Not all mercury is toxic: bluefin tuna study reveals more

Mercury (Hg) contamination in seafood is a global health concern. However, a new study has found that not all mercury is equally toxic, as its chemical form makes a big difference. The study highlights the need for more accurate measurement and makes recommendations about how mercury content should be measured.

Mercury can originate both from natural sources like volcanoes and forest fires and from human activities such as coal burning, goldmining and industrial waste incineration. Bacteria convert it into toxic methylmercury, which is biomagnified within food webs. Because tuna are top predators, eating many smaller contaminated fish, mercury accumulates in their bodies over time.

Researchers at the ESRF (European Synchrotron), together with CNRS, ENS Lyon and the Institute of Marine Research in Norway, have now unveiled how Atlantic bluefin tuna transforms the toxic form of mercury into less harmful forms. Their study, published in Environmental Science & Technology, shows that the tuna’s edible muscle contains not only toxic methylmercury, but also mercury bound in stable, non-toxic compounds.

“When evaluating the level of toxicity, we should do so by measuring the concentration of methylmercury, which can be done routinely today, instead of total mercury. Otherwise, we include forms of mercury that are sometimes present in fish but are harmless to the human body,” said study lead Alain Manceau, researcher emeritus at CNRS/ENS Lyon and a scientist at the ESRF, who has spent years studying how animals detoxify mercury.

Using the ESRF’s intense X-ray beams, the study team tracked how mercury is processed inside Atlantic bluefin tuna. They discovered that, unlike toothed whales and apex seabirds, where detoxification occurs mainly in the liver, Atlantic bluefin tuna rely primarily on the spleen to break down methylmercury. The process of detoxification takes place due to the interaction between selenium — an essential nutrient found in sea water — and mercury, as selenium binds mercury into stable mercury–selenium complexes, which are far less toxic, if at all. High-trophic marine predators detoxify methylmercury through a series of reactions involving reduced selenium in the form of a prominent selenoprotein (Selenoprotein P).

Manceau and Pieter Glatzel, an ESRF scientist, used a synchrotron technique called high-energy resolution X-ray absorption spectroscopy to obtain the results. The team found that part of mercury in edible muscle of Atlantic bluefin tuna occurs as a tetraselenolate complex — Hg(Sec)₄ — which is arguably not toxic, since this complex transforms into inert mercury selenide in the spleen.

“The reason why the muscle has no mercury selenide is simply because the mercury concentration is not that high in this tissue to begin with,” Manceau said.

The Atlantic bluefin tuna is a high trophic predator, like bigeye tuna, so the results of this research cannot be extended to lower trophic tuna species, which contain much less mercury. The smaller tuna species typically found in tuna cans, such as albacore and skipjack, are much less contaminated.

This study does however highlight the need for more precise testing of seafood, distinguishing between toxic methylmercury and less reactive mercury–selenium complexes, to provide consumers with better-informed food safety recommendations.

“One generally assumes that all mercury in fish is methylated,” Manceau said. “While this is usually the case, our results show that up to a quarter of the mercury present in the edible muscle of bluefin tuna is in less harmful forms. This proportion reaches 90% in marlin, also known as makaire. Health risks, therefore, depend not only on the total amount of mercury, but also on its chemical form.”

Image credit: iStock.com/karinsasaki