Production of methylmercury (MeHg) by a Geobacter sulfurreducens PCA mutant (top) and wild-type strain (middle) as a function of cysteine concentration. Bottom graph shows mercury (Hg) speciation calculations using Minteq computer code at cysteine concentrations of 0–1000 μM in phosphate-buffered saline.
Microbial conversion of inorganic mercury (Hg) to a neurotoxic form called methylmercury in the environment is influenced by many factors, including pH, sulfide, and complexing organic ligands and thiols. Some thiol compounds such as cysteine previously have been found to enhance Hg(II) uptake and methylation by the wild-type (WT) strain of the bacterium Geobacter sulfurreducens PCA. The prevalence of this enhancement in other strains is not clear. To better understand the complex roles of cysteine in microbial mercury uptake and methylation, we systematically examine how cysteine concentration and reaction time affect methylmercury production by G. sulfurreducens PCA and its c-cytochrome–deficient mutant, ΔomcBESTZ.
In contrast to previous findings, our results show that cysteine does not always increase mercury uptake and methylation in anaerobic microbes but, in fact, inhibits these processes in certain strains such as ΔomcBESTZ. These results provide improved understanding on how competitive mercury interactions between microbial cells and complexing thiol ligands in solution may control microbial uptake of mercury and production of methylmercury, a global pollutant.
Results demonstrate the time-dependent effect of cysteine concentration on mercury methylation by G. sulfurreducens PCA cells: addition of cysteine alters Hg(II) speciation, decreasing Hg(II) reduction and sorption but increasing the initial Hg(II) aqueous concentration (see figure). These interactions are kinetically limiting and thus may control the rate of mercury uptake and methylation by both the mutant and the WT. The demonstrated significant effect of cysteine on mercury methylation by anaerobic bacteria underscores the need to further examine the effects of complexing ligands such as thiols or thiol groups in natural organic matter on methylmercury production in the environment.
Lin, H., X. Lu, L. Liang, and B. Gu. 2015. "Cysteine inhibits mercury methylation by Geobacter sulfurreducens PCA mutant ΔomcBESTZ." Environ. Sci. Technol. Lett. 2:144–48. [DOI:10.1021/acs.estlett.5b00068]