Selenium: environmental significance, pollution, and biological treatment technologies

SUMMARY In nature, selenium is actively cycled between oxic and anoxic habitats, and this cycle plays an important role in carbon and nitrogen mineralization through bacterial anaerobic respiration. Selenium-respiring bacteria (SeRB) are found in geographically diverse, pristine or contaminated environments and play a pivotal role in the selenium cycle. Unlike its structural analogues oxygen and sulfur, the chalcogen selenium and its microbial cycling have received much less attention by the scientific community. This review focuses on microorganisms that use selenate and selenite as terminal electron acceptors, in parallel to the well-studied sulfate-reducing bacteria. It overviews the significant advancements made in recent years on the role of SeRB in the biological selenium cycle and their ecological role, phylogenetic characterization, and metabolism, as well as selenium biomineralization mechanisms and environmental biotechnological applications.

Ecology and Biotechnology of Selenium-Respiring Bacteria

Selenium species, particularly the oxyanions selenite (SeO3 (2-)) and selenate (SeO4 (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.

Microbial transformations of selenium species of relevance to bioremediation

Microbial biofilms, which characterized by their resistance to the traditional antimicrobials, are considered as a renewable source of contamination by pathogens. As alternative antimicrobial and antibiofilm agent selenium nanoparticles (SeNPs) were biosynthesized by treatment of 1 mM SeO 2 with the free-cell supernatant of Bacillus licheniformis isolated from food wastes. The biosynthesized SeNPs were characterized by their spherical shape with diameter range of 10–50 nm and a well-defined absorption peak at 263 nm in UV–vis spectra. The biosynthesized SeNPs were used to control growth and biofilm formation by six foodborne pathogens including Bacillus cereus , Enterococcus faecalis , Staphylococcus aureus , Escherichia coli O157:H7, Salmonella Typhimurium, and Salmonella Enteritidis. The MIC 90 of SeNPs against all tested bacteria was 25 μg/mL, whereas the antibiofilm concentration was 20 μg/mL against all bacteria, except B. cereus . Although the biogenic SeNPs had antimicrobial and antibiofilm effects, they did not show ability to remove the established biofilm up to 50 μg/mL. The concentration of 75 μg/mL showed slight effect on removing the established biofilm. No toxicity on Artemia larvae was demonstrated by SeNPs upto 100 μg/mL. In conclusion, SeNPs produced in the present study can be used as a promising agent for effectively preventing biofilm formation by foodborne pathogens.

Antimicrobial and antibiofilm effects of selenium nanoparticles on some foodborne pathogens

Heavy metal contamination is a global issue, where the prevalent contaminants are arsenic (As), cadmium (Cd), chromium (Cr)(VI), mercury (Hg), and lead (Pb). More often, they are collectively known as "most problematic heavy metals" and "toxic heavy metals" (THMs). Their treatment through a variety of biological processes is one of the prime interests in remediation studies, where heavy metal-microbe interaction approaches receive high interest for their cost effective and ecofriendly solutions. In this review, we provide an up to date information on different microbial processes (bioremediation) for the removal of THMs. For the same, emphasis is put on oxidation-reduction, biomineralization, bioprecipitation, bioleaching, biosurfactant technology, biovolatilization, biosorption, bioaccumulation, and microbe-assisted phytoremediation with their selective advantages and disadvantages. Further, the literature briefly discusses about the various setups of cleaning processes of THMs in environment under ex situ and in situ applications. Lately, the study sheds light on the manipulation of microorganisms through genetic engineering and nanotechnology for their advanced treatment approaches.

Bioremediation of toxic heavy metals (THMs) contaminated sites: concepts, applications and challenges.

Characterization of Pseudomonas stutzeri NT-I capable of removing soluble selenium from the aqueous phase under aerobic conditions.

Effective selenium volatilization under aerobic conditions and recovery from the aqueous phase by Pseudomonas stutzeri NT-I.

The recovery of tellurium as an important rare metal from wastes and wastewater must be promoted to satisfy its increasing demand for use in many industries. For development of a biological process for recovering tellurium from wastewater, three bacterial strains capable of reducing soluble tellurite into insoluble elemental tellurium were isolated from the drainage wastewater of a metal refinery plant. The bacterial strains, Stenotrophomonas maltophilia TI-1, Ochrobactrum anthropi TI-2 and O. anthropi TI-3, resisted up to 30 mM tellurite, and removed 86–89 % of 1 mM tellurite from water phase within 21 h in a reductive reaction. They showed a considerably higher tellurite removal rate than the previously reported tellurite-reducing bacteria. Tellurite-reducing activity was observed at 20–40 °C, pH 6.2–8.5 and 0.05–5 % NaCl (0.1 mM/h at maximum). The tellurite-reducing activities of strains TI-2 and TI-3 were not considerably different from each other, however strain TI-1 showed higher tellurite-reducing activity at low temperatures, low pH and high NaCl concentrations than the other strains. Because deposits of elemental tellurium were formed on and free from the bacterial cells, it is possible to recover tellurium from the water phase by centrifugation, ultrafiltration, or coagulation following tellurite reduction. All bacterial strains isolated in this study are inferred to be great candidates to establish bacterial tellurium recovery processes from industrial wastewater containing high concentrations of tellurite.

Isolation and Characterization of Bacteria Capable of Reducing Tellurium Oxyanions to Insoluble Elemental Tellurium for Tellurium Recovery from Wastewater

Biotreatment of Selenium Refinery Wastewater Using Pilot-Scale Granular Sludge and Swim-Bed Bioreactors Augmented with a Selenium-Reducing Bacterium Pseudomonas stutzeri NT-I

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Characterization of Pseudomonas stutzeri NT-I capable of removing soluble selenium from the aqueous phase under aerobic conditions.

Effective selenium volatilization under aerobic conditions and recovery from the aqueous phase by Pseudomonas stutzeri NT-I.

Isolation and Characterization of Bacteria Capable of Reducing Tellurium Oxyanions to Insoluble Elemental Tellurium for Tellurium Recovery from Wastewater

Biotreatment of Selenium Refinery Wastewater Using Pilot-Scale Granular Sludge and Swim-Bed Bioreactors Augmented with a Selenium-Reducing Bacterium Pseudomonas stutzeri NT-I