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 an Escherichia 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 (Fe ₂ O ₃ , Fe ₃ O ₄ , Al ₂ O ₃ , and SiO ₂ ). 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 Fe ₂ O ₃ and lower for Al ₂ O ₃ 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 Fe ₂ O ₃ 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.