Abstract
BackgroundSelenite (SeO32−) oxyanion shows severe toxicity to biota. Different bacterial strains exist that are capable of reducing SeO32− to non-toxic elemental selenium (Se0), with the formation of Se nanoparticles (SeNPs). These SeNPs might be exploited for technological applications due to their physico-chemical and biological characteristics. The present paper discusses the reduction of selenite to SeNPs by a strain of Bacillus sp., SeITE01, isolated from the rhizosphere of the Se-hyperaccumulator legume Astragalus bisulcatus.ResultsUse of 16S rRNA and GyrB gene sequence analysis positioned SeITE01 phylogenetically close to B. mycoides. On agarized medium, this strain showed rhizoid growth whilst, in liquid cultures, it was capable of reducing 0.5 and 2.0 mM SeO32− within 12 and 24 hours, respectively. The resultant Se0 aggregated to form nanoparticles and the amount of Se0 measured was equivalent to the amount of selenium originally added as selenite to the growth medium. A delay of more than 24 hours was observed between the depletion of SeO32 and the detection of SeNPs. Nearly spherical-shaped SeNPs were mostly found in the extracellular environment whilst rarely in the cytoplasmic compartment. Size of SeNPs ranged from 50 to 400 nm in diameter, with dimensions greatly influenced by the incubation times. Different SeITE01 protein fractions were assayed for SeO32− reductase capability, revealing that enzymatic activity was mainly associated with the membrane fraction. Reduction of SeO32− was also detected in the supernatant of bacterial cultures upon NADH addition.ConclusionsThe selenite reducing bacterial strain SeITE01 was attributed to the species Bacillus mycoides on the basis of phenotypic and molecular traits. Under aerobic conditions, the formation of SeNPs were observed both extracellularly or intracellullarly. Possible mechanisms of Se0 precipitation and SeNPs assembly are suggested. SeO32− is proposed to be enzimatically reduced to Se0 through redox reactions by proteins released from bacterial cells. Sulfhydryl groups on peptides excreted outside the cells may also react directly with selenite. Furthermore, membrane reductases and the intracellular synthesis of low molecular weight thiols such as bacillithiols may also play a role in SeO32− reduction. Formation of SeNPs seems to be the result of an Ostwald ripening mechanism.
Highlights
Selenite (SeO32−) oxyanion shows severe toxicity to biota
Taxonomic identification of the strain SeITE01 The bacterial strain SeITE01 was isolated from the rhizosphere of the Se-hyperaccumulator plant Astragalus bisulcatus grown on a Se-polluted soil through enrichment cultures spiked with 2.0 mM sodium selenite, as described previously [26]
Sequencing of the whole 16S rRNA gene confirmed that strain SeITE01 can be associated to the Bacillus cereus group which includes B. cereus, B. thuringensis, B. anthracis, B. mycoides, B. pseudomycoides, B. cytotoxicus and B. wheihenstephanensis [27]
Summary
Selenite (SeO32−) oxyanion shows severe toxicity to biota. Different bacterial strains exist that are capable of reducing SeO32− to non-toxic elemental selenium (Se0), with the formation of Se nanoparticles (SeNPs). Selenium contamination represents an important public health concern and requires remediation initiatives especially in those geographic locations where agricultural irrigation drainage waters transport significant amounts of Se by leaching seleniferous soils Industrial activities such as oil refining, phosphate and metal ore mining and coal fire-based power production can all contribute to the dispersion of selenium in the environment. It is worth noting that a large number of bacterial species, residing in diverse terrestrial and aquatic environments, possess the ability to reduce selenite and selenate into elemental selenium This can occur through both enzymatic or non enzymatic mechanisms, leading to the formation of Se nanostructured particles (SeNPs) which are deposited inside the cell (cytoplasmic), within the periplasm or extracellularly [9,10,11,12,13,14]. The involvement of thiol-containing proteins such as glutathione has even been identified in some Gram negative bacteria capable of anaerobic reduction of Se032− to amorphous Se0 nanoparticles [21]
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