Mercury (Hg) remobilized from the earth crust by natural and anthropogenic processes and methylated in natural environments can become bioaccumulated in aquatic food webs to concentrations of concern for human and wildlife heath. Today, Hg and MeHg concentrations in the Baltic fish and mussels exceed the thresholds compliant with good environmental status. To understand the risks and develop adequate mitigation strategies, we need to understand production and accumulation pathways related to MeHg levels in aquatic food webs. We hypothesize that Hg-methylating bacteria inhabiting guts of invertebrates in the lower food webs is an ecologically plausible source of their MeHg body burden and thus MeHg entering the food web. I will test this hypothesis using molecular techniques, PCR and sequencing, to identify the hgcAB genes and their carriers in the dissected guts of amphipods, with particular emphasis on interspecific (geographic and ontogenetic) and intraspecific variability. A laboratory experiment will be conducted to evaluate effects of dietary uptake of Hg on the MeHg levels in amphipods and, thus, examine whether endogenous Hg methylation can occur. Using field data and laboratory experiments, a relationship between the hgcAB abundance in the gut microbiome and animal MeHg body burden will be established. To our knowledge, this is a first study addressing endogenous methylation in benthic invertebrates as a MeHg source in ecosystem. These findings will provide crucial insights on the origin and pathways of MeHg in aquatic food webs.