Microbial Chain Elongation and Subsequent Fermentation of Elongated Carboxylates as H₂-Producing Processes for Sustained Reductive Dechlorination of Chlorinated Ethenes

Abstract

In situ anaerobic groundwater bioremediation of trichloroethene (TCE) to nontoxic ethene is contingent on organohalide-respiring Dehalococcoidia, the most common strictly hydrogenotrophic Dehalococcoides mccartyi (D. mccartyi). The H2 requirement for D. mccartyi is fulfilled by adding various organic substrates (e.g., lactate, emulsified vegetable oil, and glucose/molasses), which require fermenting microorganisms to convert them to H2. The net flux of H2 is a crucial controlling parameter in the efficacy of bioremediation. H2 consumption by competing microorganisms (e.g., methanogens and homoacetogens) can diminish the rates of reductive dechlorination or stall the process altogether. Furthermore, some fermentation pathways do not produce H2 or having H2 as a product is not always thermodynamically favorable under environmental conditions. Here, we report on a novel application of microbial chain elongation as a H2-producing process for reductive dechlorination. In soil microcosms bioaugmented with dechlorinating and chain-elongating enrichment cultures, near stoichiometric conversion of TCE (0.07 ± 0.01, 0.60 ± 0.03, and 1.50 ± 0.20 mmol L–1 added sequentially) to ethene was achieved when initially stimulated by chain elongation of acetate and ethanol. Chain elongation initiated reductive dechlorination by liberating H2 in the conversion of acetate and ethanol to butyrate and caproate. Syntrophic fermentation of butyrate, a chain-elongation product, to H2 and acetate further sustained the reductive dechlorination activity. Methanogenesis was limited during TCE dechlorination in soil microcosms and absent in transfer cultures fed with chain-elongation substrates. This study provides critical fundamental knowledge toward the feasibility of chlorinated solvent bioremediation based on microbial chain elongation.

Optimization of Dechlorination Experiment Design Using Lightweight Deep Learning Model

“…How to compress the model to reduce storage space and computing consumption has become a research hotspot. Present compression is mainly realized through the two methods below [ 17 , 18 ].…”

Thermophilic Dehalococcoidia with unusual traits shed light on an unexpected past

“…2D). As the organohalide-respiring metabolisms of members of the class Dehalococcoidia detoxify halogenated pollutants (e.g., 6,31,35,42,[45][46][47] ), we considered the possibility that these protein complexes may represent a dehalogenation apparatus. However, no distinct densities were observed associated with the cytoplasmic membrane, and like other members of this class outside of the Dehalococcoidales, the Tepidiforma genomes had no identifiable homologs of reductive dehalogenases in the RDaseDB 46 .…”

“…Physiologies within the Dehalococcoidia, both known and predicted, differ markedly. Though cultivated members of the class are best known for reductive dehalogenation 6,[42][43][44][45][46] or consortial degradation of recalcitrant organic carbon, and syntrophic interactions have been suggested based on environmental genomes 12,31,47 . For example, some marine Dehalococcoidia that encode genes involved in the degradation of aromatic and plant-derived organic compounds have been predicted to partially degrade terrestrial recalcitrant organic carbon for utilization by the rest of the community 14,32,55,57,58 .…”

Thermally Enhanced Biodegradation of TCE in Groundwater

“…Furthermore, the same temporary increase in methane concentration (by one order of magnitude; from 280 ± 260 to 1755 ± 2128 µg/L) induced by temperatures of 22 • C was also observed without whey addition. This delay in methane production (usually 2-3 weeks for this type of remediation [41]) is typical because methane (and reductive dechlorination as well) must precede (syntrophic) fermentation of carbon substrates to produce short fatty acids and H 2 [42]. published that at higher temperatures of 30-40 • C, methanogenesis became absolutely predominant above reductive dechlorination when dechlorinating bacteria not very successfully competed for limited sources, e.g., carbon substrates and/or reducing equivalents (no substrate added) [39,43].…”