Endogenous methylmercury production: implications for biomagnification in pelagic food webs

Mercury (Hg) is a contaminant of global concern due to its bioaccumulation capacity.

The bioavailability of Hg is increased dramatically by converting inorganic Hg to methylmercury (MeHg) by anaerobic microbes with the hgcAB gene pair.

Today, Hg and MeHg concentrations in the Baltic fish and invertebrates exceed safe levels; moreover, no direct links are seen between the inputs and biota concentrations.Recently, we identified the hgcA gene in the gut microbiomes of Baltic copepods and amphipods, essential prey for fish, which suggests a potential for endogenous MeHg production at the base of the food webs.

However, it is still unclear whether any endogenous MeHg is produced and, if yes, how the production capacity varies across invertebrate taxa and regions.We propose to evaluate endogenous MeHg production by microbiomes of primary consumers in the Baltic pelagic (copepods) and benthic (amphipods) food webs.

Our approach will include (1) a large-scale Baltic survey of their gut microbiomes using molecular techniques to identify the hgcAB genes and their carriers in the animal guts, (2) experimental studies to trace dietary uptake of Hg to the MeHg in the consumers, and (3) a spatio-temporal modeling study to link endogenous Hg methylation in prey to Hg/MeHg concentrations in the Baltic herring.

Demonstrating endogenous methylation as an additional MeHg source in aquatic food webs would improve our global understanding of Hg cycle and MeHg bioaccumulation.