Microorganisms involved in bioleaching and nucleic acid-based molecular methods for their identification and quantification

Abstract

Bioleaching is the biological conversion of an insoluble metal compound into a water soluble form. In case of metal sulfide bioleaching, metal sulfides are oxidized to metal ions and sulfate by aerobic, acidophilic Fe(II) and/or sulfurcompound oxidizing Bacteria or Archaea. Bioleaching involves chemical and biological reactions. Despite molecular oxygen being the final electron acceptor for the overall metal sulfide bioleaching process, Fe(III) ions are the relevant oxidizing agent for the metal sulfides. The metal sulfide oxidation itself is a chemical process in which Fe(III) ions are reduced to Fe(II) ions and the sulfur moiety of the metal sulfide is oxidized to sulfate, and various intermediate sulfur compounds, e.g. elemental sulfur, polysulfide, thiosulfate, and polythionates. For example the oxidation of sphalerite (ZnS) to elemental sulfur is given in the following equation: (1) ZnS+2Fe3+→Zn 2++0.125S8+2Fe2+ Because of two different groups of metal sulfides exist, two different metal sulfide oxidation mechanisms have been proposed, namely the thiosulfate mechanism (for acid-insoluble metal sulfides, such as pyrite) and the polysulfide mechanism (for acid-soluble metal sulfides, e.g. sphalerite or chalcopyrite, CuFeS2). These mechanisms explain the occurrence of all inorganic sulfur compounds which have been detected in the course of metal sulfide oxidation (for review see: Sand et al., 2001; Rohwerder et al., 2003; Schippers, 2004; Chapter 2). The role of the microorganisms in the bioleaching process is to oxidize the products of the chemical metal sulfide oxidation (Fe(II) ions and sulfur-compounds) in order to provide Fe(III) and protons, the metal sulfide attacking agents. In addition, proton production keeps the pH low and thus, the Fe ions in solution. Aerobic, acidophilic Fe(II) oxidizing Bacteria or Archaea provide Fe(III) by the following equation: (2) 2Fe2++0.5O2+2H+→ 2Fe3++H2O Aerobic, acidophilic sulfur-compound oxidizing Bacteria or Archaea oxidize intermediate sulfur compounds to sulfate and protons (sulfuric acid). Most relevant is the oxidation of elemental sulfur to sulfuric acid since elemental sulfur can only be biologically oxidized under bioleaching conditions: (3) 0.125S8+1.5O2+H2O→SO 24 +2H+ The sulfur-compound oxidizing Bacteria or Archaea produce protons which dissolve metal sulfides besides pyrite which is not acid-soluble. Pyrite is only attacked by Fe(III) ions (not by protons) and therefore only dissolved by Fe(II) oxidizing Bacteria or Archaea. This book chapter gives an update of previous excellent reviews on microorganisms involved in bioleaching (e.g. Harrison, 1984; Rossi, 1990; Rawlings, 1997, 2002; Johnson, 1998; Hallberg &Johnson, 2001). In the first part of this chapter, the metal sulfide oxidizing microorganisms are described. In the second part, acidophilic microorganisms which do not oxidize metal sulfides and their importance for bioleaching are reviewed. In the third part, nucleic-acid based methods for the identification and quantification of these microorganisms are introduced. © 2007 Springer.