The Secret to Degrading PCBs and Dioxins Is in How Bacteria Breathe

Researchers uncover a previously unknown process used to break up harmful environmental toxins.

Oct 19 2014, 7:55pm

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A team of researchers based at the University of Manchester has uncovered a new way of breaking down and neutralizing several of the most pervasive and resilient environmental pollutants. The product of 15 years of study, the breakthrough, described in today's edition of Nature, comes courtesy of tiny microorganisms that boast a natural (and necessary) talent for lowering the toxicity of pollutants.

"We already know that some of the most toxic pollutants contain halogen atoms and that most biological systems simply don't know how to deal with these molecules," David Leyes, a professor at the University of Manchester and study co-author, explained in a statement. "However, there are some organisms that can remove these halogen atoms using vitamin B12. Our research has identified that they use vitamin B12 in a very different way to how we currently understand it."

Halogens represent a uniquely toxic region of the Periodic Table, home to the elements chlorine, fluorine, bromine, iodine, and astatine. We're mostly concerned with the first two, however, which can pack the most toxic punch once organized into the right compound and/or delivered in large enough quantities. They're toxic, but not always.

Chlorine, for example, is used to sanitize drinking water when administered in very small quantities, as well as being useful as a general disinfectant. In a sense, chlorine has been a silent partner to penicillin (and its kin) in neutralizing microbial infections. In its most common form, chlorine is just a molecular component of table salt.


We all know the other side of chlorine, however: poison gas. In very recent history, it's thought to have been used as a chemical weapon in Syria and Iraq. Above a certain concentration, chlorine begins to react with water in sufficient enough quantities to produce hydrochloric acid. Above 1,000 or so parts-per-million, all it takes is a couple of deep breaths for chlorine to be fatal. Lung damage begins to occur at 60 ppm.

The current study is concerned with chlorides in organochloride form. This is where we find an atom of chloride chemically bound to some organic molecule. One example would be PCBs, or polychlorinated biphenyl compounds. PCBs, once a common component of coolant fluids, were recognized as a major environmental threat in the 1970s; in 1979, they were banned in the United States. In 2001, they were banned worldwide.

A short list of PCB health effects: liver damage, impaired immune responses, rashes and lesions, poor cognitive development. Cancer.

PCBs are very similar to compounds known as dixoins, toxic substances that also contain some organochloride element and result in effects ranging from immunotoxicity to endocrine effects to the promotion of tumors. They're mutagenic and genotoxic, scrambling DNA and increasing the likelihood of cancer or other bad things.

Despite being banned, PCBs still persist abundantly in the environment. Generally, the concentration of dioxides and PCBs has gone up over time, the result of industrial pollution and the burning of household wastes.

Some microorganisms are quite adept at trimming the halogen atoms from PCBs and dioxins, dramatically limiting their toxic potential. That these organisms exist is hardly new knowledge, but the challenge has been in growing enough of them fast enough to be able to study how they do their detoxification work. Leyes and his team were able to accomplish this by genetically modifying faster-growing organisms, swapping in bits of DNA responsible for the original organisms' detox capabilities. Then, it was just a matter of observing how the organisms accomplish halogen removal.

As the paper explains, the researchers focused on a variety of bacteria that use enzymes known as reductive dehalogenases to yank the halogens from dioxins and PCBs. The process is basically dechlorinating the toxic compounds. This is their own special variety of anaerobic respiration, known as halorespiration. Bacteria breathe our toxins, but they need some B12, a crucial component of the enzyme, to make the process happen.

What the Manchester team found is that beneath the enzymes used by bacteria for this process is basically just cobalt, a reasonably common (in chemically bonded forms) metal. As the current paper explains, cobalt has an affinity for binding with halogens, a process that, in nature, is mediated by the vitamin B12. The result is a "cleaving" of the halogen atom from its carbon partner and, thus, from the dioxin.

"Detailing how this novel process of detoxification works means that we are now in a position to look at replicating it," Leyes said. "We hope that ultimately new ways of combating some of the world's biggest toxins can now be developed more quickly and efficiently."