# Dechlorination of mixed chlorinated organophosphate esters (V6 and TCEP) and associated reductive dehalogenase gene expression by Dehalococcoides mccartyi

## TL;DR

This study shows how Dehalococcoides mccartyi can dechlorinate a common chlorinated organophosphate ester mixture, V6, and identifies key enzymes involved in the process.

## Contribution

The first study to elucidate microbial transformation pathways and mechanisms of commercial V6 by D. mccartyi.

## Key findings

- D. mccartyi dechlorinated 99.5% of TCEP and 95.4% of V6 within 10 days.
- Two reductive dehalogenase genes were co-transcribed and their enzymes co-expressed during dechlorination.
- Dechlorination produced phosphate de-esterification products, chloride ions, and ethene.

## Abstract

Dehalococcoides mccartyi (D. mccartyi) plays a critical role in the dechlorination of halogenated organic pollutants, yet its performance on mixed chlorinated organophosphate esters (Cl-OPEs) remains poorly understood. In this study, a mixed microbial culture containing D. mccartyi was developed to dechlorinate commercial V6, a mixture comprising 80.2% 2,2-bis(chloromethyl)trimethylene bis(bis(2-chloroethyl) phosphate) (V6), 9.4% tris(2-chloroethyl) phosphate (TCEP), and other Cl-OPE impurities. Within 10 days, 99.5% of TCEP and 95.4% of V6 were dechlorinated. The presence of TCEP slightly enhanced the dechlorination of V6 compared with V6 as a purified compound. Both compounds underwent cleavage of C−O and C−Cl bonds in their chloroethoxy groups (Cl–CH2–CH2–O–), yielding different phosphate de-esterification products and ethene. D. mccartyi exhibited significant growth, with two reductive dehalogenase homologous (rdhA) genes co-transcribed during the dechlorination of commercial V6, purified V6, or TCEP. Metaproteomic analysis revealed that the enzymes encoded by these two genes were significantly expressed, suggesting that they may be the key enzymes mediating the dechlorination of mixed Cl-OPEs. Overall, this study provides insights into the role of D. mccartyi and its reductive dehalogenases in the natural attenuation of mixed Cl-OPEs in contaminated environments.

Commercial V6, a chlorinated organophosphate esters mixture, is widely used in polyurethane foam and has been detected in various environmental matrices. This study is the first to elucidate the microbial transformation pathways and mechanisms of commercial V6. A mixed culture containing Dehalococcoides mccartyi was found to dechlorinate V6 and tris(2-chloroethyl) phosphate (TCEP) into phosphate de-esterification products, chloride ion, and ethene. Notably, two reductive dehalogenase genes were simultaneously transcribed and their corresponding enzymes co-expressed, indicating a key role of D. mccartyi in the natural attenuation of commercial V6 in the environment.