Methylmercury production in high arctic wetlands

Abstract

Mercury is present at elevated levels in the top predators living in High Arctic ecosystems. Because only methylmercury (MeHg) bioaccumulates in food chains, the sources need to be identified. In temperate environments, wetlands are considered to be the principal sources of MeHg, with sulfate-reducing bacteria (SRB) thought to be responsible. The present study investigated whether High Arctic wetlands produced MeHg and whether SRB were involved in MeHg formation. Frozen soil was collected from 18 High Arctic wetlands before ground thaw, and when analyzed for MeHg, values were low, averaging 0.065 ng/g. When soils were incubated for 30 and 60 d at typical summer Arctic soil temperatures (4 degrees C and 8 degrees C), MeHg increased up to 100-fold. These laboratory observations were consistent with field measurements of wetland surface water, where MeHg concentrations increased from near detection limits (0.02 ng/L) at the inflow to an average of 1.21 ng/L at the outflow. Both laboratory and field data showed MeHg production in High Arctic wetlands. The prevalence of SRB in soil was low, however, and DNA analysis of the dissimilatory sulfate-reductase gene specific to SRB was positive at only one site. The present study showed that wetlands in the High Arctic can produce MeHg but that SRB may not the dominant mercury methylators.

The fate of mercury in Arctic terrestrial and aquatic ecosystems, a review

“…A study of Arctic wetland soils showed that MeHg concentrations increased 100-fold after thawing and incubating at typical Arctic summer temperatures (4 to 8 8C). [91] Although sulfatereducing bacteria are thought to be the predominant MeHg producers in temperate anoxic environments, the genes responsible for dissimilatory sulfate-reduction could not be detected from all the wetland sites studied. [91] This suggested either a methodological issue or that sulfate-reducers are in fact not the dominant microbial methylators in Arctic wetlands.…”

“…[91] Although sulfatereducing bacteria are thought to be the predominant MeHg producers in temperate anoxic environments, the genes responsible for dissimilatory sulfate-reduction could not be detected from all the wetland sites studied. [91] This suggested either a methodological issue or that sulfate-reducers are in fact not the dominant microbial methylators in Arctic wetlands. Results from a study of the biogeochemical cycling of MeHg in lakes and tundra watersheds of Alaska (688N) showed that the principal source of MeHg was in situ benthic production (80 to 91 % of total inputs), and that contributions from the tundra watershed snowpack and soils were modest.…”

“…[90] Sulfatereducing bacteria are able to methylate Hg II species under anaerobic conditions in Arctic sediments and wetlands. [77,91] Therefore, bacteria in Arctic snow, ice and aquatic (marine and fresh water) environments may play a critical role in the conversion of deposited Hg into MeHg or Hg 0 depending on several environmental factors including the level of oxygen present, the redox conditions and the presence (or absence) of sunlight. Although a correlation between heterotophic bacteria enumeration and MeHg concentrations in snow has been observed in subarctic ecosystems, [16] there is currently no report on the isolation of Hg-methylating bacteria from the cryosphere.…”

“…[34,35] In the case of the Beaufort Shelf an annual delivery of 2.2 t year À1 of THg and 15 kg year À1 of MeHg was estimated for the Mackenzie River alone. [35] Rivers clearly provide a conduit for terrestrial Hg and MeHg from wetlands and snowmelt [91] to enter estuaries. Arctic rivers also provide DOC, POC and suspended sediment, which may sequester Hg and MeHg thus preventing their entry into estuarine food webs.…”

A synthesis of atmospheric mercury depletion event chemistry in the atmosphere and snow

“…Sulfate reducing bacteria (SRB) are considered to be the main Hg methylators in aquatic temperate ecosystems and may play a role in the Arctic soils, streams and lakes; however this assumption requires further testing. Loseto et al (2004b) collected sediments in Arctic wetlands prior to spring thaw. The MeHg concentration in the frozen sediments was low when they were transported to the laboratory (average 0.065 ng/g) but after incubation at 6 and 8 • C for 30 and 60 days, values increased up to 100 fold in some samples.…”