Abstract:

Background and Aim: Contamination of soil and water resources with selenium oxyanions is a serious public health and environmental threat. Long-term exposure to high selenite concentrations can result in skin disorders, hair loss, liver and kidney necrosis, breathing problems, deformed nails and cell death.  In this regard, isolation and identification of indigenous selenite-resistant yeast strains can be used as safe and natural catalysts in the bio-remediation of the toxic selenite (SeO₃^-2).

Materials and Methods: In this experimental study, collected wastewater effluent and activated sludge samples are cultured into the selective Rose Bengal Chloramphenicol Agar medium containing selenite for selective screening of target strains of yeast. Agar dilution method and disk diffusion testing was performed to determine the intrinsic tolerance isolated yeast strains to selenite. The doubling time and the percentage of growth inhibition of the resistant isolated yeast strains in the presence of selenite were determined from standard growth curves. A colorimetric method using 3, 3-Diaminobenzidine hydrochloride has been developed for the microbial selenite removal. Phenotypic and phylogenetic analyses were used in the characterization of the potent yeast strain.

Results: Confirmation of the results from selenite tolerance pattern was performed on turbidity growth curves of the isolated yeast strains. The results showed that the strain se29w, isolated from waste rubber plant, exhibited maximum tolerance of 18 g/l to selenite. The strain se29w was classified as Candida tropicalis (GenBank accession number KR150680).  Reduction of selenite to elemental red selenium was also observed in the strain se29w.  Based on the results of selenite removal experiments, the resting cells of C. tropicalis se29w is capable remove 92.5% of selenite with initial concentration of 1 g/l at 30 C, pH 7.2, agitation 180 rpm, 20 g/l of wet biomass and after 60h incubation.

‍Conclusion: The present research is the first report on the removal and reduction of selenite with C. tropicalis. The current project have proposed a variety of screening methods for identifying targeted microbial catalysts for bio-remediation of selenite and to improve environmental and human health.