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).