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UBC Theses and Dissertations

Biological reduction of selenium oxyanions in the presence of nitrate anions using anaerobic microbes Subedi, Gaurav


Biological selenium reduction has emerged as a viable solution for the removal of toxic selenium from the environment. However, the presence of nitrate hinders selenium reduction by acting as a competitive electron acceptor. The present thesis investigated the use of local mine-impacted sediment as an inoculum for selenium reduction and studied the affect of nitrate on the removal of selenium. Sediment samples, impacted by mining activities, were collected from two vastly different sites of the Elk River Valley. These sediments namely; Goddard Marsh and Mature Tailing Coal, were enriched for selenium reducing bacterial consortium under high selenium and varying nitrate concentrations to put additional selection pressure. Ultimately, two cultures from Goddard Marsh enriched under low and high nitrate condition as well as one culture from Mature Tailing Coal enriched under moderate nitrate condition were used to access the affect of nitrate on selenium reduction using central composite design matrix. The extent of Se reduction was highest in the Goddard Marsh enrichment with no nitrate while enrichment with moderate and high nitrate reduced selenium poorly. ANOVA results from the CCD experiment in Goddard Marsh enrichment with no nitrate indicated no affect of nitrate in Se reduction. Two primer sets targeting the selenate redutase (serA) from Thauera selenatis and nitrite reductase (nirK) from denitrifying population were used to quantify the population of selenium reducing and denitrifying population in the CCD experiment. Q-PCR assay successfully quantified serA genes in the cultures and correlated well with the initial Se concentration. Furthermore, the selenium reducing ability of enrichment cultures were compared with the bio-stimulated native sediments. Native sediments efficiently removed selenium from the culture medium while enrichment cultures preferentially removed nitrate over selenium. Metagenomic sequencing revealed the presence of many putative selenium reducers in the native sediments while Pseudomonas were more prevalent in the enrichment cultures. Denitrification, sulfate reduction and selenium assimilation genes were abundant in most sequences indicating its role in the reduction of selenium and nitrate. Thus, our study shows that efficient reduction of selenium in the presence of nitrate is possible with biological system.

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