# Influence of Macrophyte and Gut Microbiota on Mercury Contamination in Fish: A Microcosms Study
## 1 Introduction  
Mercury (Hg) is considered to be an important pollutant for aquatic systems since its organic form, i.e., methylmercury (MeHg), is easily bioaccumulated and biomagnified along food webs. It has been demonstrated that the aquatic plant rhizosphere are favorable ecological niches for Hg transformations. Several studies have demonstrated high Hg methylation and demethylation potentials in the periphton associated with floating macrophye roots in tropical ecosystems [1; 2; 3; 4; 5], in boreal regions [6], and in temperate areas [7]. Microorganisms, especially sulfate-reducing prokaryotes (SRP), were identified as the main elements responsible for MeHg production. Indeed, SRP were detected in periphton by 16S rDNA probing [3; 8], real-time polymerase chain reaction (RT-PCR) [4], and by the cultural method [8]. However, other functional groups such as iron reducers [9; 10], methanogens [11], and some fermenters [12] seem to also methylate Hg. Moreover, microorganisms able to methylate Hg seem to have the specific gene cluster _hgcAB_[13], which was spotted in a large diversity of prokaryotes and environments [14; 12]. For the time being, the link between the level of expression of these genesand the biotic MeHg production remains to be seen [15; 16], still requiring a cultural method to decide on the capacity of a strain to methylate. Aquatic roots supply a physical and organic-rich support for the development of microbial communities. Since periphyto accumulates metals through adsorption on inorganic and organic particulate material [17], including Hg [18; 19], the activity of these microorganisms in such a matrix can be a source of contamination for food webs [20; 21]. This conclusion is supported by the fact that macrophytes are considered to be an important food source for herbivous and detritivorous invertebrates and fish [22]. In southwestern France, invasive macrophytes like _Ludwigia_ sp. colonize several freshwater lakes and rivers, causing damage to aquatic ecosystems [23; 24] and interacting with Hg biogeochemistry and bioaccumulation. The evaluation of net MeHg budgets in these temperate ecosystems suggests that the aquatic rhizosphere is a hotspot for MeHg production due to SRP activity, and may represent an important source of contamination for the aquatic food chain [7]. In this context, the first objective of this study was to determine the bioaccumulation and bioavailability of Hg species, particularly MeHg formed in the rhizosphere of this invasive macrophyte in the food web, especially for fish. In this work, isotopically enriched inorganic mercury \({}^{199}\)IHg was used to monitor the distribution and conversion into \({}^{199}\)MeHg in different reconstituted aquatic compartments (water, sediment, plant, fish) within microcosm experimentations. The use of isotopically enriched tracers is a sensitive and reliable technique [25; 26] to follow Hg reactivity during laboratory experimentations, allowing researchers to work at low concentrations of Hg, close to environmental levels. Since ten years ago, the intestinal microbiota has been recognized as a major player in the health of its host as it takes part in major biological functions, such as nutrition [27], immunity [28], and the metabolism of xenobiotics [29]. Indeed, it could play a major role in the exposure of the host to certain chemical pollutants. Thus, the intestinal microbiota of aquatic organisms arose very recently as a potential compartment involved in the contamination of these organisms, via Hg methylation [30]. This endogenous methylation is therefore a major question in relation to the environmental problem of Hg (exposure of species and impacts) and to the understanding of the dynamics of this pollutant in ecosystems. However, there is currently a significant lack of knowledge on the subject [31]; the intestinal microbiota is therefore not taken into account today in Hg risk assessments, either in terms of the environment or human health.