Methylation of Mercury in Earthworms and the Effect of Mercury on the Associated Bacterial Communities

Abstract

Methylmercury compounds are very toxic for most organisms. Here, we investigated the potential of earthworms to methylate inorganic-Hg. We hypothesized that the anaerobic and nutrient-rich conditions in the digestive tracts of earthworm's promote the methylation of Hg through the action of their gut bacteria. Earthworms were either grown in sterile soils treated with an inorganic (HgCl2) or organic (CH3HgCl) Hg source, or were left untreated. After 30 days of incubation, the total-Hg and methyl-Hg concentrations in the soils, earthworms, and their casts were analyzed. The impact of Hg on the bacterial community compositions in earthworms was also studied. Tissue concentrations of methyl-Hg in earthworms grown in soils treated with inorganic-Hg were about six times higher than in earthworms grown in soils without Hg. Concentrations of methyl-Hg in the soils and earthworm casts remained at significantly lower levels suggesting that Hg was mainly methylated in the earthworms. Bacterial communities in earthworms were mostly affected by methyl-Hg treatment. Terminal-restriction fragments (T-RFs) affiliated to Firmicutes were sensitive to inorganic and methyl-Hg, whereas T-RFs related to Betaproteobacteria were tolerant to the Hg treatments. Sulphate-reducing bacteria were detected in earthworms but not in soils.

Comparison of mercury and methylmercury bioaccumulation in earthworms (<i>Bimastus parvus</i>) native to landfill-leachate-contaminated forest soil

“…Earthworms (Eisenia foetida) were found to potentially methylate Hg in their digestive tract (Hinton and Veiga, 2002). In addition, earthworms (Lumbricus terrestris) showed MeHg synthesis in vivo due to the sulfatereducing bacteria found in earthworms but not in soils (Rieder et al, 2013). In our study, MeHg concentrations in the forest soil and native B. parvus earthworms were expected to increase due to the higher soil THg concentrations in the leachate-contaminated forest sites compared to the reference sites.…”

“…It has been generally accepted that inorganic forms of Hg can be methylated by soil bacterial activity (mostly under anaerobic conditions); methylation enhances bioavailability and bio-toxicity and increases potential threats to soil ecosystem functions (Shao et al, 2012). However, recent studies have found that inorganic Hg is methylated within the guts of earthworms and not by soil bacterial action, based on the fact that sulfate-reducing bacteria were detected in the gut tract of earthworms (Kaschak et al, 2014;Rieder et al, 2013).…”

Total mercury and methylmercury concentrations over a gradient of contamination in earthworms living in rice paddy soil

“…Other soil properties, such as texture, or redox conditions, may also be important factors. Furthermore, earthworms may vary within and between species in their capacity to methylate inorganic Hg in their digestive systems , and these differences may be producing the high levels of variation that were observed (although we note that the earthworm communities were similar at different sites). The present study is unable to answer questions about in vivo methylation processes in earthworms, and this issue should be a priority research direction in the future.…”

Is gastrointestinal microbiota relevant for endogenous mercury methylation in terrestrial animals?

“…(2014) In vitro incubation of gut inhabiting bacteria of the earthworm Lumbricus terrestris treated with HgCl 2 or CH 3 HgCl in sterile and non-sterile soils Rieder et al (2013) In vitro incubation of intestinal microbiota of the earthworm Eisenia foetida and intestinal sulfate reducing-bacteria in pure cultures with HgCl 2 in sterile and non-sterile soils time. They are based on the MeHg extraction by the Westöö method and the identification of the cyanide and iodide salts by thin-layer chromatography (TLC) which involve many limitations for a correct isolation and determination of MeHg.…”

“…Hence, these results cannot confirm that methylation takes place in the guts so that further experiments in this field should be performed. Rieder et al (2013) conducted a very interesting study with natural soils but sterilized, so it was possible to discriminate MeHg produced in the soil or in the earthworm gut. They performed two independent experiments with the earthworm Lumbricus terrestris and sterile soils.…”