Microbial Bioremediation of Uranium

Anaerobic bacteria like Geobacter and Shewanella use c-type cytochromes to convert soluble hexavalent uranium into insoluble tetravalent uranium. This reduced form precipitates as uraninite, a solid mineral that immobilizes uranium and prevents its migration through groundwater, containing contamination at the source.

Biosorption is a passive, non-metabolic process where uranium ions bind to functional groups like carboxyl and phosphate on microbial cell surfaces, even in dead biomass. Bioaccumulation requires active uptake into living cells, where uranium is stored internally, often bound to polyphosphate granules, making it metabolically dependent.

Phosphatase enzymes, particularly acid phosphatases like PhoN, enzymatically release phosphate ions that react with uranyl ions to form stable uranium-phosphate minerals such as autunite. Microbes including Serratia, Caulobacter, and Microbacterium facilitate this biomineralization process, effectively immobilizing uranium even at low environmental concentrations.

Hexavalent uranium is highly water-soluble, allowing it to migrate easily through groundwater and contaminate surrounding areas. Tetravalent uranium, produced through microbial bioreduction, is insoluble and precipitates as uraninite, dramatically reducing its mobility and preventing further environmental spread of contamination at the source.

Geobacter and Shewanella species are primary bioreducers, using U(VI) as a terminal electron acceptor through complex electron transport systems. Desulfovibrio species also facilitate uranium reduction under sulfate-reducing conditions via cytochrome c₃. These anaerobic bacteria are most efficient when electron donors like acetate are available.

pH, availability of electron donors such as acetate, phosphate concentration, and microbial cytochrome composition all affect uranium immobilization efficiency. Acidophilic and alkaliphilic strains show variation in uranium uptake depending on external conditions, and mineralization efficiency varies with pH and phosphate availability in diverse geochemical settings.

Uranium bioremediation uses immobilization mechanisms like bioreduction and biomineralization, similar to how microbes address other contaminants. Like microbial bioremediation of pesticides, uranium remediation leverages microbial metabolic pathways to transform toxic compounds into less mobile or less toxic forms in situ.