Fig. 5: Illustration of methylmercury bioavailability controlled by dissolved organic matter thiol concentrations.

Conceptual diagram depicting that dissolved organic matter’s thiol ligand content (DOM-RSH), not carbon content (DOC), dictates methylmercury (MeHg) bioavailability to plankton cells across the terrestrial-marine aquatic continuum. a shows how the DOM-RSH concentration may control uptake by a direct equilibrium partitioning of MeHg between DOM-RSH and plankton cells (case A) and/or by an equilibrium between MeHg(DOM-RS) and MeHgCl complexes internalized by cells with species-specific rates (case B). b DOM-RSH (nM) and DOC (µM) concentrations across the terrestrial-marine aquatic continuum are shown using system-averaged bar plots (full dataset presented in Table 1). The numerical values above the bars are the system specific RSH/DOC (nM/µM) ratios. The substantial and systematic change in DOM-RSH concentrations across the continuum is driven by a decrease in both RSH/DOC ratios and DOC concentrations, resulting in a larger range of observed DOM-RSH compared to DOC. The implications of a and b are conceptualized in c. Namely, the bioavailability of MeHg (depicted as volume concentration factors (VCF) in plankton) increases in the order: marsh < river ≈ estuary < shelf because of the decreasing DOM-RSH concentration in water. Source data are provided as a Source Data file.