Enzymically mediated bioprecipitation of uranium by a Citrobacter sp.: a concerted role for exocellular lipopolysaccharide and associated phosphatasein biomineral formation

A Citrobacter sp. accumulated uranyl ion (UO22+) via precipitation with pho sphate ligand liberated by phosphatase activity. The onset and rate of uran yl phosphate deposition were promoted by NH4+, forming NH4UO2PO4, which has a lower solubility product than NaUO2PO4. This acceleration decoupled the rate-limiting chemical crystallization process from the biochemical phospha te ligand generation. This provided a novel approach to monitor the cell-su rface-associated changes using atomic-force microscopy in conjunction with transmission electron microscopy and electron-probe X-ray microanalysis, to visualize deposition of uranyl phosphate at the cell surface. Analysis of extracted surface materials by P-31 NMR spectroscopy showed phosphorus reso nances at chemical shifts of 0.3 and 2.0 p.p.m., consistent with monophosph ate groups of the lipid A backbone of the lipopolysaccharide (LPS). Additio n of UO22+ to the extract gave a yellow precipitate which contained uranyl phosphate, while addition of Cd2+ gave a chemical shift of both resonances to a single new resonance at 3 p.p.m. Acid-phosphatase-mediated crystal gro wth exocellularly was suggested by the presence of acid phosphatase, locali zed by immunogold labelling, on the outer membrane and on material exuded f rom the cells. Metal deposition is proposed to occur via an initial nucleat ion with phosphate groups localized within the LPS, shown by other workers to be produced exocellularly in association with phosphatase. The crystals are further consolidated with additional, enzymically generated phosphate i n close juxtaposition, giving high loads of LPS-bound uranyl phosphate with out loss of activity and distinguishing this from simple biosorption, or pe riplasmic or cellular metal accumulation mechanisms. Accumulation of 'tethe red' metal phosphate within the LPS is suggested to prevent fouling of the cell surface by the accumulated precipitate and localization of phosphatase exocellularly is consistent with its possible functions in homeostatis and metal resistance.