Methylmercury (MeHg) can be bioaccumulated and biomagnified at high levels in food webs, particularly in fish and rice, and is thus a significant threat to human health and the environment.  

It is known that MeHg is produced predominantly by some anaerobic microorganisms possessing the HgcAB proteins located in the cytoplasm. However, Hg(II) uptake by these organisms is passive or active remains unclear.  

Recently, scientists from the Institute of Applied Ecology (IAE) of the Chinese Academy of Sciences and Oak Ridge National Laboratory (ORNL) carried out a series of experiments to examine whether Hg(II) can be taken up passively or actively in the absence of thiols, using D. desulfuricans ND132 as a model Hg-methylating organism.  

The kinetics and dynamics of concurrent Hg(II) adsorption, uptake, and methylation by both viable and inactivated cells or spheroplasts of the D. desulfuricans ND132 were determined.  

The researchers found that more than 60% of the added Hg(II) (25 nM) was taken up passively in 48 h by live and inactivated cells and also by cells treated with the proton gradient uncoupler, carbonylcyanide-3-chlorophenylhydrazone (CCCP) without addition of thiols. Heat treatment, cell starvation, or CCCP treatment increased cell surface adsorption of Hg(II) and halted Hg(II) methylation but did not stop cellular Hg(II) uptake.  

Similarly, CCCP-treated spheroplasts did not produce MeHg but continued taking up to 55% of the Hg(II). Spheroplasts showed a greater capacity to adsorb Hg(II) than whole cells, and the level of cytoplasmic membrane-bound Hg(II) correlated well with MeHg production, as Hg(II) methylation was associated with cytoplasmic HgcAB.  

The results indicate that active metabolism is not required for cellular Hg(II) uptake. Although active uptake of Hg(II) cannot be completely ruled out, the passive uptake or permeation of Hg(II) is an important mechanism for ND132 cells to take up Hg(II).   

The study entitled "Mercury Uptake by Desulfovibrio desulfuricans ND132: Passive or Active?" was published in Environmental Science & Technology.