Effect of divalent metals on Hg(II) uptake and methylation by bacteria

The production of methylmercury by some bacteria is a key first step in the accumulation and biomagnification of this toxic substance in aquatic food webs, a major human health concern. By direct measurement of cellular Hg(II) uptake in model iron and sulfate reducing bacteria, we have observed that specific trace metals, such as Zn(II) and Cd(II), inhibit uptake and methylation in these organisms, whereas other metals, such as Ni(II), Co(II), or Fe(II), do not. The inhibition of Hg(II) methylation by Zn(II) was competitive in nature and related to the concentration of inorganically complexed Zn(II) (Zn′). The inhibition of Hg(II) methylation was alleviated by decreasing the free Zn′ concentration through complexation with nitrilotriacetic acid without altering the speciation of Hg(II). The inhibitory effect by Zn(II) was observed when either Hg-cysteine complexes or neutral HgCl₂ dominated the speciation of Hg(II), demonstrating that both charged and neutral species are transported into the cytosol by an active rather than passive process. We propose that Hg(II) uptake is the result of its accidental uptake by metal transporter(s), possibly one effecting the transport of Zn(II).