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Geomicrobiology: Transformations of Metals and Minerals by Microorganisms, Bioweathering and Bioremediation
Microorganisms are intimately involved in the biogeochemical processes underpinning metal and mineral transformations in the environment, as well as the cycling of related substances like organometals, metalloids and radionuclides. A variety of processes determine metal and radionuclide mobility and bioavailability and these influence transfer to other environmental components and other living organisms including plants and humans. These processes are also intimately associated with such global phenomena as bioweathering, soil formation, and the biodeterioration of rocks, minerals and building materials. The balance between mobilization and immobilization varies depending on the organisms involved, their environment and physico-chemical conditions. Metal mobilization can arise from leaching mechanisms, complexation by metabolites such as organic acids and siderophores, and methylation where this results in volatilization. Immobilization can result from sorption, transport and intracellular sequestration or precipitation as organic and inorganic compounds and biominerals, e.g. oxalates, carbonates and sulfides. In addition, reduction of higher-valency species may effect mobilization, e.g Fe(III) to Fe(II), Mn(IV) to Mn(II), or immobilization, e.g. U(VI) to U(IV), Cr(VI) to Cr(III) (see Gadd, G.M. et al. (2005), Microorganisms in Earth Systems - Advances in Geomicrobiology, Cambridge University Press). As well as being of global environmental significance, microbial metal transformations are relevant to plant productivity and human health, with application to the treatment of pollution (bioremediation). For bioremediation, solubilization of metal contaminants provides a means of removal from soils, sediments, and solid industrial wastes. Alternatively, immobilization processes may enable metals or radionuclides to be transformed in situ and are applicable to removing metals from aqueous solution.
Research in this laboratory concentrates on the fundamental chemical and biological mechanisms by which microorganisms change the mobility and toxicity of metals, and the importance of such mechanisms in natural environments and their application for treatment of pollution. Current research interests relate to microbial interactions with toxic metals, metalloids and radionuclides. Most research concentrates on free-living and mycorrhizal fungal systems, but other key organisms include sulphate-reducing bacteria. This research has ranged from cellular, biochemical and molecular aspects to the environment and biotechnology, and has lead to an understanding of the processes underlying accumulation, detoxification and tolerance, as well as microbial roles and involvement in environmental biogeochemistry. The environmental and biotechnological significance of these phenomena is a consistent focus, particularly in studies on bioweathering, mineral formation and dissolution, and in bioremediation applications for metals and radionuclides. We are also interested in microbial responses to metal and organic co-pollution, as well as fungal morphogenesis especially in the context of metal-mineral interactions and heterogeneous environments. Current projects relating to the functional consequences and modelling of fungal growth in heterogeneous environments are in collaboration with Dr Fordyce Davidson, Division of Mathematics.
Biomineral formation by fungi and sulphate-reducing bacteria. (A) a cord-forming fungus growing on copper phosphate (B) light micrograph of moolooite crystals (copper oxalate, CuC2O4.xH2O) around the hyphal cords (C) scanning electron micrograph of moolooite crystals associated with hyphal cord and mucilaginous sheath (Fomina, M. et al. (2005) Applied and Environmental Microbiology 71, 371-381) (D) a crust of calcium oxalate (weddelite and whewellite) crystals and tubular crystalline sheath around fungal hyphae (ESEM dry mode) (Gadd, G.M. et al. (2006) In: Fungi in the Environment, Cambridge University Press. (E) hydrated sulphate-reducing bacterial biofilm (Desulphomicrobium sp.) transforming selenite to abundant Se/S granules. Inset shows granules associated with the surface of an individual bacterium and precipitation in the extracellular matrix (Hockin, S.L. & Gadd, G.M. (2003) Applied and Environmental Microbiology 69, 7063-7072).
Diagram of an integrated microbial process for the bioremediation of metal-contaminated soil. For this, microbially-catalyzed reactions of the natural sulfur cycle were used. Bioleaching of contaminated soil was achieved using sulfur-oxidizing bacteria which produce sulfuric acid. Precipitation of leachate metals as insoluble metal sulfides was subsequently carried out using an anaerobic bioreactor containing a mixed culture of sulfate-reducing bacteria. Metals were effectively removed from the soils and leachates resulting in an effluent liquor with a very low metal content (see White, C. et al. (1998) Nature Biotechnology 16: 572-575). Other research has characterised important process variables as well as the application of SRB-biofilms in metal and metalloid bioprecipitation (see White, C. & Gadd, G.M. (1998) Microbiology 144, 1407-1415; Bridge, T.A.M. et al. Microbiology (1999) 145, 2987-2995; White, C. et al. (2003) Biodegradation (2003) 14, 139-151).