Abstract :
[en] Mercury (Hg) is a persistent toxic compound whose amount in the global biosphere has
at least tripled since industrialization. The biogeochemical cycle of mercury is particularly
complex because elemental mercury is very volatile, has a long residence time in the
atmosphere and thus can potentially reach locations that are very distant from the initial
emission source. Since the 70’s, many international organisations (OECD, EU, UNEP)
have implemented different policies to reduce Hg pollution. However, for these policies
to be efficient, the polluters/Hg sources have to be fully assessed and identified. So, there
is a crucial need to trace Hg sources and to assess the quantitative impact of local versus
global Hg sources on ecosystems.
Recently, the study of the seven Hg stable isotopes has emerged as a new promising
technique affording to explore the Hg cycle both in situ and in laboratory. Mercury stable
isotopes display both mass dependent fractionation (MDF, reported as δ202Hg) and mass
independent fractionation (MIF, reported as Δ199Hg and Δ201Hg). The combination of both
values should allow to trace back sources and pathways of Hg and methylmercury
(MeHg). But, so far, few studies have considered Hg isotopes in marine vertebrates.
Thus, the general objective of this thesis was to study a marine predator, the European
seabass, Dicentrarchus labrax, to explore the possibility of using mercury stable isotopes
to investigate Hg sources and pathways in European coastal waters. Our specific objectives
were to characterize the Hg profile (THg, MeHg and isotopy) of wild populations of
seabass, to assess whether Hg isotopes differed between populations and could give
indications on Hg sources, with special attention paid to differentiate local versus global
Hg sources. We also wanted to explore internal variation of Hg profiles by comparing
several tissues. And, we wanted to validate our interpretations of in situ results by
experimentally evaluating the potential fractionation between muscle and liver tissues of
D. labrax, and between the diet and fish tissues.
To fulfill our objectives, we split the thesis in 3 axes. First, juvenile seabass were collected
in seven geographically distinct locations: the Agiasma lagoon in the northern Aegean Sea
(AeS), the North Sea (NS) along Belgian and English coasts, the Seine estuary (SE), the
Turkish coast of the Black Sea (BS), the Marano and Grado lagoons in the northern
Adriatic Sea (NAS), the Portuguese lagoon, Ria de Aveiro, at two distinct sites: a very
contaminated one (RAC) and a least contaminated one (RAR).
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Our results showed seabass displayed extremely variable THg (Total mercury)
concentrations amongst locations. Environmental contamination of the fish habitats
seemed to be the main driving factor of THg concentrations in their tissues. Then, we
showed that the populations also had distinct Hg isotopic niches (using SIBER on Hg stable
isotopes, a premiere!), and that Hg isotopes (Δ199Hg and δ202Hg) could be used as a
discriminating tool. We showed that Hg isotopes also told more about the origin of THg
contaminating juvenile seabass: Δ199Hg values indicated a rather coastal MeHg source
while δ202Hg could be linked with the global versus local Hg origin. Some populations like
AeS were thought to be affected mainly by background, global Hg source, while other
sites such as RAC (and SE and NAS) that were more heavily contaminated, would undergo
a strong influence of local contamination.
The previous observations were made in muscle tissue. Since different tissues often have
distinct turnover times and different affinity for pollutants, the second axis of this thesis
aimed at comparing liver and muscle results. So, we investigated the liver tissue of the
very same individuals than in axis 1. The THg RATIO (THgliver/THgmuscle) was very variable
amongst populations. We found that Hg organotropism (affinity for different organs) was
influenced by the overall contamination level and maybe also by the food regime (via the
%MeHg in diet). The Hg isotope composition also differed between muscle and liver of
wild seabass. Hg speciation was most probably not the only cause of such a difference, and
there was certainly an internal fractionating process (MDF). We even found serious
indication of mercury demethylation happening in seabass, although demethylation in
fish had yet to be proven.
This is what our 3rd axis addressed. We exposed captive juvenile seabass to
environmentally relevant THg concentrations through the diet. Observations confirmed
in situ results: Hg organotropism depends on the %MeHg in diet, and THg RATIOs < 1
are to be related to the extremely small proportion of inorganic Hg in the seabass diet.
Most of all, we found strong and concordant indications of demethylation process
occurring in seabass that would be responsible for the systematically distinct δ202Hg values
observed between muscle and liver.
In conclusion, our findings constitute the first large scale Hg stable isotope study, on a
single fish species, from European coastal waters. They demonstrate the interest and
relevance of using Hg stable isotopes to investigate the Hg cycle and sources on both small
and large scales and show the possibility to differentiate between global and local Hg
sources. This takes a crucial sense in the current context where tracing Hg contamination
sources is necessary to implement efficient environmental policies.
