Another toxin for the Baltic Sea?
Scientists have identified a new class of naturally produced marine toxins that may add to the dioxin load of Baltic Sea animals.
The Baltic Sea, long plagued by contaminants and toxic cyanobacteria blooms, may have a new class of marine toxins to add to its environmental problems, according to results published today on ES&T’s Research ASAP website (DOI: 10.1021/es0624725).
Called polybrominated dibenzo-p-dioxins (PBDDs), the compounds have been documented previously in blue mussels and shellfish (Environ. Sci. Technol. 2005, 39, 8235–8242). PBDDs are produced naturally, probably by algae or cyanobacteria, and accumulate in organisms as high on the food chain as fish and shellfish, says lead author Peter Haglund of Umeå University (Sweden).
Levels of PBDDs were low in freshwater species but increased farther offshore in the marine environment. In some mussels, concentrations were in the range of nanograms per gram of tissue—greater than levels of the most abundant PCB contaminants. “We’re afraid it might have some ecological impacts on aquatic species, since mussels account for 90% of the animal biomass in some of these ecosystems,” says Haglund.
Brominated dioxins act on the same receptor system as the better-known chlorinated dioxins and could have similarly harmful effects on reproduction, development, and neurology, the authors say. However, the exact nature of PBDDs is important in determining potential health effects. Compounds with fewer bromine atoms are less toxic and are metabolized faster, says Martin van den Berg of Utrecht University (The Netherlands). And when bromine is found in certain positions around the outside of a molecule, it packs more of a toxic wallop. “I don’t think the di- and tribrominated dioxins are posing a risk for higher organisms and humans,” van den Berg says, but he adds that certain tetrabrominated compounds, which were not found at high levels in this study, could pose health risks.
The researchers measured increasing levels of PBDDs over the past decade. The toxins may be on the rise, the authors say, thanks to global warming and eutrophication (loading of excess nutrients from sewage treatment and agriculture). In a warm, nutrient-rich broth, the organisms at the bottom of the food chain that likely make PBDDs tend to ramp up activity. “It’s alarming that we are adding these on top of the chlorinated dioxins that are already at high levels,” Haglund says. Already, some Baltic Sea fish can’t be sold because they exceed European Commission thresholds for total dioxins, he adds.
The toxins may have always been present, but they escaped notice until Haglund and colleagues stumbled across them. In 2000, the group was concerned about polybrominated diphenyl ether (PBDE) flame retardants. “There had been indications that brominated diphenyl ethers might be pyrolyzed to dioxins and could be emitted to the environment by incinerators for municipal solid waste, so we started a broad screening for brominated dioxins and furans,” Haglund says.
Their results surprised them. Even though fish from only one of nine sites tested positive for the brominated compounds, all eight samples from that site had similar levels of di-, tri-, and tetrabromodioxins. The compounds were inconsistent with breakdown products of PBDEs and seemed to have appeared out of nowhere.
The team found that on the basis of geographical distribution, the Baltic PBDDs are likely to be produced naturally in coastal waters. The next step was to develop a possible pathway for biological synthesis of PBDDs.
“Analytically, it’s a slam dunk,” says Christopher Reddy of Woods Hole Oceanographic Institute. “They made a convincing case that these compounds are natural. I’m not surprised that they’re natural, but I’m impressed with their detective work,” he says.
The discovery that organisms make PBDDs begs a question, Reddy says: “Why are they making these things?” Many marine organisms make toxins as chemical defenses, but why algae would produce dioxins is not clear, he adds.
“This study raises a lot of questions,” says Sheryl Tittlemier of the food research division of Health Canada, who has studied natural organohalogens. “How do concentrations in mussels vary over a year?” she asks, noting that cyanobacteria and mussel populations, and thus risk, could fluctuate seasonally.
Although the research points out a natural source for brominated dioxins that is unassociated with flame retardants, several researchers say that doesn’t let chemical manufacturers off the hook. “In mass-balance terms, we have to keep in mind that industry still delivers a significant amount,” Reddy says. Haglund and van den Berg agree, noting that the toxicity of other brominated compounds is well known.
Gordon Gribble of Dartmouth College says that two colleagues told him that they found similar brominated compounds but did not publish the data because the origins were a mystery. “We’ve known that chlorinated dioxins can form when you burn wood that contains certain chlorides,” he says, “But brominated dioxins are rare.” The next step, he and Haglund agree, will be to study the toxicity of PBDDs and learn whether their concentrations pose serious risks.
