Although selenium is an essential trace element for living organisms, selenium oxyanions (selenite and selenate) are highly toxic at high concentrations. Some microorganisms can reduce the oxidized forms of selenium into the elemental form which is insoluble, strongly resistant to oxidation and, therefore less toxic and mobile. Ralstonia metallidurans strain CH34, a soil bacterium characteristic of metal-contaminated biotopes, has been recently described to resist up to 6 mM selenite. This resistance could be due to a mechanism of bioreduction of selenite followed by an immobilization of elemental selenium in the biomass. This confers a strong interest for this strain in application for environmental bioremediation.
The main purpose of this study was to investigate the ability of R. metallidurans CH34 to resist, incorporate and reduce selenite or selenate as a function of the selenium oxide concentrations. Selenium concentration in the bacteria was determined using inductively coupled plasma - mass spectrometer analysis and compared to the concentrations of total selenium in the culture media over microbial growth. Our results suggest an adaptation of the bacteria to both selenite and selenate stress. Kinetics of bioaccumulation of the selenium oxyanions by R. metallidurans CH34 have been quantified for the first time.
The results suggest that R. metallidurans CH34 will tolerate and survive at very high selenite or selenate concentrations (respectively 2 and 10 mM). It was found to resist high selenium concentrations by accumulating selenite and to a lesser extend selenate and by reducing toxic Se (IV) into the insoluble, immobilized Se (0) form.
* UMR5075 UJF/CEA/CNRS Laboratoire des Protéines Membranaires, IBS, 41 Rue Jules Horowitz, 38027 Grenoble Cedex 1, France