Biostimulation of trichloroethene dechlorination by organohalide respiring bacteria with enzymatically produced organochlorides

Abstract

Anaerobic microcosms were set up for trichloroethylene (TCE) dechlorination with solvent extractable or water extractable organic matter (OM) reacted with the enzyme chloroperoxidase (CPO). CPO-reacted water-soluble OM was more suitable for bio-stimulating TCE dechlorination than CPO-reacted solvent-extracted OM. Microcosms were set up with additional variables such as carbon source and soil type. Results showed that inhibition (vs. stimulation) of CPO-reacted-OM also occurs. This inhibition was influenced by the amended electron donor: Methanol>Acetate>Molasses. CPO-reacted-OM clearly was an inhibitor in reactors seeded with contaminated aquifer or stream sediment. Overall, the results indicate CPO-reacted-OM can develop a dechlorinating microbial community but may be a competitive inhibitor with TCE. Microbial analyses (16S rRNA gene amplicon sequencing and qPCR) were conducted to determine the interaction between TCE, CPO reacted OM, and the microbial communities. 16S rRNA gene sequencing and qPCR assays showed consistent results for Dehalobacter. Inoculation material such as contaminated aquifer and stream sediments supported the growth of different bacteria during TCE dechlorination (such as Methanosarcina) compared to the Dehalobacter and Clostridium which dominated the growth in microcosms with other soil amendments. Microcosms with Dehalobacter growth during TCE dechlorination degraded TCE faster as well. Toxicity was conducted to determine the effect of natural organochlorines on bacterial communities. Under anaerobic conditions, increasing the amount of CPO reacted OM 5-fold caused the methane production to decrease from 79.78% to 20.79% of the methane production observed in the control. Aerobic toxicity tests indicated that adding any amount of CPO reacted OM or the OM control would cause the oxygen consumption to decrease more than half of the control with no amended OM added. These results show that the CPO reacted OM inhibits methanogens but not heterotrophic aerobes. Thus, either CPO-OM is toxic to only certain microbes or the toxicity is based on a redox-type of mechanism. The results of this dissertation point to a number of complexities with regards to the natural dechlorinating niche and the possibility to use natural organochlorides as biostimulants in the bioremediation of TCE.