The role of ligands in the microbial uptake of Hg(II): Insights from x-ray absorption spectroscopy
Mercury (Hg) is a metal that occurs naturally in atmospheric and aquatic environments at low concentration levels. However, anthropogenic activities have contributed to mobilize further this element through mining, precious metal extraction, industrial processes and commercial products, and above all: coal combustion. As a result of its biogeochemical cycle and the biomagnification of one of its chemical species through aquatic food webs, it is considered by the World Health Organization as one of the foremost chemicals of concern. These concerns are directly linked to the formation in anaerobic and suboxic environments of a potent neurotoxin: mono-methylmercury (MeHg, or CH3Hg+). Some bacteria and archaea - such as sulfate reducing prokaryotes - are the primary organisms responsible of the methylation of Hg. However, these prokaryotes will only produce MeHg after the biouptake of Hg(II). To gain insight into the processes that are controlling the bacterial uptake of Hg
(II), we have combined various experimental approaches relying on manipulating its chemical speciation in exposure media, measuring biouptake using a genetically engineered strain of Escherichia coli, probing cell physiology, performing microscopic observations, and investigating the average coordination environment of Hg(II) associated with bacterial cells using x-ray absorption spectroscopy. Our results show that: 1./ the uptake of Hg(II) can be promoted or suppressed by complexation with the presence of ligands in the exposure medium, 2./ ligand exchange reactions with sulfhydryl groups associated with the bacterial membrane play an important role in mediating Hg(II) transfer, and 3./ there is a tight interplay between sulfur metabolism and metal uptake.