thriving in these extreme environments (