Genomics, physiology and applications of cold tolerant acidophiles

Abstract: Psychrotolerant acidophiles have gained increasing interest because of their importance in biomining operations in environments where the temperature falls well below 10°C during large parts of the year.Acidithiobacillus ferrivorans is the only characterized acidophile with the ability to live a psychrotrophic lifestyle and is able to oxidize ferrous iron and inorganic sulfur compounds at low temperature. The A. ferrivorans SS3 genome sequence mirrors its low temperature chemolithotrophic lifestyle and indicates ecological flexibility. Two enzyme systems for the synthesis of the protective molecule trehalose as well as multiple cold shock proteins suggest its psychrotolerance. In addition to genes coding for ferrous iron and inorganic sulfur compound oxidation enzymes, candidate genes for molecular hydrogen utilization were identified. Also, A. ferrivorans SS3 was suggested to have the ability to switch between nitrogen fixation and nitrogen uptake. Characterization of ferrous iron and inorganic sulfur compound oxidation during low temperature growth showed that both substrates were efficiently oxidized and revealed the potential of using ferric iron as an alternative electron acceptor. Gene transcript analyses also revealed constitutive expression of genes involved in ferrous iron oxidation and their rapid increase in expression when ferrous iron became available. Growth experiments further suggested ferrous iron was preferred over inorganic sulfur compounds during bioleaching. This phenomenon was especially evident during A. ferrivorans-mediated bioleaching of chalcopyrite as sulfur accumulated and eventually inhibited further leaching. A potential way to alleviate this problem is addition of low temperature, obligate inorganic sulfur compound oxidizing acidophiles. Although, these microorganisms have not been identified, analysis of a cold, acidic biofilm from Kristineberg mine suggested additional psychrotolerant inorganic sulfur compound oxidation oxidizers might be present. Psychrotolerant acidophiles from Kristineberg mine have also been demonstrated to remove inorganic sulfur compounds from mining process water at in situ temperatures. This use of indigenous microorganisms for removal of environmental pollutants is a big step towards greener mining.