Chemical Forms of Selenium in the Metal-Resistant Bacterium Ralstonia metallidurans CH34 Exposed to Selenite andSelenate

Abstract

Ralstonia metallidurans CH34, a soil bacterium resistant to a variety of metals, is known to reduce selenite to intracellular granules of elemental selenium (Se 0 ). We have studied the kinetics of selenite (Se IV ) and selenate (Se VI ) accumulation and used X-ray absorption spectroscopy to identify the accumulated form of selenate, as well as possible chemical intermediates during the transformation of these two oxyanions. When introduced during the lag phase, the presence of selenite increased the duration of this phase, as previously observed. Selenite introduction was followed by a period of slow uptake, during which the bacteria contained Se 0 and alkyl selenide in equivalent proportions. This suggests that two reactions with similar kinetics take place: an assimilatory pathway leading to alkyl selenide and a slow detoxification pathway leading to Se 0 . Subsequently, selenite uptake strongly increased (up to 340 mg Se per g of proteins) and Se 0 was the predominant transformation product, suggesting an activation of selenite transport and reduction systems after several hours of contact. Exposure to selenate did not induce an increase in the lag phase duration, and the bacteria accumulated approximately 25-fold less Se than when exposed to selenite. Se IV was detected as a transient species in the first 12 h after selenate introduction, Se 0 also occurred as a minor species, and the major accumulated form was alkyl selenide. Thus, in the present experimental conditions, selenate mostly follows an assimilatory pathway and the reduction pathway is not activated upon selenate exposure. These results show that R. metallidurans CH34 may be suitable for the remediation of selenite-, but not selenate-, contaminated environments.Microorganisms play a major role in the biogeochemical cycle of selenium in the environment (12). Certain strains that are resistant to selenium oxyanions, and reduce selenite (Se IV ) and/or selenate (Se VI ) to the less available elemental selenium (Se 0 ) (7), could be potentially used for the bioremediation of contaminated soils, sediments, industrial effluents, and agricultural drainage waters.Ralstonia metallidurans CH34 is a soil bacterium characteristic of metal-contaminated biotopes. It is resistant to a variety of heavy metals and metalloids including Cr VI , Co II , Ni II , Cu II , Zn II , As V , Cd II , Hg II , and Pb II . The genes for metal resistance are located in two large plasmids (pMOL28 and pMOL30). Their function and regulation are well understood for some of these elements (18). This bacterial strain is also resistant to selenite, and detoxification is realized by the incorporation of this oxyanion and its subsequent reduction to red Se 0 , as shown by X-ray absorption spectroscopy (24). This study also revealed that the Se 0 granules were localized mainly in the cytoplasm. In contrast to previously cited metals and metalloids, the genes involved in selenite resistance have not yet been identified, and the exact mechanism of selenite bioreduction is still unknown. R....

Selenium Speciation in Biofilms from Granular Sludge Bed Reactors Used for Wastewater Treatment

“…Figure 2 shows the normalized Se K-edge XANES spectra collected for a selection of model compounds with characteristic selenium valences. The Se K-edge position is sensitive to the valence of Se [3,[7][8][9][10]. In contrast to previous studies, no simple relation is found between the Se K-edge position and the selenium valence.…”

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Genetic and Biochemical Evidence for the Involvement of a Molybdenum-Dependent Enzyme in One of the Selenite Reduction Pathways ofRhodobacter sphaeroidesf. sp.denitrificansIL106

“…In a few species of bacteria, selenate and selenite act as electron acceptors in an anaerobic form of respiration (16,33,34), whereas for most of the microorganisms studied so far, reduction of selenium oxyanions is not a bioenergetic process. Some species, including E. coli (36), Thauera selenatis (4,29), and Enterobacter cloacae SLD1a-1 (15), have been reported to reduce both selenate and selenite; others, like R. sphaeroides (38) and Ralstonia metallidurans (28), can only reduce selenite.…”

Effective bioreduction of hexavalent chromium–contaminated water in fixed-film bioreactors

“…In addition, various sequences were related to bacteria able to reduce a variety of elements and compounds, including ( et al, 1989;Wang et al, 1990;Rege et al, 1997); (v) selenium oxyanions, such as Enterobacter cloacae (Watts et al, 2003;Losi and Frankenberger, 1997;Ridley et al, 2006;Yee et al, 2007) and Ralstonia sp. (Valls et al, 2000;Sarret et al, 2005);and (vi) arsenic, such as Flavobacterium sp. (Macur et al, 2004).…”