Competitive Binding Kinetics of Methylmercury–Cysteine with Dissolved Organic Matter: Critical Impacts of Thiols on Adsorption and Uptake of Methylmercury by Algae

Complexes with low-molecular-weight thiols are crucial species of methylmercury (MeHg) excreted by anaerobic Hg-methylating microbes, notably, MeHg–cysteine (MeHg–Cys). As MeHg–Cys diffuses into surface water, it would undergo a ligand exchange process with dissolved organic matter (DOM) under nonsulfidic conditions, inevitably altering MeHg speciation and bioavailability to phytoplankton. In this study, we investigated the competitive binding kinetics between MeHg–Cys and Suwannee River natural organic matter, and their influence on the adsorption and uptake of MeHg by the cyanobacterium, Synechocystis sp. PCC6803. Liquid chromatography-inductively coupled plasma mass spectrometry was employed to monitor the kinetics processes involving competition of DOM with Cys for MeHg binding, which revealed that competitive binding kinetics were dictated by the abundance of thiol moieties in DOM. Thiol concentrations of 0.97 and 49.34 μmol of thiol (g C)−1 resulted in competitive binding rate constant (k values) of 0.30 and 3.47 h–1, respectively. Furthermore, the time-dependent competitive binding of DOM toward MeHg–Cys significantly inhibited MeHg adsorption and uptake by cyanobacteria, an effect that was amplified by an increased thiol abundance in DOM. These findings offer valuable insights into the kinetic characteristics of MeHg's fate and transport, as well as their impact on bioconcentration in aquatic organisms within natural aquatic ecosystems.