200 College St.; Online
Abstract
Bioaugmentation has emerged as an effective way to remediate groundwater of anthropogenic contaminants, such as chloroform (CF) and dichloromethane (DCM). The Dehalobacter genus can anaerobically respire many of these chlorinated compounds using reductive dehalogenases, often as part of a heterogenous microbial community. One such community is SC05, which dechlorinates CF completely to carbon dioxide and hydrogen. Despite its effective use at contaminated sites, prior to this work SC05 remained unstudied in terms of taxonomy and broader metabolism, without identification of the active DCM degrader(s?). This thesis seeks to ascertain key microbes in the culture and their metabolic mechanisms using experimental, metagenomic, and metabolic modelling approaches.
A unique characteristic of “self-feeding” is first established in SC05, wherein electron equivalents produced from DCM mineralization are harnessed for CF dechlorination. An SC05 subculture continually dechlorinated CF for over 1400 days with no exogenous electron donor. Dehalobacter was the only bacterial genus that grew in either the CF dechlorination or DCM mineralization phase, implicating it as a key mediator of both CF and DCM degradation. Dehalobacter expressed a single reductive dehalogenase that dechlorinates CF to DCM but has no activity on DCM, as well as the mec cassette—core proteins for DCM degradation. These two modules were within 10 kb in a single genomic neighbourhood.
Two unique Dehalobacter genomes were ultimately assembled, each of which encoded the acd- mec neighbourhood. When assessed pangenomically, this region was designated as a mobile genetic element resulting from horizontal gene transfer between Dehalobacter strains. Each strain could employ this shared genetic cargo to dechlorinate CF and mineralize DCM, with differing dynamics dependant on culture conditions. Genome-scale metabolic models of each strain were curated to predict and compare metabolism during each remediation step.
Overall, this work elucidates some of the former mysteries of SC05, informing considerations for field application such as electron donor demand. It also highlights the importance of hydrogen cycling and microbial syntrophy in anaerobic DCM degradation. Fundamentally, it expands the typical assumptions of the metabolic rigidity of Dehalobacter genus and posits mechanisms of evolution and horizontal gene transfer as it pertains to adaptation of microbial communities to anthropogenic chemicals.
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