Industry is generally blamed for generating the persistent chemical compounds, such as polychlorinated biphenyls (PCBs), which can accumulate in the bodies of humans and animals. But now it appears that nature is not above generating similar pollutants.

A team of marine chemists from the Woods Hole Oceanographic Institution, Falmouth, US, has discovered that a bioaccumulating compound found in fish and marine mammals, which is structurally very similar to an industrial compound used as a flame retardant, is produced by a natural source.

The researchers were exploring the origin of methoxylated poly-brominated diphenyl ethers (MeO-PBDEs). Although these compounds have been isolated from natural sources, such as sponges and algae, they have a similar biochemical structure to PBDEs, which are known bioaccumulating compounds that are produced by industry for use as flame retardants.

To determine whether the MeO-PBDEs found in fish and marine mammals had a natural or manmade source, the researchers tested for the presence of radiocarbon (¹⁴C). This isotope is generated by collisions between cosmic-ray neutrons and ¹⁴N. Following its oxidation to ¹⁴CO₂, radiocarbon is incorporated into plants via photosynthesis. Radiocarbon has a half-life of just under 6000 years, so, in geological terms, it quickly transforms to ¹³C through radioactive decay. If MeO-PBDEs have a natural source then they should contain radiocarbon, but if they have an industrial source then they won't.

After six months studying blubber from a dead whale found at False Cope, Virginia, US, the researchers discovered that two MeO-PBDE compounds isolated from the blubber did indeed contain radiocarbon. This indicates a natural source for the MeO-PBDEs, which would have found their way into the whale through its diet.

The researchers argue that this may help answer questions about how animals metabolise industrially-generated compounds even though these compounds have only appeared in their environment fairly recently. 'Animals have been exposed to industrial compounds for years, and having natural compounds of similar chemical structure may help toxicologists explain how and why enzymes have the ability to metabolise compounds like PCBs,' explains team member Emma Teuten.

The researchers are now planning to use this radiocarbon method to determine the origins of other bioaccumulating compounds.

Jon Evans