The Geomicrobiology of Acidic Pit Lakes

Acidic pit lakes formed in coal and metal open-pit mines are currently attracting an increasing scientific attention from the geomicrobiological community due to their high potential to host novel microbial metabolisms of biotechnological and astrobiological interest. These lakes represent unusual environments where geochemical and ecological features are intermediate between typical acid mine drainage systems and volcanic crater lakes. Acidophilic microalgae specially adapted to extremely low light conditions are the main primary producers in the photic zone. The transitional redoxclines are inhabited by a diverse microbial community which fuels iron and sulfur cycling. The deep anoxic waters are particularly interesting and may contain significant and invaluable biological dark matter. Although these environments have been largely unexplored, recent “omics” research has shown a high abundance of bacterial and archaeal metabolisms involved in iron and sulfur cycling. The reported presence of sulfate-reducing, sulfur-reducing and sulfur-disproportionating bacteria shows that biosulfidogenesis is feasible and even common in these extreme environments. Most microorganisms found in acid pit lakes are polyextremophiles adapted to extreme acidity, salinity and metal concentrations, and may include moderate thermophiles and halophiles.