Particle-Bound Hg(II) is Available for Microbial Uptake as Revealed by a Whole-Cell Biosensor

Particle-bound mercury (Hg_P), ubiquitously present in aquatic environments, can be methylated into highly toxic methylmercury, but it remains challenging to assess its bioavailability. In this study, we developed anEscherichia coli-based whole-cell biosensor to probe the microbial uptake of inorganic Hg(II) and assess the bioavailability of Hg_P sorbed on natural and model particles. This biosensor can quantitatively distinguish the contribution of dissolved Hg(II) and Hg_P to intracellular Hg. Results showed that the microbial uptake of Hg_P was ubiquitous in the environment, as evidenced by the bioavailability of sorbed-Hg(II) onto particulate matter and model particles (Fe₂O₃, Fe₃O₄, Al₂O₃, and SiO₂). In both oxic and anoxic environments, Hg_P was an important Hg(II) source for microbial uptake, with enhanced bioavailability under anoxic conditions. The composition of particles significantly affected the microbial uptake of Hg_P, with higher bioavailability being observed for Fe₂O₃ and lower for Al₂O₃ particles. The bioavailability of Hg_P varied also with the size of particles. In addition, coating with humic substances and model organic compound (cysteine) on Fe₂O₃ particles decreased the bioavailability of Hg_P. Overall, our findings highlight the role of Hg_P in Hg biogeochemical cycling and shed light on the enhanced Hg-methylation in settling particles and sediments in aquatic environments.