Environmental context. Mercury (Hg) presents an environmental concern owing to its transformation to the potent neurotoxin methylmercury (CH₃Hg⁺). The environmental factors that control bacterial methylation of mercury are poorly understood, but we know that methylmercury is bioaccumulated and biomagnified in aquatic food webs. We show that, even at low concentrations (~3 mg L^–1), natural dissolved organic matter strongly complexes with ionic Hg²⁺ and CH₃Hg⁺, thereby influencing biological uptake and methylation of Hg in aquatic environments.

Abstract. Complexation of the mercuric ion (Hg²⁺) and methylmercury (CH₃Hg⁺) with organic and inorganic ligands influences mercury transformation and bioaccumulation in aquatic environments. Using aqueous geochemical modelling, we show that natural dissolved organic matter (DOM), even at low concentrations (~3 mg L^–1), controls the Hg speciation by forming strong Hg-DOM and CH₃Hg-DOM complexes through the reactive sulfur or thiol-like functional groups in DOM in the contaminated East Fork Poplar Creek at Oak Ridge, Tennessee. Concentrations of neutral Hg(OH)₂, Hg(OH)Cl, CH₃HgCl, and CH₃HgOH species are negligible. Of the coexisting metal ions, only Zn²⁺, at concentrations of 1.6–2.6 × 10^–7 M, competes with Hg²⁺ for binding with DOM, causing decrease in Hg-DOM complexation but having little impact on CH₃Hg-DOM complexation. DOM may thus play a dominant role in controlling the transformation, biological uptake, and methylation of Hg in this contaminated ecosystem.