Potential for Mercury Reduction by Microbes in the High Arctic

Abstract

The contamination of polar regions due to the global distribution of anthropogenic pollutants is of great concern because it leads to the bioaccumulation of toxic substances, methylmercury among them, in Arctic food chains. Here we present the first evidence that microbes in the high Arctic possess and express diverse merA genes, which specify the reduction of ionic mercury [Hg(II)] to the volatile elemental form [Hg(0)]. The sampled microbial biomass, collected from microbial mats in a coastal lagoon and from the surface of marine macroalgae, was comprised of bacteria that were most closely related to psychrophiles that had previously been described in polar environments. We used a kinetic redox model, taking into consideration photoredox reactions as well as mer-mediated reduction, to assess if the potential for Hg(II) reduction by Arctic microbes can affect the toxicity and environmental mobility of mercury in the high Arctic. Results suggested that mer-mediated Hg(II) reduction could account for most of the Hg(0) that is produced in high Arctic waters. At the surface, with only 5% metabolically active cells, up to 68% of the mercury pool was resolved by the model as biogenic Hg(0). At a greater depth, because of incident light attenuation, the significance of photoredox transformations declined and merA-mediated activity could account for up to 90% of Hg(0) production. These findings highlight the importance of microbial redox transformations in the biogeochemical cycling, and thus the toxicity and mobility, of mercury in polar regions.

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

“…[55] A model for Hg 0 with depth in the water column incorporating photoreduction, photooxidation, bioreduction and biooxidation was recently developed. [60] The model results suggest that because of: (i) light energy attenuation with depth and (ii) the presence of chloride, biologically mediated processes are likely to dominate the production of Hg 0 . This research focussed on the mercury resistance mer-operon, but there are likely other processes involved such as phytoplankton blooms as was initially postulated.…”

“…[60] There is good evidence that microbes are metabolically active at subzero temperatures in snow [54,83,84] and sea ice. [85] This raises the question of whether deposited Hg II can be actively transformed into other species (GEM or MeHg) by microbes in the Arctic cryosphere (snow, sea ice, freshwater ice).…”

“…O c e a n s e d im e n ts O c e a n b a c te ri a A lg a e o r p h y to p la n k to n Methylmercury as a percentage of total mercury Methyl mercury (ppm) 60 80 100…”

“…One of the dominant mechanisms by which carbon controls the ultimate fate of Hg is through absorption of light in the water column. [2,60,[92][93][94] C-DOM (coloured dissolved organic matter), a powerful absorber of UV radiation, provides a particularly important limit on photodemethylation and photo-reduction rates. As a consequence, an inverse relationship has been observed between DOC levels and DGM formation in Arctic lakes.…”

A mass balance inventory of mercury in the Arctic Ocean

“…Summertime evasion could become a major Hg removal process from continental shelves in a warmer Arctic Ocean, as it is presently in the Baltic Sea. [56,58] There is already an active microbial community present in Arctic seawater capable of reducing environmentally significant amounts of inorganic Hg II , [110] and the shrinking area of sea-ice will increase UV radiation penetration and therefore probably photoreduction of Hg II to Hg 0 . The present latitudinal gradient of oceanic GEM evasion can be used to project future increases for the Arctic.…”