Enzymically mediated bioprecipitation of uranium by a Citrobacter sp.: a concerted role for exocellular lipopolysaccharide and associated phosphatasein biomineral formation
Le. Macaskie et al., Enzymically mediated bioprecipitation of uranium by a Citrobacter sp.: a concerted role for exocellular lipopolysaccharide and associated phosphatasein biomineral formation, MICROBIO-UK, 146, 2000, pp. 1855-1867
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.