Divalent base cations hamper Hg(II) uptake.

Despite the alarming trends of declining base cation concentrations in boreal lakes, no studies have attempted to predict the consequences of this decline on the geochemical cycle of mercury, a top priority contaminant worldwide. In this study, we used a whole-cell gram-negative bioreporter to evaluate the direction and magnitude of changes in net accumulation of Hg(II) by bacteria in response to changing base cation concentrations. We show that regardless of the speciation of Hg(II) in solution, increasing divalent base cation concentrations decrease net Hg(II) accumulation by the bioreporter, suggesting a protective effect of these cations. Our work suggests that the complexity of the cell wall of gram-negative bacteria must be considered when modeling Hg uptake pathways; we propose that base divalent cations contribute to hamper net Hg(II) accumulation by decreasing outer membrane permeability and, therefore, the passive diffusion of Hg(II) species to the periplasmic space. This work points to an unsuspected and likely harmful consequence of a delay in recovering from acidification in boreal lakes, in that uptake of Hg(II) by bacteria is not only enhanced by the reduced pH but can also be enhanced by a decline in base cation levels.