Thiol-mediated reaction kinetics between dissolved organic matter and Hg(II) regulate Hg(II) bioavailability: Evidence from whole-cell biosensors and methylating bacteria.

Dissolved organic matter (DOM), particularly its thiol moieties, kinetically controls Hg(II) speciation, thereby regulating the pool of bioavailable Hg(II). However, how variations in thiol abundance within DOM influence Hg(II)-DOM reaction kinetics and Hg(II) bioavailability remains unclear. Here, we investigated the Hg(II)-DOM reaction kinetics driven by DOM thiol abundance under dark/light conditions, as well as their effects on microbial Hg(II) uptake and methylation. This was achieved by employing an Escherichia coli-based whole-cell biosensor and a Geobacter sulfurreducens PCA methylation assay. Hg(II) bioavailability declined with longer Hg(II)-DOM reaction times, with high-thiol DOM exhibiting greater suppression than low-thiol DOM. This kinetics- and thiol abundance-dependent decrease in bioavailability was attributed to the formation of strong Hg-thiol complexes or Hg-sulfide nanoparticles (HgSNP). Notably, light exposure further reduced Hg(II) bioavailability in low-thiol DOM but had little effect in high-thiol systems, where HgSNP formed rapidly (≤1 h) even in darkness. This contrast arose from light-induced HgSNP formation in low-thiol DOM. Additionally, DOM thiols also inhibited Hg(0) generation throughout the reaction. Given the natural variability in thiol abundance across aquatic environments, our findings highlight its critical role in kinetically regulating Hg(II) bioavailability. These results offer key mechanistic insights into microbial Hg(II) uptake and methylation.