Reductive debromination of polybrominated diphenyl ethers by anaerobic bacteria from soils and sediments

Abstract

Polybrominated diphenyl ethers (PBDEs) have attracted attention recently due to their proven adverse effects on animals and their increasing concentrations in various environmental media and biota. To gain insight into the fate of PBDEs, microcosms established with soils and sediments from 28 locations were investigated to determine their debromination potential with an octa-brominated diphenyl ether (octa-BDE) mixture consisting of hexa- to nona-BDEs. Debromination occurred in microcosms containing samples from 20 of the 28 locations when they were spiked with octa-BDE dissolved in the solvent trichloroethene (TCE), which is a potential cosubstrate for stimulating PBDE debromination, and in microcosms containing samples from 11 of the 28 locations when they were spiked with octa-BDE dissolved in nonane. Debromination products ranging from hexa- to mono-BDEs were generated within 2 months. Notably, the toxic tetra-BDEs accounted for 50% of the total product. In sediment-free culture C-N-7* amended with the octa-BDE mixture and nonane (containing 45 nM nona-BDE, 181 nM octa-BDEs, 294 nM hepta-BDE, and 19 nM hexa-BDE) there was extensive debromination of the parent compounds, which produced hexa-BDE (56 nM), penta-BDEs (124 nM), and tetra-BDEs (150 nM) within 42 days, possibly by a metabolic process. A 16S rRNA gene-based analysis revealed that Dehalococcoides species were present in 11 of 14 active microcosms. However, unknown debrominating species in some of the microcosms debrominated the octa-BDE mixture in the absence of other added halogenated electron acceptors (such as TCE). These findings provide information that is useful for assessing microbial reductive debromination of higher brominated PBDEs to less-brominated congeners, a possible source of the more toxic congeners (e.g., penta- and tetra-BDEs) detected in the environment.

FIG. 1.

Debromination of an octa-BDE mixture dissolved in nonane by culture C-N-7* amended with acetate-H₂. (A) Chromatographs after different incubation periods. The shaded areas show the debromination products, while the unshaded area shows the substrates. (B) Amounts of substrate removed and products generated in culture C-N-7*. (p), products; (s), substrate. (Inset) Total PBDE concentrations in a control bottle during the incubation period.

FIG. 2.

Profile of debromination of octa-BDE/nonane by culture U-N-1* amended with acetate-H₂. The shaded area shows the debromination products, while the unshaded area shows the substrates.

FIG. 3.

Comparison of the profiles for debromination of an octa-BDE mixture by microcosm C-T-7 or C-N-7 spiked with various organic carbon sources and carrier solvents. The shaded areas show the debromination products, while the unshaded area shows the substrates after 28 days of incubation.

FIG. 4.

Direct and nested PCR using genus-specific Dehalococcoides primers and group-specific Chloroflexi primers. (A) Dehalococcoides. Lanes 1 and 10, 100-bp ladders (Promega); lanes 2 to 9, direct PCR; lanes 11 to 16, nested PCR; lane 17, negative control; lane 18, positive control (Alameda Naval Air Station [ANAS] enrichment). (B) Chloroflexi. Lanes 1 and 11, 100-bp ladders (Promega); lanes 2 to 10, direct PCR; lanes 12 to 16, nested PCR; lane 17, negative control; lane 18, positive control (ANAS enrichment).