Field Evidence for Intrinsic Aerobic Chlorinated Ethene Cometabolism by Methanotrophs Expressing Soluble Methane Monooxygenase

Abstract

Idaho National Laboratory's Test Area North is the site of a trichloroethene (TCE) plume resulting from waste injections. Previous investigations revealed that TCE was being attenuated relative to two codisposed internal tracers, tritium and tetrachloroethene, with a half-life of 9 to 21 years. Biological attenuation mechanisms were investigated using a novel suite of assays, including enzyme activity probes designed for the soluble methane monooxygenase (sMMO) enzyme. Samples were analyzed for chlorinated solvents, tritium, redox parameters, primary substrates, degradation products, bacterial community methanotrophic potential, and bacterial DNA. The enzyme probe assays, methanotrophic enrichments and isolations, and DNA analysis documented the presence and activity of indigenous methanotrophs expressing the sMMO enzyme. Three-dimensional groundwater data showed plume-wide aerobic conditions, with low levels of methane and detections of carbon monoxide, a by-product of TCE cometabolism. The TCE half-life attributed to aerobic cometabolism is 13 years relative to tritium, based on the tracer-corrected method. Similarly, a half-life of 8 years was estimated for cisdichloroethene (DCE). Although these rates are slower than most anaerobic degradation processes, they can be significant for large plumes. This investigation is believed to be the first documentation of intrinsic aerobic TCE and DCE cometabolism in an aquifer by indigenous methanotrophs.

Proteomic and targeted qPCR analyses of subsurface microbial communities for presence of methane monooxygenase

“…The results presented may overestimate the number of methanotrophs because the genomes of some species (e.g., M. capsulatus Bath) may contain more than two copies of pmo genes (McDonald et al 2008). The qPCR results are consistent with previous findings that methanotrophs are present in the Snake River Plain aquifer (Newby et al 2004), and supports previous findings that these cells are responsible for some of the observed natural attenuation of TCE that occurs in the aquifer (Wymore et al 2007). All planktonic samples from TAN showed the presence of peptides from subunit PmoB indicating active intrinsic methane oxidation, and TCE cometabolic potential, throughout the medial and distal zones of the TCE plume as well as outside the plume.…”

“…This evidence supports previous findings using enzyme activity probes for MMO and microcosm experiments that intrinsic methane oxidation and TCE co-metabolism is actively occurring in the medial and distal zones of the TAN site (Sorenson et al 2000;Wymore et al 2007;Conrad et al 2010). Our detection of enzymes that are definitive for a specific degradation process is critical for accepting natural attenuation as a long-term strategy for remediating an aquifer (Madsen 1991).…”

“…12°C from Wymore et al (2007)). The FTISR consisted of six 1 m 9 2.5 cm stainless steel cylindrical reactors filled with crushed basalt (Conrad et al 2010a;Lehman 2007).…”

“…Planktonic samples were collected from TAN-29 and TAN-35 (medial zone), TAN-36 (distal zone) and TAN-58 (outside the distal zone). Basalt biofilm samples were collected from FTISR reactors that were incubated in the groundwater in TAN-35 c Recently, Wymore et al (2007) reevaluated the TCE degradation rates at TAN using the tracer corrected method and confirmed the likelihood of aerobic degradation of TCE with a half-life of 13 years. Additionally, the same study used enzyme activity probes for MMO enzyme and found sMMO activity, using phenylacetylene as a differential inhibitor of sMMO, from samples collected from wells in the distal zone of TAN providing additional evidence of intrinsic TCE co-metabolism at TAN.…”

Geomicrobiology: Low life

“…A community of microbes naturally present in the basaltic aquifer there uses methane in the subsurface as an energy source and co-metabolizes the contaminant, thereby slowly cleaning the water 13 . Colwell expects that these microbes, and other naturally occurring communities in the aquifer that metabolize organic compounds, could reduce clean-up costs by some US$7 million over coming decades.…”