Developing a predictive understanding of Hg transformation in ecosystems requires knowledge of exchange and feedback processes at Critical Interfaces. Task 2 involves biogeochemical mechanisms controlling mercury uptake and methylation.

Task 2: Biogeochemical Mechanisms Controlling Mercury Uptake and Methylation

Overarching Objectives

The overarching goal of Task 2 is to gain a fundamental understanding of the key geochemical and biochemical mechanisms controlling mercury sorption, uptake, and transformation at the microbe-fluid and particulate (mineral) interfaces.

Questions to be Addressed

  • What are the key geochemical and biochemical variables and their interactions affecting Hg-DOM complexation, Hg-cell surface interactions, cellular uptake, and methylation?
  • What are the specific molecules in DOM and proteins in and out of the cell membrane that complex Hg?
  • How does photo-redox transformation of Hg-DOM influence Hg reactivity and bioavailability?
  • Under what conditions do suspended particles become a net sink or source of bioavailable Hg in EFPC?

Hypotheses to Test

  • Only a small set of DOM molecules form strong complexes with Hg and dominates Hg speciation. Identification of these specific DOM molecules (e.g., by high-resolution mass spectrometry [HR-MS]) is likely the key to understand how Hg is taken up by bacteria and transformed in the environment.
  • Deletion of hgcAB affects the expression of membrane proteins that are required for Hg redox transformation, bindings, and uptake. Understanding such unexpected consequences of targeted gene deletion could provide new insight into the native function of hgcAB and the cell physiology.
  • Outer membrane proteins play a major role in Hg-cell surface binding and exchange and may thus control cellular Hg uptake and methylation.
  • Hg-DOM complexation and particulate interactions in natural water constrains Hg photo-redox transformation and bioavailability, which may ultimately determine how Hg speciate, transform, and cycle in aquatic systems.

Biogeochemical Mechanisms

Recent Accomplishments

SFA research previously found that microbial methylation is strongly influenced by specific thiol ligands and that the uptake process is energy dependent. We also identified the strain-specific, bacterially mediated oxidation and methylation of dissolved elemental Hg(0) under anoxic incubations and the unexpected effects of gene deletion on mercury interactions with the methylation-deficient mutant ΔhgcAB. Our subsequent studies (FY2014) focused on coupled mercury reduction, oxidation, cell-surface adsorption, and methylation by G. sulfurreducens PCA. We found that mercury methylation was positively correlated with mercury sorption but negatively correlated with mercury reduction. These reactions depended on the ratio of mercury to cellular thiols: Increasing this ratio shifts the major reaction from oxidation to reduction, and vice versa. We also found that the c-cytochrome deletion mutant, ΔomcBESTZ, decreases mercury reduction but increases methylation. Additionally, we recently reported the influences of cysteine on time-dependent Hg(II) reduction, sorption, and methylation by the wild-type (WT) strain of PCA and its mutant ΔomcBESTZ (see research highlight, Cysteine Inhibits Production of Methylmercury Neurotoxin by Geobacter Mutant ΔomcBESTZ). Without cysteine, the mutant methylated twice as much Hg(II) as the WT, but the addition of cysteine inhibited mercury methylation regardless of reaction time or cysteine concentration. These results contrast with the common belief that cysteine increases mercury uptake and methylation and suggest that the role of cysteine in microbial mercury methylation is more complicated than previously thought.

We also investigated the rates and mechanisms of methylmercury photodegradation and Hg(0) photo-oxidation. All DOM and organic ligands are found to increase methylmercury photolysis under solar irradiation, but the first-order rate constants vary depending on DOM oxidation state and the type and concentration of the complexing ligands20. Compounds containing both thiol and aromatic moieties within the same molecule (e.g., thiosalicylate and reduced DOM) increased methylmercury photodegradation rates far greater than those containing only aromatics or thiols (e.g., salicylate, glutathione, or their combinations). In other words, the synergistic effects of thiols and aromatics in DOM can lead to greatly enhanced methylmercury photodegradation. The SFA research team also identified a new mechanism for direct energy transfer from an excited triplet state of DOM to break the mercury–carbon bond in methylmercury, thus answering a long-standing question about why DOM enhances methylmercury photodegradation. Additionally, we identified a new pathway of Hg(0) photo-oxidation by carbonate radicals (CO3• –). Photo-oxidation of Hg(0) is affected by reactive ionic species (e.g., DOM, carbonate, and nitrate). Using scavengers and enhancers for singlet oxygen (1O2) and hydroxyl (HO) radicals, as well as electron paramagnetic resonance spectroscopy, we showed that carbonate radicals primarily drive the Hg(0) photo-oxidation in EFPC water. These findings are of great significance in understanding mercury chemical speciation, transformation, and transfer at the water-air interface in the environment.

The ORNL Mercury SFA is sponsored by the Subsurface Biogeochemical Research (SBR) program within the U.S. Department of Energy's Office of Biological and Environmental Research.