Microorganisms play a critical role in the transformation and immobilization of uranium in contaminated environments through four main pathways: bioreduction, biosorption, bioaccumulation, and biomineralization. These mechanisms reduce uranium’s toxicity and prevent its migration through groundwater systems, offering sustainable approaches for in situ bioremediation.
Bioreduction of Uranium
Bioreduction is driven by anaerobic bacteria such as certain strains of Geobacter and Shewanella, which use U(VI) as a terminal electron acceptor. Through complex electron transport systems involving c-type cytochromes (e.g., OmcA, MtrC), these microbes enzymatically reduce soluble and mobile U(VI) to insoluble U(IV), precipitating as uraninite (UO₂). Environmental conditions such as pH, presence of acetate or other electron donors, and the specific cytochrome composition influence the efficiency of this process. Desulfovibrio species, particularly under sulfate-reducing conditions, also facilitate U(VI) reduction via cytochromes such as cytochrome c₃.
Biosorption and Bioaccumulation
Biosorption involves passive binding of uranium to functional groups like carboxyl, hydroxyl, and phosphate moieties on microbial cell surfaces. This non-metabolic process is effective even in dead biomass. On the other hand, bioaccumulation requires active or facilitated uptake of uranium into cells, often followed by intracellular binding or precipitation. Notably, acidophilic and alkaliphilic strains from uranium-contaminated environments show variation in uranium uptake and retention capacity depending on external conditions.
Biomineralization via Phosphatases
Biomineralization relies on the enzymatic release of phosphate, which reacts with uranyl ions to form stable minerals such as autunite. Phosphatases—especially acid phosphatases like PhoN—mediate this process. Studies highlight the role of strains such as Serratia, Caulobacter, and Microbacterium in facilitating uranium-phosphate precipitation, even under low uranium concentrations (<1 µM), relevant to environmental standards. The location and efficiency of mineralization vary with pH and phosphate availability, emphasizing the adaptability of microbial pathways in diverse geochemical settings.
Microorganisms immobilize uranium from contaminated environments by transforming it into less soluble forms through mechanisms such as bioreduction, biosorption, bioaccumulation, and biomineralization.
Hexavalent uranium is water-soluble, which migrates easily through groundwater and contaminates surrounding areas. In bioreduction, microbes such as Geobacter and Shewanella use their c-type cytochromes to convert hexavalent uranium into a tetravalent form.
The tetravalent uranium then precipitates as solid uraninite, reducing its mobility and containing the contamination.
Biosorption involves the passive binding of uranium ions to functional groups such as carboxyl, phosphate, and hydroxyl groups on microbial surfaces, reducing the contamination.
In bioaccumulation, microbes absorb uranium and transport it inside their cells, where it is stored, often bound to polyphosphate granules.
Some microbes, including Microbacterium and Caulobacter, trigger biomineralization by enzymatically releasing phosphate, which reacts with uranium ions to form stable uranium-phosphate minerals.
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