Phosphatase-mediated heavy metal accumulation by a Citrobacter sp. and related enterobacteria

Abstract

A Citrobacter sp. was reported previously to accumulate heavy metals as cell-bound heavy metal phosphates. Metal uptake is mediated by the activity of a periplasmic acid-type phosphatase that liberates inorganic phosphate to provide the precipitant ligand for heavy metals presented to the cells. Amino acid sequencing of peptide fragments of the purified enzyme revealed significant homology to the phoN product (acid phosphatase) of some other enterobacteria. These organisms, together with Klebsiella pneumoniae, previously reported to produce acid phosphatase, were tested for their ability to remove uranium and lanthanum from challenge solutions supplemented with phosphatase substrate. The coupling of phosphate liberation to metal bioaccumulation was limited to the metal accumulating Citrobacter sp.; therefore the participation of species-specific additional factors in metal bioaccumulation was suggested.

Biomineralization of Uranium by PhoY Phosphatase Activity Aids Cell Survival in Caulobacter crescentus

“…Given that U precipitates formed in the presence of cells contained larger amounts of P i than did the abiotic control (Table 3), we hypothesized that P i production by phosphatases facilitates U biomineralization, which has been observed in other systems (17)(18)(19). Whole-cell phosphatase activity of C. crescentus grown in PYE medium with and without U was tested with three different organic phosphate substrates: glucose-6-phosphate, fructose-1,6-bisphosphate, and glycerol-2-phosphate (see Fig.…”

“…S2 in the supplemental material) (18). Nevertheless, PhoY and PhoK belong to a group that is clearly distinct from acid phosphatases (e.g., PhoN in Salmonella enterica) that have been implicated in U biomineralization as well as other annotated phosphatases in C. crescentus (17,19,20).…”

“…strain N14), Sphingomonas sp. BSAR-1, Arthrobacter, Rahnella, and Bacillus has been found to facilitate U(VI) precipitation through the formation of uranium phosphate complexes (17)(18)(19). Phosphatases from these organisms have been cloned and expressed in Escherichia coli and other organisms, and the resulting engineered strains have been reported to efficiently precipitate uranium (20)(21)(22).…”

Localization of enzymically enhanced heavy metal accumulation by Citrobacter sp. and metal accumulation in vitro by liposomes containing entrapped enzyme

“…However, little is known of the cellular localization of this phosphatase in vivo and the mechanism(s) by which enzymic activity contributes to the metal bioaccumulation process. Factors other than phosphate release alone are implicated in effective metal removal (Macaskie et al, 1995a); indeed, some other enterobacteria containing phoN removed little heavy metal from solution (Macaskie et al, 1994b).…”

“…This organism has been applied to the removal of uranium from mine water (Roig et al, 1995;Macaskie et al, 1996Macaskie et al, ,1997 and to the accumulation of radiotoxic elements such as americium and plutonium (Macaskie et al, 1994a. Metal uptake requires a cellular phosphatase, homologous to the phoN product of Salmonella and some other enterobacteria (Macaskie et al, 1994b), which liberates inorganic phosphate from a supplied organic phosphate substrate to precipitate HPO;. with metals (M) extensively as cell-bound MHPO, (Macaskie et al, 1992Tolley et al, 1995;Yong & Macaskie, 1995).…”