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, it remains controversial whether Hg(II) uptake by these organisms passively or actively.

Recently, scientists from the Institute of Applied Ecology (IAE) of 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 is associated with cytoplasmic HgcAB.

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

The research was supported by the Key Laboratory of Pollution Ecology and Environmental Engineering, CAS, the Program of Young Visiting Scholar, CAS, the National Natural Science Foundation of China, the Office of Biological and Environmental Research within the Office of Science of the U.S. Department of Energy (DOE).

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