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,
immobilization by bioprecipitation
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 22.214.171.124), 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 126.96.36.199) 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
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).
For more information:
Medline for cadmium metabolism AND bacteria
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.