Recent advances in the study of mercury methylation in aquatic systems

Abstract

With increasing input of neurotoxic mercury to environments as a result of anthropogenic activity, it has become imperative to examine how mercury may enter biotic systems through its methylation to bioavailable forms in aquatic environments. Recent development of stable isotope-based methods in methylation studies has enabled a better understanding of the factors controlling methylation in aquatic systems. In addition, the identification and tracking of the hgcAB gene cluster, which is necessary for methylation, has broadened the range of known methylators and methylation-conducive environments. Study of abiotic factors in methylation with new molecular methods (the use of stable isotopes and genomic methods) has helped elucidate the confounding influences of many environmental factors, as these methods enable the examination of their direct effects instead of merely correlative observations. Such developments will be helpful in the finer characterization of mercury biogeochemical cycles, which will enable better predictions of the potential effects of climate change on mercury methylation in aquatic systems and, by extension, the threat this may pose to biota.

Increase in Nutrients, Mercury, and Methylmercury as a Consequence of Elevated Sulfate Reduction to Sulfide in Experimental Wetland Mesocosms

“…The relatively large increase in surface water MeHg in response to increased (SO 4 ) Depl in this experiment supports the assumption that MSR was responsible for most of the observed production of MeHg. It is likely that increased SO 4 loading to low‐SO 4 aquatic systems with organic sediment will result in increased Hg methylation even though the relative importance of Hg methylation in the environment by different groups of bacteria is still a subject of debate (Paranjape & Hall, ).…”

Biotic formation of methylmercury: A bio–physico–chemical conundrum

“…Comprehensive evaluations of the chemical and physical processes that govern Hg distribution and fate among the major environmental compartments can be found in the literature (Chételat et al ; Sundseth et al ; Kim et al ; Bjørklund et al ; Dranguet et al ; Paranjape and Hall ; Klapstein and Driscoll ). Briefly, in the water column Hg II can (1) be reduced to Hg 0 and reemitted back to the atmosphere, (2) methylated to the organic form MMHg, or (3) bind to organic matter (OM) as well as inorganic particles and directly deposit to bottom sediments.…”

“…Besides the presence and diversity of Hg II methylating microbes, Hg II methylation depends on the amount of Hg II bioavailable for methylation (Schaefer et al ; Jonsson et al ), which is determined by chemical speciation of Hg II , solubility of the Hg‐S particles (Hsu‐Kim et al ; Liem‐Nguyen et al ) as well as the availability of electron donors and acceptors for Hg II methylating microorganisms (Desrochers et al ). Hg II methylation is a bio‐physico‐chemical conundrum because both the amount of Hg II available for methylation and the activity of microorganisms involved in the process are determined by multiple physico‐chemical variables such as sulfur (Skyllberg et al ; Drott et al ), iron (Bravo et al ) and OM concentration and speciation (Schartup et al ; Bravo et al ) as well as Eh, pH, nutrient availability, and temperature (Ullrich et al ; Paranjape and Hall ) (Fig. ).…”

Survey of mercury in boreal chorus frog (Pseudacris maculata) and wood frog (Rana sylvatica) tadpoles from wetland ponds in the Prairie Pothole Region of Canada

“…Factors that influence HgII methylation and MeHg bioavailability are important controls in MeHg bioaccumulation in aquatic organisms (Wiener 2010;Li and Cai 2013;Hsu-Kim et al 2013;Paranjape and Hall 2017). We explored water chemistry and physical parameters that are often useful in explaining the variation of MeHg and THg in whole water (such as DOC, pH, total anion concentrations; see Methods section for complete list), and thus Hg in aquatic animals, using PCA (Fig.…”

“…They are also important breeding and nesting habitats for avian wildlife in North America (Batt et al 1989). Atmospherically deposited Hg may be readily methylated (Hoggarth et al 2015) and accumulated (Sando et al 2007;Hall et al 2009) in these environments because prairie wetlands have potentially high dissolved organic carbon (DOC) concentration, a warm temperature in the summer months, and low dissolved oxygen; all variables which lead to anoxic conditions in which the main methylating organisms (sulfate and iron reducing bacteria and methanogens (Gilmour et al 2013;Parks et al 2013)) thrive (Paranjape and Hall 2017).…”