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Differential Stimulation of Haloreduction by Carbon Addition to Subsurface Soils (PDF)

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Highly substituted ethenes such as PCE and TCE are susceptible to microbial reductive dehalogenation and halorespiration under anaerobic conditions. The objective of this study was to evaluate whether soil from a bulk aerobic aquifer, historically contaminated with chlorinated ethenes (CEs), could be made anaerobic by carbon source addition to enhance haloreduction, and to evaluate how the addition of these different carbon sources affects microbial community structure.

We hypothesized that sulfate-reducing bacteria (SRBs) were responsible for dechlorination in this study because of the near-shore location of the site. Using soil and water from the aquifer, microcosms were constructed to examine the effects of various electron donors on reductive dehalogenation. Six electron donor treatments were compared including acetate, propionate, lactate, ethanol, vegetable oil (soy oil), and hydrogen release compound (HRC, Regenesis).

An SRB-inhibited control (using Na2MoO4) and an abiotic control (using NaN3) were included with each treatment. Concentrations of CEs in the aqueous phase were measured via headspace analysis and GC/MS detection. With addition of excess electron donor, reduction of PCE and TCE to cis-1,2-dichloroethene (cis-1,2-DCE) occurred in the microcosms.
When acetate, propionate, ethanol, or HRCwere added, PCE and TCE concentrations quickly decreased (~20 days) and cis-1,2-DCE was formed. With the lactate and vegetable oil treatments, cis-1,2-DCE was formed only after a considerable lag period (~60 days). For the lactate, ethanol, vegetable oil, and HRCtreatments, PCE and TCE reduction was observed
neither in the abiotic nor in the SRB-inhibited control. This finding suggests that SRBs are involved in dechlorination with those carbon sources. In the acetate and propionate treatments, reduction of PCE and TCE to cis-1,2-DCE again was not observed in the abiotic control but was observed in the SRB-inhibited control. This suggests that reductive dehalogenation using acetate or propionate as a carbon source is not contingent on the presence of sulfate-reducing
microorganisms. On the other hand, since reduction of CEs still occurred in the non-inhibited microcosms, other microbial groups were possibly stimulated.

In our effort to understand the key microbial groups required for reductive dehalogenation and monitor structural changes in microbial community as a response to the addition of different carbon sources, DNA profiles based on Terminal Restriction Fragment Length Polymorphism (T-RFLP) analysis were compared. As expected, different bacterial community profiles were obtained depending on carbon source. When sulfate reduction was inhibited or yeast extract was added, a dramatic change in the microbial community structure occurred.

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