Abstract
BackgroundSelenium (Se) is an essential trace element, but is toxic at high concentrations. Depending upon the geological background, the land use or on anthropogenic pollution, different amounts of Se may be present in soil. Its toxicity is related to the oxyanions selenate and selenite as they are water soluble and bioavailable. Microorganisms play an important role in Se transformations in soil and its cycling in the environment by transforming water-soluble oxyanions into water insoluble, non-toxic elemental Se (0). For this study, soil samples were collected from selenium-contaminated agricultural soils of Punjab/India to enrich and isolate microbes that interacted with the Se cycle.ResultsA mixed microbial culture enriched from the arable soil of Punjab could reduce 230 mg/l of water soluble selenite to spherical Se (0) nanoparticles during aerobic growth as confirmed by SEM-EDX. Four pure cultures (C 1, C 4, C 6, C 7) of Gram negative, oxidase and catalase positive, aerobic bacteria were isolated from this mixed microbial consortium and identified by 16 S rDNA gene sequence alignment as two strains of Duganella sp. (C 1, C 4) and two strains of Agrobacterium sp.(C 6, C 7). SEM/TEM-EDX analyses of the culture broth of the four strains revealed excretion of uniformly round sharply contoured Se (0) nanoparticles by all cultures. Their size ranged from 140–200 nm in cultures of strains C 1 and C 4, and from 185–190 nm in cultures of strains C 6 and C 7. Both Duganella sp. revealed better selenite reduction efficiencies than the two Agrobacterium sp.ConclusionsThis is the first study reporting the capability of newly isolated, aerobically growing Duganella sp. and Agrobacterium sp. from soils of Punjab/India to form spherical, regularly formed Se (0) nanoparticles from water soluble selenite. Among others, the four strains may significantly contribute to the biogeochemical cycling of Se in soil. Bioconversion of toxic selenite to non-toxic Se (0) nanoparticles under aerobic conditions in general may be useful for detoxification of agricultural soil, since elemental Se may not be taken up by the roots of plants and thus allow non-dangerous fodder and food production on Se-containing soil.
Highlights
Selenium (Se) is an essential trace element, but is toxic at high concentrations
In this study we report the isolation of pure cultures of two Duganella species and two Agrobacterium species from Se rich sediments of Punjab
Enrichment of aerobic selenite reducing bacteria from soil of the Punjab area in India From a soil slurry in enrichment medium (EM) a mixed culture free of soil particles was obtained after several transfers into fresh EM that could grow in the presence of 40 mg/l of Se (IV), or reduce this amount of Se (IV) in less than 2 days
Summary
Selenium (Se) is an essential trace element, but is toxic at high concentrations. Depending upon the geological background, the land use or on anthropogenic pollution, different amounts of Se may be present in soil. Within the four inorganic Se oxidation states (−II, 0, +IV, +VI) found in nature, the oxyanions selenate and selenite are most mobile and detrimental as they are bioavailable and can be taken up by the plants from Se-rich soil or Se-containing irrigation water and enter the plant-animal/human food chain, posing a health risk for animals and humans [4,6] Diseases, such as skin lesions and hair fall as a result of consuming water or plants grown in north-east Punjab, India have been associated with the high Se concentration in soil and irrigation water of that area [4,7]. The variety of Se nanoparticle-forming aerobic bacteria is less known compared to anaerobic/anoxic bacteria and is confined mainly to species of Pseudomonas and Bacillus such as
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