Bacteria may break down popular flame retardant to produce toxics
By removing bromine atoms from the large Deca PBDE flame retardant used in computers and televisions, microbes may be able to transform it into banned compounds.
Research published today on ES&T’s ASAP website (DOI: 10.1021/es052508d) documents that microbes can break down the large molecules of the widely used Deca PBDE (polybrominated diphenyl ether) flame retardant. The paper raises concerns about the Deca flame retardant’s safety by showing that various bacteria can work in concert to remove the bromine atoms from the Deca compound to produce the smaller PBDE compounds that have been banned in the EU and discontinued in the U.S.
The paper is the first to identify species of bacteria capable of breaking down the main constituent of the Deca flame retardant formulation, Deca-BDE, which contains 10 bromine atoms. The study builds on previous research showing that the Deca flame retardant could be transformed during sewage treatment in anaerobic environments, which contain no oxygen.
The Deca mixture is found in electronic products such as computers and televisions, and it is the only PBDE formulation currently in use. Because of the Deca-BDE molecule’s large size, it is considered relatively inert, but the smaller PBDE compounds, or congeners, that have been banned and discontinued are persistent and bioaccumulative. The levels of these compounds have been rising throughout the world, especially in North America, and their neurotoxic effects are similar to those of PCBs, which they resemble chemically.
Some of the smaller PBDE compounds are associated with tumors and thyroid hormone imbalances, and some have been shown to impact developing rodent brains and impair male hormones. The PBDE compounds with 5 bromine atoms , which are considered the most toxic, recently have been found to alter the development of male gonads in rats.
In the new paper, Lisa Alvarez–Cohen and her colleagues at the University of California, Berkeley, describe research they conducted with bacteria known to be able to break down large molecules containing chlorine. Sulfurospirillum multivorans are able to break down trichloroethylene, and the different species of Dehalococcoides used in the experiments can attack dioxins and vinyl chloride. The new study firmly establishes that the Dehalococcoides bacteria can break down brominated compounds, says Lorenz Adrian, who is with the Technical University of Berlin’s Institute for Biotechnology and who first showed that the bacteria could attack dioxins.
Alvarez-Cohen’s team documented the S. multivorans bacteria’s ability to decompose the Deca-BDE molecules into smaller PBDE compounds containing 8 and 9 bromine atoms.
The Dehalococcoides bacteria could not attack the large Deca-BDE molecules, but they could break down PBDE compounds containing 8 bromines to produce PBDE compounds with 6, 5, and 4 bromines. The breakdown products included BDE-99, which contains 5 bromines and is often found to bioaccumulate in people and animals. Although these tests took place in a laboratory, Alvarez-Cohen says that “it is highly likely that we’ll see this kind of sequential transformation in the environment.”
Other researchers agree. “We saw this same type of sequential breakdown [by bacteria] with PCBs,” points out Linda Birnbaum, director of the experimental toxicology division at the U.S. EPA’s National Health and Environmental Effects Research Laboratory. The research raises the question of whether “continued production and use of the Deca may lead to ongoing exposure of wildlife and people to the lower brominated congeners for which we have toxicity concerns,” she adds.
Andreas Gerecke, a project leader in the analytical chemistry department of Switzerland’s National Materials Science & Technology Laboratory (EMPA), was the first scientist to report that Deca-BDE was being broken down in sewage treatment plants. He says he thinks that it is likely that microbes are breaking down the Deca-BDE molecule in oxygen-free environments such as contaminated underwater sediments. However, he points out that the rates documented in the paper are quite slow.
Alvarez-Cohen acknowledges that this is true but says that she is currently involved in studies with additional bacteria showing “much [more rapid] rates of degradation.”
However, scientists from the Bromine Science and Environmentalal Forum, an industry group, point out that “no degradation was found without TCE being added as a fuel, along with other substrates. Since TCE is not normally present in the environment at high concentrations (it oxidizes to another substance), the environmental relevance of this study is questionable; i.e., the conditions under which degradation was forced to occur are not likely to be found in the environment.”
Even so, scientists interviewed for this article agree that the paper’s findings are significant. “High levels of Deca-BDE have been detected in aquatic sediments and anaerobic environments such as Baltimore Harbor,” points out Heather Stapleton, an assistant professor of environmental sciences and policy at Duke University’s Nicholas School of the Environment and Earth Sciences. Deca-BDE is also “detected at elevated levels in sewage sludge [and] biosolids, which can be home to multiple strains of bacteria. Considering that land application of biosolids and soil amendment is an increasing practice, [this new paper’s findings warrant] further investigation.”
