Cadmium is a highly toxic heavy metal element that is ubiquitous in the Earth's crust (ToxFAQs: Cadmium). Despite its toxicity, cadmium is widely used in industry. More than one half of the sites on the Environmental Protection Agency's National Priorities List are contaminated with cadmium. Microbial interactions with cadmium have been explored extensively in a wide variety of prokaryotes, fungi, and algae. Research has focused on cadmium transport and detoxification mechanisms, and cadmium immobilization by bioprecipitation and biosorption.

No essential biological function for cadmium has been identified, but a cadmium-dependent carbonic anhydrase has been reported in the marine diatom Thalassiosira weissflogii (Lane and Morel, 2000). Molecular mechanisms of cadmium uptake in microorganisms have not been well characterized, but uptake may occur via magnesium, manganese, or calcium transport systems (reviewed by Nies, 1999). Plasmid -based cadmium efflux systems in bacteria have been identified. Gram-positive bacteria export cadmium via P-type ATPases (EC 3.6.3.3), and Gram-negative bacteria use Czc (cadmium, zinc, cobalt) divalent cation transporters for cadmium efflux (reviewed by Silver, 1998).

Cadmium-binding proteins have an important role in moderating cadmium toxicity in some fungi and bacteria. A metallothionein encoded by the CUP1 gene binds Cd²⁺ in Candida glabrata, the yeast cadmium factor (YCF1, EC 3.6.3.46) mediates accumulation of cadmium -glutathione complexes in Saccharomyces cerevisiae vacuoles, and metal -binding peptides ("phytochelatins") sequester cadmium in subcellular organelles in Schizosaccharomyces pombe and C. glabrata (reviewed by Perego and Howell, 1997). Cadmium-chelating proteins have also been discovered in filamentous fungi (Razak, 1989). Cadmium-binding metallothioneins have been identified in cyanobacteria (reviewed by Turner and Robinson, 1995), and cadmium-binding proteins have been reported in Pseudomonas putida (Higham et al, 1986) and Escherichia coli (Khazeli and Mitra, 1981).

The potential for microorganisms to immobilize or volatilize soluble cadmium has been explored. Direct reduction of cadmium has not been observed, but biomethylation of cadmium by polar marine bacteria has been reported (Pongratz and Heumann, 1999). Several microorganisms have been shown to precipitate soluble cadmium as insoluble sulfides (Holmes et al, 1997), phosphates (Montgomery et al, 1995), or carbonates (Cunningham and Lundie, 1993). Biomass from several bacterial, fungal, and algal species has been evaluated as biosorbents for the removal of soluble cadmium from solution (Volesky and Holan, 1995).

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Khazeli MB, Mitra RA. Cadmium-binding component in Eschericia coli during accommodation to low levels of this ion. Appl Environ Microbiol. 1981;41:46-50.

Pongratz R, Heumann KG. Production of methylated mercury, lead, and cadmium by marine bacteria as a significant natural source for atmospheric heavy metals in polar regions. Chemosphere. 1999;39:89-102.