The role of bacteria in the formation of iron sulfide minerals under low pH conditions
Zusammenfassung der Projektergebnisse
During a two-year research project at the University of Ottawa, Canada, I studied microbial sulfate reduction and the formation of secondary minerals under conditions mimicking low pH environments. Acidic environments, both natural and anthropogenic, are rich in dissolved metals. Due to alkalinity generation and hydrogen sulfide production, microbial sulfate reduction may lead to the formation of secondary minerals, such as aluminum hydroxides and iron sulfides. This renders microbial sulfate reduction useful for the biotechnological application in the remediation of acid mine drainage sites. Sulfate-reducing microorganisms present a phylogenetically diverse group of Prokaryotes that gain their energy by anaerobic respiration with sulfate as terminal electron acceptor. They are, however, generally considered to be inhibited by low pH. The first task was, therefore, to retrieve sulfatereducing microorganisms that could grow over a pH range of 3 – 6. The selective enrichment of sulfate-reducing microorganisms from sediments of an acidic mine pit lake with molecular hydrogen as electron donor and in the presence of elevated concentrations of Al and Fe(II) yielded Desulfosporosinus spp. at pH 5 – 6. These sulfate reducers are commonly found in acid mine drainage sites. More surprising was the finding that Thermodesulfobium ssp. which are only known from hot springs dominated in the enrichments at pH 3 – 4. Electron microscopic studies revealed the formation of amorphous Al-rich precipitates on the cell surfaces of these bacteria. The precipitates sometimes covered the cell surfaces completely and compact aggregates of cells and precipitates were formed. The Al precipitation process may function as a detoxification mechanism which could be the reason why Thermodesulfobium spp. outcompeted Desulfosporosinus spp. at low pH conditions. In contrast to the Al-rich precipitates, iron sulfides did not precipitate on cell surfaces and were similar irrespective of prevailing pH or species of sulfate reducers present. Iron sulfide particles were monosulfidic and crystalline but they could not be assigned to a specific mineral based on electron diffraction and calculated d-spacings. Overall, it was found that mineral or precipitate formation did not have an inhibiting effect on cell activity. How the cells cope with a dense coating of Al-rich precipitates or how they may even benefit from it still needs further investigation. Furthermore, it is still unclear why iron sulfides did not form on cell surfaces and if metabolically active microorganisms possibly have a mechanisms preventing direct mineral precipitation on the cell surface.
Projektbezogene Publikationen (Auswahl)
- (2007) Abatement of acid mine drainage: sulfate reduction and iron sulfide formation at low pH conditions. Mining and the Environment – International Conference, Sudbury, Ontario, Canada
Meier J, Piva A, Lutzu G, Fortin D
- (2007) Iron sulfide formation under low pH conditions in sulfate reducing enrichment cultures obtained from acidic pit lake sediments. 17th Annual Goldschmidt Geochemistry Conference, Cologne, Germany
Meier J, Piva A, Fortin D
- (2007) pH as a selecting factor for the enrichment of sulfate reducing prokaryotes from sediments of an acidic pit lake. International Symposium of Environmental Biogeochemistry (ISEB) 18, Taupo, New Zealand
Meier J, Piva A, Monteiro C, Fortin D
