Biotic methylation of mercury by intestinal and sulfate-reducing bacteria and their potential role in mercury accumulation in the tissue of the soil-living Eisenia foetida

•Thin sections of Eisenia foetida show a Hg gradient from the gut to cuticle.•Intestinal bacteria activity led to MeHg accumulation in earthworm tissue.•9 of 14 tested Sulfate-reducing bacteria produced 100–1200 pg MeHg mL−1.•Mercury methylation by Desulfotomaculum ruminis was demonstrated for the first time.

Monomethylmercury as one of the most toxic mercury species influences the health and development of higher organisms and tends to accumulate in the tissue of animals and humans. The aim of this study was to explore the mercury methylating capability of (1) intestinal microbiota of the soil-living earthworm Eisenia foetida (E. foetida) and (2) intestinal sulfate reducing-bacteria in pure cultures. After exposing animals to inorganic mercury chloride (4 mg kg−1 Hg2+) in soil and sterile soil for ten days, the amount of methylmercury in tissue was measured. Despite sterilization of soil, the accumulation of the organic mercury species in tissue was 51 ng g−1. To elucidate the potential of mercury methylation by the intestinal bacterial community, the methylation of inorganic mercury (0.1 mg L−1 Hg2+) in growth media inoculated with soil, gut content and gut epithelia was compared. The media with microorganisms of gut content and gut epithelia reached 710–980 pg mL−1 methylmercury while the methylmercury amount in samples inoculated with soil was below the detection limit. This lead to the assumption that, in this experiment, the soil microbiota is not the main source for methylmercury. Furthermore, we investigated the distribution of mercury in tissue thin-sections of worms kept in soil containing mercury chloride using laser ablation inductively coupled plasma mass spectrometry. Thereby, a mercury gradient with highest signal intensities in the intestines and lowest intensities in the epidermis was measured. These results indicated that the biotic production of methylmercury in the gut of the soil-living earthworm E. foetida is mainly related to the intestinal and, to a much lesser extent, to the soil microbiota in their habitat. Moreover, we demonstrated the capability of intestinal sulfate-reducing bacteria in pure cultures to be a source for methylmercury. Ten of 14 intestinal sulfate-reducing bacteria were found to be able to produce methylmercury in growth medium. In the presence of 0.1 mg L−1 inorganic HgCl2 bacterial cultures produced 100–1200 pg mL−1 methylmercury in 12 h. The mercury methylating species were Desulfovibrio spp., Desulfovibrio piger, Desulfovibrio giganteus, Desulfovibrio termitidis, Desulfotomaculum ruminis and Desulfobulbus propionicus, and two enrichment cultures, Zy1 and EF4, originating from the gut of a damselfly and E. foetida, respectively. To the best of our knowledge, this is the first study concerning biotic methylation including several pure and enrichment strains of intestinal microorganisms and it is a new insight that mercury methylating capability was found within the genus Desulfotomaculum.