LETTERS PUBLISHED ONLINE: 17 APRIL 2011 | DOI: 10.1038/NGEO1134 Methylation of inorganic mercury in polar marine waters Igor Lehnherr 1 * , Vincent L. St. Louis 1 , Holger Hintelmann 2 and Jane L. Kirk 1 † Monomethylmercury is a neurotoxin that accumulates in marine organisms, with serious implications for human health 1 . The toxin is of particular concern to northern Inuit peoples, for example, whose traditional diets are composed primar- ily of marine mammals and fish 2 . The ultimate source of monomethylmercury to marine organisms has remained uncer- tain, although various potential sources have been proposed, including export from coastal 3 and deep-sea 4 sediments and major river systems 5,6 , atmospheric deposition 7 and water- column production 8,9 . Here, we report results from incubation experiments in which we added isotopically labelled inorganic mercury and monomethylmercury to seawater samples col- lected from a range of sites in the Canadian Arctic Archipelago. Monomethylmercury formed from the methylation of inorganic mercury in all samples. Demethylation of monomethylmer- cury was also observed in water from all sites. We de- termined steady-state concentrations of monomethylmercury in marine waters by incorporating the rate constants for monomethylmercury formation and degradation derived from these experiments into a numerical model. We estimate that the conversion of inorganic mercury to monomethylmercury in the water column accounts for around 47% (±62%, standard deviation) of the monomethylmercury present in polar marine waters, with site-to-site differences in inorganic mercury and monomethylmercury levels accounting for most of the vari- ability. We suggest that water-column methylation of inor- ganic mercury is a significant source of monomethylmercury in pelagic marine food webs in the Arctic, and possibly in the world’s oceans in general. Various potential sources of monomethylmercury (MMHg) to marine waters have been postulated, including in situ production in the water column 8,9 . It has been hypothesized that inorganic Hg(ii) is methylated in marine waters, because depth profiles of methylated Hg (MMHg + dimethylmercury (DMHg; a toxic volatile form of Hg)) in the Pacific 9,10 and Atlantic 11,12 Oceans, the Mediterranean 13,14 and Black 15 Seas and the Canadian Arctic Archipelago (CAA; ref. 16) all show a peak in concentration in subthermocline waters, above and below which methylated Hg concentrations are typically lower. This vertical distribution of methylated Hg is consistent with the notion that there is (1) a net source of methylated Hg to intermediate waters as a result of either in situ Hg(ii) methylation or the import of methylated Hg from sinking particulate matter, and (2) loss of methylated Hg, through net demethylation and/or export, in surface and deep waters 9,13 . Furthermore, laboratory experiments have also demonstrated that pure bacterial cultures isolated from polar marine waters are capable of methylating various metals including Hg (ref. 17). Despite this circumstantial evidence, virtually no in situ 1 Department of Biological Sciences, University of Alberta, Edmonton, Alberta, T6G 2E9, Canada, 2 Department of Chemistry, Trent University, Peterborough, Ontario, K9J 7B8, Canada. † Present address: Canada Centre for Inland Waters, Environment Canada, Burlington, Ontario, L7R 4A6, Canada. *e-mail: lehnherr@ualberta.ca. Atmosphere Degradation DMHg MMHg Algae and bacteria k d + k pd k ? Chlorophyll maximum MMHg Hg(II) DMHg Hg(II) MMHg DMHg Oxycline Deposition Gas exchange Microbial POC remineralization k m2 k d k m1 k m3 k m2 k ? k m1 k m3 Sinking POC Zooplankton Figure 1 | Conceptual model of Hg methylation/demethylation in marine waters. The various Hg methylation and (photo)demethylation pathways (thin arrows), each governed by their respective rate constants (k; k m = methylation, k d = demethylation, k pd = photodemethylation), along with associated biogeochemical processes (thick arrows), such as air–water gas exchange of DMHg and remineralization of POC, and MMHg bioaccumulation/biomagnification (dashed arrows). Note that owing to space constraints, and because the focus is on Hg methylation/ demethylation, redox reactions involving Hg(0) are not shown in this schematic diagram. For definitions of k m1 , k m2 and k m3 see Table 1. measurements of Hg(ii) methylation exist to test the hypothesis that marine waters are an important source of methylated Hg species. However, recent analytical advances 18 have spurred the use of Hg stable isotopes as tools to examine Hg biogeochemical transformations in the environment, enabling us to now directly test this hypothesis. We used two Hg stable-isotope spikes, 198 Hg(ii) and MM 199 Hg, to examine methylation (formation of MM 198 Hg, DM 198 Hg and DM 199 Hg), and demethylation (loss of MM 199 Hg, including total and photo-demethylation) processes in marine waters of the CAA (Fig. 1). Seawater samples were collected at five stations across the CAA (Supplementary Fig. S1) between 27 September and 18 October 2007, while aboard the CCGS Amundsen icebreaker. At each station, 12 l Teflon-lined Niskin bottles (General Oceanics) mounted on the ship’s rosette system were used to obtain samples from two biologically important depths, the subsurface chlorophyll maximum (SCM, the depth of maximum chlorophyll a fluorescence and hence algal biomass) and the lower oxycline (a zone of high heterotrophic respiration and organic carbon decomposition) (Supplementary Fig. S2). Potential rates of Hg 298 NATURE GEOSCIENCE | VOL 4 | MAY 2011 | www.nature.com/naturegeoscience © 2011 Macmillan Publishers Limited. 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