Organic ligands control microbial uptake of Hg(II): effects on Hg(II) speciation and bacterial physiology

How organic ligands regulate microbial Hg(II) bioavailability remains a critical knowledge gap in elucidating methylmercury production and subsequent bioaccumulation in biota. Six model ligands with different functional groups (ethylenediaminetetraacetate [EDTA], citrate [CTA], cysteine [CYS], glutathione [GSH], 2, 3-dimercapto-1-propanesulfonate [DMPS], and 3-mercapto-1-propanesulfonate [MPS]) were applied to probe their impacts on Hg(II) bioavailability, by direct measurement of active cellular Hg(II) and cell physiology using a Hg(II)-specific (Escherichia coli PRL) and a constitutive biosensor (E. coli PJL), respectively. Hg(II) uptake was significantly inhibited by EDTA, CTA, DMPS, and MPS, whereas enhanced by CYS and GSH. These findings indicate co-occurred uptake mechanism other than the widely accepted membrane-ligand exchange model, since bioavailability did not inversely correlate with Hg-binding strength (CTA < EDTA < GSH < CYS < DMPS). Furthermore, ligand-induced bacterial physiological changes (e.g., thiolate inhibition vs. carboxylate neutrality) were not the primary driver of bioavailability variations. Based on Hg(II) speciation analysis and thermodynamic modelling, it was implied that sulfate reduction and CYS degradation generated sulfide corona near bacterial cells, promoting the formation of Hg sulfide nanoparticles (HgS_NP) or Hg-S clusters during Hg(II) uptake. Enhanced bioavailability of Hg(II)-CYS/GSH likely arose from passive diffusion and intracellular dissolution of these HgS species. Ambient oxygen levels further modulate Hg(II) bioavailability, with enhanced Hg(II)-CYS uptake observed under anoxic conditions, likely due to oxygen-dependent transporters expression and bacterial metabolisms. Overall, these findings highlight ligand-specific mechanisms governing Hg(II) bioavailability, emphasizing the interactions of Hg-binding strength, cell physiology, and sulfide-driven speciation in Hg biogeochemical cycling.