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Seabass populations can be differentiated by their Mercury Isotope Distribution

(13.10.2017)


Background:
Mercury is a toxic element that is accumulated in organisms and biomagnified across the food chain, particularly as its organic form, methylmercury.  Mercury is emitted to the environment mainly by human activities such as fossil fuel combustion, mining and industrial activities (such as chlor-alkali industry), enhancing the total amount of mercury cycling in the environment by an estimated factor  of 3 to 5 since industrialization. While generally recognized as a global pollutant posing considerable health risks, global activities to restrict the emission of mercury have only started (e.g. Minamata convention) but have not changed the situation up to now.

Emitted in the form of elemental mercury (Hg0) to the atmosphere, it can travel long distances impacting the environment at a global scale. Emitted in the oxidized form (Hg(II)), mercury is more reactive and therefore is impacting the environment more closely to the emission source. Transformation of the inorganic mercury to organic mercury species is performed by microbes that can methylate inorganic mercury species mainly taking place in aquatic sediments. Degradation of organo mercury species can also take place via microbial activity but also by photochemical processes induced by sunlight. Together these processes control the mercury speciation and the subsequent accumulation and biomagnification resulting from the bioavailability of the mercury species.

Recently, analyzing the mercury isotope distribution has provided information of the sources of mercury and the processes dictating its speciation. Mercury stable isotope distribution is a result of both mass dependend fractionation (MDF, measured as δ202Hg) and mass independen fractionation (MIF, measured as Δ199Hg and Δ201 Hg). Mass dependent fractionation is occuring in all biotic and abiotic chemical reactions that have been investigated so far, such as biomethylation, demethylation, passive diffusion and photochemical reactions. During the last decade,  δ202Hg and Δ199Hg values have successfully been used to study the processes in the environment and to trace contamination sources and pathways in marine food webs.

The new study:
In this new study, a group of European researchers use Hg stable isotopes, Hg concentration and speciation together with carbon and nitrogen stable isotops to investigate the sources of mercury in a marine carnivore fish across Europe. The European seabass was sampled from 7 different coastal sites throughout Europe spanning different latitudes and water quality, including areas impacted by pollution sources. After sampling, fish were kept in freezers until final analysis. Muscle tissue was analyzed, known to accumulate mercury and closely related to human health risk.


Photo: Dicentrarchus labrax (seabass)
Citron / CC BY-SA 3.0

Total mercury was analyzed by a direct mercury analyzer based on  atomic absorption spectrometry. Mercury speciation analysis was performed by using GC-ICP-MS with quantification by isotope dilution analysis.  Mercury isotopic composition analysis was performed by CVG-ICP-MS with a multicollector instrument.
The Stable Isotope Bayesian Ellipse in R (SIBER) package was used for data analysis.

Both total mercury as well as MeHg concentrations were extremely variable between individual fish samples (from 85 to 3560 ng/g) with highest values observed in fish collected from contaminated sites. As already known from  previouis studies, MeHg was the main species comprising more than 90% of THg. Interestingly, mercury concentration was neither related to fish age or body mass nor to trophic position in the food web. Therefore the authors suggest that the level of site contamination is likely the best explanation for the observed differences.

The SIBER plot of MDF versus MIF values shows that four groups of fish populations can be differentiated based on their core isotopic niche. With respect to the isotopic signature, the result of the study indicate that processes responsible for MDF related to trophic transfer might be negligible in the case of juvenile seabass. 
The authors conclude that Hg isotopes can indeed help discriminating local versus global contamination.



The original study:

Alice Cransveld, David Amouroux, Emmanuel Tessier, Emmanuil Koutrakis, Ayaka A Ozturk, Nicola Bettoso, Claudia L. Mieiro, Sylvain Berail, Julien P.G. Barre, Nicolas Sturaro, Joseph Schnitzler, Krishna Das, Mercury Stable Isotopes Discriminate Different Populations of European Seabass and Trace Potential Hg Sources around Europe, Environ. Sci. Technol., Article ASAP (2017). DOI: 10.1021/acs.est.7b01307



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