b Selenium species, particularly the oxyanions selenite (SeO 3 2؊ ) and selenate (SeO 4 2؊ ), are significant pollutants in the environment that leach from rocks and are released by anthropogenic activities. Selenium is also an essential micronutrient for organisms across the tree of life, including microorganisms and human beings, particularly because of its presence in the 21st genetically encoded amino acid, selenocysteine. Environmental microorganisms are known to be capable of a range of transformations of selenium species, including reduction, methylation, oxidation, and demethylation. Assimilatory reduction of selenium species is necessary for the synthesis of selenoproteins. Dissimilatory reduction of selenate is known to support the anaerobic respiration of a number of microorganisms, and the dissimilatory reduction of soluble selenate and selenite to nanoparticulate elemental selenium greatly reduces the toxicity and bioavailability of selenium and has a major role in bioremediation and potentially in the production of selenium nanospheres for technological applications. Also, microbial methylation after reduction of Se oxyanions is another potentially effective detoxification process if limitations with low reaction rates and capture of the volatile methylated selenium species can be overcome. This review discusses microbial transformations of different forms of Se in an environmental context, with special emphasis on bioremediation of Se pollution. Since the discovery in 1954 by Pinsent that the oxidation of formate by cell suspensions of Escherichia coli requires growth medium containing molybdate and selenite, there has been a growing interest in the biochemical role of selenium in microorganisms (1). Se is an essential component of selenoamino acids, such as selenomethionine and selenocysteine (the 21st proteinogenic amino acid), that occur in certain types of prokaryotic enzymes. Indeed, the requirement for selenite in E. coli growing on formate is linked to the fact that formate dehydrogenase contains selenocysteine. Other prokaryotic enzymes that contain selenocysteine include glycine reductase in several clostridia, formate dehydrogenases in diverse prokaryotes, including Salmonella, Clostridium, and Methanococcus, as well as hydrogenases in Methanococcus and other anaerobes. In addition, other bacterial Se-dependent enzymes, in which the selenium is part of the active site molybdenum-containing cofactor, include nicotinic acid dehydrogenase and xanthine dehydrogenase, which is present in certain clostridial species (2-4).Reactions that are involved in the cycling of Se in soil, including those influenced by microbes, are diagrammatically summarized in Fig. 1 2Ϫ and was spatially separated from sulfate reduction in the environment despite the presence of substantial concentrations of sulfate where it occurred. Thus, it can be concluded that Se and S have different reductive biogeochemical cycles and appear to involve distinct populations of microorganisms.With respect to the remediation of sel...

Methane-oxidizing bacteria are well known for their role in the global methane cycle and their potential for microbial transformation of wide range of hydrocarbon and chlorinated hydrocarbon pollution. Recently, it has also emerged that methane-oxidizing bacteria interact with inorganic pollutants in the environment. Here, we report what we believe to be the first study of the interaction of pure strains of methane-oxidizing bacteria with selenite. Results indicate that the commonly used laboratory model strains of methane-oxidizing bacteria, Methylococcus capsulatus (Bath) and Methylosinus trichosporium OB3b, are both able to reduce the toxic selenite (SeO3 2−) but not selenate (SeO4 2−) to red spherical nanoparticulate elemental selenium (Se0), which was characterized via energy-dispersive X-ray spectroscopy (EDXS), X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The cultures also produced volatile selenium-containing species, which suggests that both strains may have an additional activity that can transform either Se0 or selenite into volatile methylated forms of selenium. Transmission electron microscopy (TEM) measurements and experiments with the cell fractions cytoplasm, cell wall and cell membrane show that the nanoparticles are formed mainly on the cell wall. Collectively, these results are promising for the use of methane-oxidizing bacteria for bioremediation or suggest possible uses in the production of selenium nanoparticles for biotechnology.Electronic supplementary materialThe online version of this article (doi:10.1007/s00253-017-8380-8) contains supplementary material, which is available to authorized users.

Microorganisms such as Stenotrophomonas bentonitica could influence the safety of the deep geological repository system by producing nanoparticles and volatile compounds of selenium.

Aerobic methane-oxidizing bacteria are ubiquitous in the environment. Two well-characterized strains, Mc. capsulatus (Bath) and Methylosinus trichosporium OB3b, representing gamma- and alphaproteobacterial methanotrophs, respectively, can convert selenite, an environmental pollutant, to volatile selenium compounds and selenium-containing particulates. Both conversions can be harnessed for the bioremediation of selenium pollution using biological or fossil methane as the feedstock, and these organisms could be used to produce selenium-containing particles for food and biotechnological applications. Using an extensive suite of techniques, we identified precursors of selenium nanoparticle formation and also found that these nanoparticles are made up of eight-membered mixed selenium and sulfur rings.

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Despite the wide-ranging proscription of hexavalent chromium, chromium (VI) remains among the major polluting heavy metals worldwide. Aerobic methane-oxidizing bacteria are widespread environmental microorganisms that can perform diverse reactions using methane as the feedstock. The methanotroph Methylococcus capsulatus Bath, like many other microorganisms, detoxifies chromium (VI) by reduction to chromium (III). Here, the interaction of chromium species with M. capsulatus Bath was examined in detail by using a range of techniques. Cell fractionation and HPLC-inductively coupled plasma-mass spectrometry (HPLC-ICP-MS) indicated that externally provided chromium (VI) underwent reduction, and was then taken up into the cytoplasmic and membranous fractions of the cells. This was confirmed by X-ray photoelectron spectroscopy (XPS) of intact cultures that indicated negligible chromium on the surface of, or outside, the cells. Distribution of chromium and other elements within intact and sectioned cells, as observed via transmission electron microscope (TEM) combined with energy-dispersive X-ray spectroscopy (EDX), and electron energy loss spectroscopy (EELS), was consistent with the cytoplasm/membrane location of the chromium (III), possibly as chromium phosphate. The cells could also take up chromium (III) directly from the medium in a metabolically dependent fashion, and accumulate it within the cells. These results indicate a novel pattern of interaction with chromium species distinct from that observed previously with other microorganisms. They also suggest that M. capsulatus, and similar methanotrophs may contribute directly to chromium (VI) reduction, and accumulation in mixed communities of microorganisms that are able to perform methane-driven remediation of chromium (VI). Importance M. capsulatus Bath is a well characterised aerobic methane-oxidising bacterium that has become a model system for biotechnological development of methanotrophs to perform useful reactions for environmental clean-up, and making valuable chemicals and biological products using methane gas. Interest in such technology has increased recently owing to increasing availability of low-cost methane from fossil, and biological sources. Here, it is demonstrated that the ability of this versatile methanotroph to reduce the toxic contaminating heavy metal chromium (VI) to less toxic chromium (III) form occurs at the same time as accumulation of the chromium (III) within the cells. This is expected to diminish the bioavailability of the chromium, and make it less likely to be re-oxidised to the toxic chromium (VI). Thus M. capsulatus has the capacity to perform methane-driven remediation of chromium-contaminated water, and other materials, and to accumulate the chromium in the low-toxicity chromium (III) form within the cells.

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