Cysteine Inhibits Mercury Methylation by Geobacter sulfurreducens PCA Mutant ΔomcBESTZ

Cysteine enhances Hg uptake and methylation by the Geobacter sulfurreducens PCA wild-type (WT) strain in short-term assays. The prevalence of this enhancement in other strains remains poorly understood. We examined the influence of cysteine concentration on time-dependent Hg­(II) reduction, sorption, and methylation by PCA WT and its c-type cytochrome-deficient mutant (ΔomcBESTZ) in phosphate-buffered saline. Without cysteine, the mutant methylated twice as much Hg­(II) as the PCA WT, whereas addition of cysteine inhibited Hg methylation, regardless of the reaction time. PCA WT, however, exhibited both time-dependent and cysteine concentration-dependent methylation. In a 144 h assay, nearly complete sorption of the Hg­(II) by PCA WT occurred in the presence of 1 mM cysteine, resulting in our highest observed level of methylmercury production. The chemical speciation modeling and experimental data suggest that uncharged Hg­(II) species are more readily taken up and that this uptake is kinetically limiting, thereby affecting Hg methylation by both the mutant and WT.