Nickel is often found in biological compounds as Ni(II). This element composes only 0.008% of the Earth's crust and thus, like cobalt, it is less available to biological systems than iron. The role of nickel in biological systems has been appreciated only since 1975, when urease was shown to be a nickel enzyme (Dixon et al, 1975). Although the source of the urease in that study was jack bean, recent work has demonstrated that bacterial urease also contains nickel (reviewed by Mobley et al, 1995). Since 1975, six more nickel-containing enzymes have been discovered in bacteria and/or archaea: hydrogenase, methyl-S-coenzyme M reductase, carbon monoxide dehydrogenase, nickel superoxide dismutase, glyoxylase I, and a putative nickel cis-trans isomerase (reviewed by Watt and Ludden, 1999).

Nickel transport and homeostatic mechanisms have been studied in a variety of microorganisms (reviewed by Watt and Ludden, 1999). Non-specific nickel influx occurs via the CorA system in bacteria and Saccharomyces cerevisiae (reviewed by Nies, 1999). High-affinity nickel transport in bacteria occurs via ABC-type transporters and HoxN-type permeases (reviewed by Eitinger and Mandrand-Berthelot, 2000). Nickel resistance in some bacteria is based on nickel efflux via RND-type transporters (Liesegang et al, 1993; Schmidt and Schlegel, 1994) and evidence suggests that excess nickel in Staphylococcus aureus is bound by polyphosphate (Gonzalez and Jensen, 1998). Nickel urease function is essential for the colonization of gastric mucosa by Helicobacter pylori, and this organism has all of the known bacterial nickel transport systems (reviewed by Nies, 1999). In fungi, nickel uptake can occur via magnesium transport systems, and nickel detoxification is achieved by sequestration of nickel (possibly complexed with histidine) in the vacuole (reviewed by Joho et al, 1995).

Nickel toxicity is of some environmental concern (ToxFAQs: Nickel). Bioreduction of nickel has not been documented, but microorganisms can immobilize soluble nickel by bioaccumulation and bioprecipitation. Magyarosy et al (2002) reported that a strain of Aspergillus niger selectively removed nickel from solution in the presence of other metals, which resulted in the precipitation of nickel oxalate crystals on the cell wall. Sar et al (2001) found that Pseudomonas aeruginosa accumulated nickel in the cell envelope as nickel phosphide and nickel carbide crystals. Escherichia coli expressing a cloned Citrobacter sp. alkaline phosphatase immobilized nickel in a complex with hydrogen uranyl phosphate (Basnakova et al, 1998). Mogollon et al (1998) evaluated the biosorptive capabilities of fungal biomass from several genera.

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