Dissolved Organic Carbon Thresholds Affect Mercury Bioaccumulation in Arctic Lakes

Abstract

Dissolved organic carbon (DOC) is known to affect the Hg cycle in aquatic environments due to its overriding influence on complexation, photochemical, and microbial processes, but its role as a mediating factor in the bioaccumulation of Hg in aquatic biota has remained enigmatic. Here, we examined 26 tundra lakes in Canada's western Arctic that span a large gradient of DOC concentrations to show that total Hg (HgT) and methyl mercury (MeHg) accumulation by aquatic invertebrates is defined by a threshold response to Hg-DOC binding. Our results showed that DOC promotes HgT and MeHg bioaccumulation in tundra lakes having low DOC (<8.6 - 8.8 mg C L(-1); DOC threshold concentration, TC) whereas DOC inhibits HgT and MeHg bioaccumulation in lakes having high DOC (>DOC TC), consistent with bioaccumulation results in a companion paper (this issue) using a microbial bioreporter. Chemical equilibrium modeling showed that Hg bioaccumulation factors were elevated when Hg was associated mainly to fulvic acids, but became dramatically reduced when DOC was >8.5 mg C L(-1), at which point Hg was associated primarily with strong binding sites on larger, less bioaccessible humic acids. This study demonstrates that the biological uptake of Hg in lakes is determined by binding thresholds on DOC, a water quality variable predicted to change markedly with future environmental change.

Influence of rice straw amendment on mercury methylation and nitrification in paddy soils

“…The DOC derived from decomposition of rice straw may increase Hg availability to Hg methylating microbes by forming DOMeHg complexes (Driscoll et al, 2012;Graham et al, 2013). This is consistent with previous studies suggesting an enhanced Hg methylation under low sulfidic (Graham et al, 2012a) or low DOC conditions (French et al, 2014). Thus, the effect of DOM on Hg uptake and methylation by microbes may strongly depend on environmental conditions (Schartup et al, 2015).…”

“…Previous study has showed highly diverse anaerobic microbes in paddy soils that had potential to convert inorganic Hg into methylmercury (MeHg) (Liu et al, 2014a), which were predominantly distributed in Deltapreteobacteria, Firmicutes, Euryarchaeota and several unclassified groups based on analysis of HgcAB orthologs. In natural ecosystems, however, Hg methylation could not be completely predicted by hgcAB carrying microbes, which may be modulated by Hg availability, soil variables such as redox status, SO 4 2À and characteristics of dissolved organic carbon (DOC) (French et al, 2014;Graham et al, 2013Graham et al, , 2012bLiu et al, 2014b). Rice straw is commonly incorporated into fields to enhance soil nutrient (Mandal et al, 2004;Yadvinder-Singh et al, 2004), and now this agricultural practice is getting more intensive than before in China due to a new policy that prohibits burning straw as this is considered to be one of the most important sources of air pollution.…”

“…Previous studies have suggested that decomposition of rice straw by anaerobic breakdown was the main source of organic carbon in paddy soils (Liesack et al, 2000;Ye et al, 2015), which may affect Hg uptake and methylation by microbes due to its strong binding with Hg 2þ that reduce Hg bioavailability and reactivity (Barkay et al, 1997;Zhu et al, 2015). However, recent studies argued that dissolved organic matter (DOM) enhanced Hg methylation at relatively low DOM content or under low sulfidic conditions (French et al, 2014;Graham et al, 2012a). This is partially consistent with the recent field study indicating a positive correlation between MeHg and soil organic matter content in paddy fields around Hg mining area (Liu et al, 2014a).…”

Towards Mechanistic Understanding of Mercury Availability and Toxicity to Aquatic Primary Producers

“…For example, CH 3 Hg content in the diatom Cyclotella meneghiniana decreased in the presence of DOM; however the effect was more pronounced in the presence of the highly aromatic hydrophobic fraction of DOM than in the presence of the transphilic one. [35] By contrast, 8 mg/L DOM promoted Hg uptake in aquatic invertebrates [37] and bacteria. [38] Low molecular weight DOM fractions enhanced Hg accumulation in plankton, while high molecular weight reduced it.…”

A Comparison of Mercury Biomagnification through Lacustrine Food Webs Supporting Brook Trout (Salvelinus fontinalis) and Other Salmonid Fishes

“…Dissolved organic carbon (DOC) quantity and quality influence the delivery of Hg to lakes from the catchment, the availability of Hg(II) for methylation, and the uptake of MeHg into primary producers and their consumers, though not always in a consistent manner (Greenfield et al, 2001;Haitzer et al, 2003;Weech et al, 2004;Kamman et al, 2005;Simonin et al, 2008;Gabriel et al, 2009;Lescord et al, 2015). There have been contradictory results with regard to DOC as a predictor of biotic Hg concentrations (e.g., Miskimmin et al, 1992;Driscoll et al, 1995;Ravichandran, 2004) and recent evidence of a DOC threshold for bioavailability of MeHg to lower-trophic-levels (Deison et al, 2012;French et al, 2014).…”