Canadian Forest Service Publications
Molecular correlations of dissolved organic matter with inorganic mercury and methylmercury in Canadian boreal streams. 2022. Mangal, V.; Lam, W.Y,; Huang,H.; Emilson, E.J.S.; Mackereth, R.W.; Mitchell. Biogeochemistry 160: 127-144.
Issued by: Great Lakes Forestry Centre
Catalog ID: 40773
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The molecular composition of dissolved organic matter (DOM) is increasingly recognized as fundamentally important to mercury transport and transformations, with numerous approaches undertaken to examine DOM characteristics beyond dissolved organic carbon concentrations. In this study, we use a high-resolution mass spectrometry approach, Fourier-transform ion cyclotron resonance mass spectrometry, to characterize DOM compound classes, DOM aromaticity (AImod), and the nominal oxygenation state of carbon (NOSC) across thirteen small boreal forest streams in central Canada. We then relate the relative abundance of hundreds of different DOM molecules with inorganic mercury and methylmercury (MeHg) concentrations across late spring and fall seasons. The number of significant correlations and the classes of DOM compounds significantly correlating with inorganic mercury and MeHg concentrations differs substantially across seasons and between mercury forms. For inorganic mercury, the abundance of nitrogen and sulfur containing DOM are most often positively correlated (mean ρ = 0.80) in the late spring, whereas during the fall, the abundance of low-oxidized lignins is more important, though with weaker correlations (mean ρ = 0.51). For MeHg, low-oxidized lignins and hydrolysable tannins, likely sourced from conifer throughfall and litter, account for up to 83% of all DOM-MeHg correlations regardless of season. Further network analyses reveal that the strongest and most significant inorganic mercury-DOM correlations are found across a wide range of NOSC values, indicating that DOM involved with the transport of inorganic mercury encompasses a wide range of polarities and thermodynamic stabilities. In contrast, DOM molecules exclusively correlated with MeHg concentrations have more positive NOSC and AImod values, implying the preferential transport of MeHg with more thermodynamically stable and aromatic DOM molecules. DOM molecules significantly correlated with both inorganic mercury and MeHg concentrations are found exclusively in the late spring. Overall, this non-targeted approach may help to inform further targeted investigations, especially as it relates to the underrepresented importance of plant biomolecules in facilitating mercury transport.
Plain Language Summary
The molecular composition of dissolved organic matter (DOM) is recognized as fundamentally important to mercury transport and transformations within forested ecosystems, and thus toxicity in aquatic systems. In this study, we characterize DOM compound classes, DOM aromaticity, and the nominal oxygenation state of carbon across thirteen small forested streams in central boreal Canada, and relate the relative abundance of different forest DOM molecules with mercury concentrations. We find that the relationships between mercury and DOM differ substantially with season, with nitrogen and sulfur containing DOM playing a strong role in the late spring, and low-oxidized lignins playing a bigger role in the fall. The most toxic form, however, was driven by low-oxidized lignins and hydrolysable tannins, likely sourced from conifer throughfall and litter and accounting for up to 83% of all DOM-MeHg correlations regardless of season. Overall, this non-targeted approach may help to inform further targeted investigations, especially as it relates to the underrepresented importance of forest-derived plant biomolecules in facilitating mercury transport, suggesting further required studies in the realm of forest management and mercury transport.