Bioremediation of selenium species in solution by methanotrophic bacteria

Abstract

Selenium species, particularly the oxyanions selenite and selenate, are significant pollutants in the environment that leach from rocks and are also released by anthropogenic activities. In the environment, microbial transformations of selenium play important roles in the biogeochemical cycles of the element. For instance, microbial reduction of the toxic and water-soluble selenium oxyanions to nanoparticulate elemental selenium greatly reduces the toxicity and bioavailability of selenium and has a major role in bioremediation. Also, microbial methylation after reduction of selenium oxyanions is another potentially effective detoxification process if limitations with low reaction rates and capture of the volatile methylated selenium species can be overcome. 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, the selenium-transforming properties of methane-oxidizing bacteria have been investigated for the first time. The interaction of selenium containing chemical species has been studied with pure strains of the commonly used laboratory model strains of methane-oxidizing bacteria, Methylococcus capsulatus (Bath) and Methylosinus trichosporium OB3b. The two strains were both able to convert the toxic selenite but not selenate or DL-selenocystine to extracellular red spherical nanoparticulate elemental selenium, which was confirmed by X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The selenium nanoparticles were characterized by a variety of techniques, including transmission electron microscopy (TEM) energy dispersive X-ray (EDX) spectrometry, high-angle annular dark-field (HAADF) scanning TEM (STEM), X-ray photoelectron spectroscopy (XPS), SDS-PAGE analyses, Fourier-transform infrared spectroscopy (FTIR), zeta potential and Raman spectroscopy. The results showed that the reduction process is an enzymatic reaction and mediated by cell wall-associated proteins. The elemental red selenium nanoparticles formed during selenite reduction were found to be amorphous containing a certain amount of sulfur. The results also indicated that the produced selenium nanoparticles are coated with organic materials, likely to be proteins and extracellular polymeric substances (EPS). The cultures also produced volatile selenium-containing species when challenged with selenite, which shows that both strains have an additional activity that can transform either elemental selenium or selenite into volatile methylated forms of selenium, including dimethyl selenide, dimethyl diselenide, dimethyl selenenyl sulphide, methylselenol and methylselenoacetate. Selenate (at concentrations higher than 100 μg mL-1) or DL-selenocystine-amended cultures of Methylococcus capsulatus (Bath) (but not Methylosinus trichosporium OB3b) produced volatile selenium-containing species. From a biotechnological standpoint, these results are promising for the use of methane-oxidizing bacteria for bioremediation of selenium-contaminated environments and suggest possible uses in the production of selenium nanoparticles for technological applications.