Fish consumption and exposure to methylmercury

Photo : Citron/CC-BY-SA

Photo : Citron/CC-BY-SA

A study conducted by a team of international researchers, including Krishna Das from the Laboratory of Biological Oceanology ( FOCUS Research Unit), presents new data on the level of mercury in seven distinct populations of European seabass (Dicentrarchus labrax), an edible species whose flesh is highly appreciated by gourmets. Where does this element come from? Is it present locally or more globally? The study, published in the journal Environmental Science & Technology (1), provides some answers and presents very different surveys according to the fishing area.

We now eat twice as much fish as we did in 1995, or 17 kilograms per capita. On a global scale, this is equivalent to the consumption of 132 billion Kg of fish each year, or nearly 4,200 Kg of fish consumed every second in the world. The seas and oceans are becoming increasingly polluted and fish are therefore increasingly exposed to these pollutants, such as mercury, a persistent toxic element in particular when it occurs in its organic form - methylmercury (MeHg) - which poses serious health risks.

Although naturally present in ecosystems, human activities have increased the amount of active-cycle mercury by an estimated factor of 3-5 since industrialization. Despite current regulations, the levels of mercury still detected in marine predators still exceed environmental quality standards.

Where does this mercury come from? Is it local contamination related to an anthropogenic source very close to the fish's habitat or is it a more global redistribution related to the mercury cycle in the environment? A more complex issue than it seems to be, especially since mercury is a toxic metal, especially in its organic form. This question has been asked by Krishna Das , a F. R. S. -FNRS Senior Research associate in the Laboratory of Biological Oceanology ( FOCUS Research Unit) for many years, she has observed very high concentrations of mercury in many species of fish and marine mammals. How to find the origin of mercury in these organisms’ By a rather special isotopic approach!

Mercury has seven stable isotopes. These isotopic values can be used to trace the sources and transformation processes that characterize the highly complex biogeochemical cycle of mercury. The analyse of these isotopic ratios require an expertise and cutting edge equipment whom David Amouroux, CNRS researcher at the University of Pau and Pays de l' Adour, has.  Thanks to this collaboration and many others, common seabass (Dicentrachus labrax) have been sampled in the North Sea, Atlantic, Mediterranean, Adriatic and Black Seas. The results of the analyses of these samples were surprising: mercury concentrations in the flesh of the European seabass differed according to the region of analysis, with concentrations sometimes multiplied by 40 between the least contaminated fish and the most contaminated fish. Unsurprisingly, the most contaminated fish were caught in the Northern Adriatic Sea and near the Ria de Aveiro, in Portugal.

Isotopic values of mercury in the European seabass from the present study.

« In this study, we used stable isotopes of mercury (Hg) combined with the concentration and speciation of mercury, as well as the stable isotopes of carbon (C) and nitrogen (N) to study the sources and exposure pathways of methylmercury in the European seabass (Dicentrarchus labrax) across Europe," explains Krishna Das. To achieve our objectives, we first compared mercury levels between sites and investigated the possible causes of the observed variability."

The analyses demonstrate the relevance of the use of stable mercury isotopes as tools for discrimination. Isotopic values of mercury also provided an overview of sources of mercury contamination of the biota (all living organisms) in the coastal environment. It was suggested that the presence of mercury at European level was mainly linked to global air contamination. Throughout this study, the results for the Black Sea seabass population were distinguished, displaying a mercury cycle similar to that of freshwater lakes. "Our results highlight the potential to use stable isotopes of mercury to discriminate against distinct populations, explore the large-scale mercury cycle and distinguish sites contaminated by global or local sources of mercury. »

The isotopic approach used in this research mainly reveals that the most polluted sites have been influenced by intense industrial and mining activities." Some sites, such as the Marano and Grado lagoons along the Adriatic Sea, show local historical contamination related to mining activities in Idrija, Slovenia. ", continues Krishna Das.

The United Nations Environment Programme (UNEP) has published several scientific reports (2) in order to assess an inventory on the issue of remediation of polluted sites." For example, mercury concentrations in several animal species in the Arctic are 10 to 12 times higher than they were before the industrial era (before 1800), the researcher worries. This means that 92% of the mercury in the tissues of marine predators (birds, seals and whales) is of anthropogenic origin. » UNEP's findings are compelling: it is essential to implement global environmental monitoring as soon as possible, not only to try to better control mercury sources but also to better understand the complexity of the mercury cycle in the ocean environment, to better protect man and the environment.

Scientific references

(1) Alice A.E. Cransveld, David Amouroux, Emmanuel Tessier, Emmanuil Koutrakis, Ayaka Amaha Ozturk, Nicola Bettoso, Cláudia L Mieiro, Sylvain Berail, Julien P. G. Barre, Nicolas Sturaro, Joseph G. Schnitzler, and Krishna Das, Mercury stable isotopes discriminate different populations of European seabass and trace potential Hg sources around Europe, Environmental Science & Technology, 2017 - Consult in ORBi

(2) Global Mercury Assesment - UN Environment