Bacterial strategies for obtaining chemical energy by degrading sulfide minerals

Abstract

Reduced inorganic sulfur compounds are oxidized by members of two domains of living beings namely Archaea and Bacteria. The sulfur compounds are mostly oxidized to sulfate and energy is, thereby, obtained for carbon dioxide fixation and cell growth. A number of genera have been shown to be involved in the sulfur cycle and in biomineralization. They include Acidothiobacillus, Leptospirillum, Acidiphilum, Sulfobacillus, Ferroplasma, Sulfolobus, Metallosphaera and Acidianus. Remarkably, these microorganisms survive in extremely inhospitable environments. They are usually highly acid tolerant and many can grow in environments of pH 1 or even lower. They can be subdivided into mesophilic (20-40°C temperature optima), moderately thermophillic (40-60°C), and (extremely) thermophilic (above 60°C). In some cases, both reduced sulfur and iron (Fe2+) are used as energy sources, while some prokaryotes can oxidize only one of these species. In many ore deposits the iron- and the sulfur species are readily available in the form of the mineral iron pyrite (FeS2). The reaction of pyrite with oxygen and water occurs exergonically with production of heat but at a very slow rate. The enthalpy of formation of pyrite is ΔHf = -37.4 kcal mol-1 and that of formation of FeSO4 is ΔHf = -222 kcal mol-1. The TΔSf values at 300 K are 3.79 and 8.6 kcal mol-1 respectively. The conversion of pyrite into two iron sulfate molecules thus involves a free-energy turnover of ΔGf=402.3 kcal mol-1 for pyrite, to which approximately 10% of solvation energy has to be added. The process of reaction with oxygen is kinetically inhibited at ambient temperature. For this reason pyrite crystals are found to be quite stable in natural environments. The slow chemical oxidation process (Eq. 13.1) is greatly enhanced by bacteria that harvest part of the chemical energy, which would otherwise be dissipated as heat: (Equation presented) Sulfuric acid is generated when ferric iron reacts with additional sulfide. An acid environment is thereby generated, which provides a favorable ecosystem for acid-loving bacteria. The acidity of the environment supports the leaching process by allowing protons to break additional chemical sulfur bonds during formation of interfacial -SHδ-. groups and by keeping Fe3+ complexes and iron hydroxide/iron oxides in solution, thus avoiding sedimentation and obstruction of the sulfide interface. Sulfide-oxidizing bacteria interfere in the natural sulfide oxidation process with oxygen by recovering part of the free energy of reaction in the form of chemical energy.