# Soil Hg Contamination Impact on Earthworms'
## 1 Introduction  
Mercury (Hg) is one of the most toxic non-essential trace metals in the environment and is globally considered a pollutant with high persistence in the environment. Hence, it remains a major threat to both aquatic and terrestrial ecosystems and to public health [1; 2; 3]. The toxicity of Hg in the soil is highly dependent on its chemical speciation. Of all its forms, methyl-Hg (MeHg) is a potent neurotoxin, which has a high potential for bioaccumulation along the food chain. The ecotoxicological impact of Hg in the soil also depends on its bioavailable fraction. Hg can adsorb to soil particles and minerals, and only a minor fraction of total Hg is soluble and thus available for microbial transformations [4]. Furthermore, soil organic matter and pH have been shown to be key abiotic factors in Hg bioavailability [5; 6; 7]. Similarly, the methylation of Hg into MeHg is affected by soil organic matter content and pH, as well as the ionic strength of soil particles [8; 9]. Earthworms constitute up to 90% of the total faunal biomass in terrestrial ecosystems [10]. They are considered soil keystone species because they are an essential component of the soil-litter food-web, contributing to the decomposition of organic matter, the cycling of nutrients, the mixing of the organic and inorganic fractions of soil, the formation of stable aggregates, and the aeration and water infiltrationof soil via their burrows. Earthworms are in direct contact with the soil, either by skin contact when moving through their habitat or by direct soil ingestion and metabolism, and are therefore potentially affected by Hg contamination. Earthworms have been observed to be relatively resistant to Hg concentrations: during an incubation experiment, they survived 28 days in a soil spiked with extremely high Hg concentrations (up to 262 mg kg\({}^{-1}\); [4]). Nevertheless, Hg accumulation in earthworm tissues interfered negatively with their growth, reproduction, biochemistry, and genetics [4]. Furthermore, soil properties such as soil organic content, concentrations of organic acids, and pH have been shown to enhance the uptake of Hg and increase the MeHg accumulation in the organisms [11; 12]. Interestingly, the ratio of methyl-Hg to total Hg (MeHg:THg) has been observed to deviate drastically in earthworm tissue compared to values in the soil [13]; it was up to 160 times higher in _Eisenia fetida_ earthworm tissues than in the surrounding substrate [12]. However, the mechanisms behind the bioaccumulation of MeHg and THg in earthworms are still elusive. Increased MeHg:THg in earthworms could originate from a more efficient accumulation of MeHg over THg due to higher lipo-solubility [14; 15]. Other lines of evidence support the idea that the methylation is controlled by the earthworm intestinal microbiome [16]. Supporting this last explanation, microbes isolated from the earthworm gut and epithelia have been observed to produce more MeHg than microbes isolated from the surrounding soil [17]. The capacity of microbes to methylate Hg relies on certain genes including the HgcAB cluster [18]. This genetic marker is phylogenetically widely dispersed within the bacterial kingdom [19]. Syntrophic and fermentative Firmicutes and Bacteroidetes, as well as many anaerobic iron- and sulfur-reducing bacteria have been found to carry these genes in their genomes [19; 20]. Moreover, the abundance of these taxa has been shown to be positively correlated with total Hg content [21], implying that they are also resistant to total Hg contamination. The central gene cluster involved in the resistance mechanisms and detoxification of MeHg in microorganisms is the mer operon [22]. Among other genes, the merA gene encodes mercury reductase, which transforms Hg(II) into volatile Hg(0), and the merB gene is associated with the demethylation of Hg [23]. MerA gene abundance was found to increase drastically with soil Hg content [24], especially in soils with an acidic pH (<7), when a higher proportion of the Hg was bioavailable [5].