Introduction

Bacterial leaching of metals (bioleaching, biomining) from mineral resources has a very long historical record (Rossi, 1990; Ehrlich, 1999). Metals have been mobilized from sulfide minerals using processes that involved autotrophic sulfur-oxidizing microorganisms, for example Thiobacillus spp., although the involvement of microorganisms in this process was demonstrated only in the 1920s (Rudolfs & Helbronner, 1922; Waksman & Joffe, 1922). In 1947, Thiobacillus ferrooxidans was identified in acid mine drainage as part of a microbial community that also included several fungi (e.g. Spicaria sp.) (Colmer & Hinkle, 1947). Several industrial processes have been developed based on these findings for the mining of cobalt, copper, nickel, uranium, zinc and gold (Bosecker, 1997; Rawlings, 1997). However, all industrial applications to obtain metals from a series of solid materials depend on the activities of sulfur-oxidizing microorganisms.

Bioleaching is mainly based on three mechanisms. Besides proton-induced metal solubilization and metal reduction or oxidation, metals can also be mobilized from solid materials by ligand-induced metal solubilization. Organic acids from heterotrophic microorganisms represent such ligands. This is particularly important in the biohydrometallurgical treatment of silicate, carbonate and oxide minerals since these materials cannot be directly attacked by sulfur-oxidizing microorganisms. Further developments should enable heterotrophic leaching to be used to extract metals from non-sulfide ores (Ehrlich, 1999). The broad diversity of heterotrophic organisms provides a huge industrial potential that has been hardly investigated.