The effect of natural organic matter on the adsorption of mercury to bacterial cells

Abstract

We investigated the ability of non-metabolizing Bacillus subtilis, Shewanella oneidensis MR-1, and Geobacter sulfurreducens bacterial species to adsorb mercury in the absence and presence of Suwanee River fulvic acid (FA). Bulk adsorption and X-ray absorption spectroscopy (XAS) experiments were conducted at three pH conditions, and the results indicate that the presence of FA decreases the extent of Hg adsorption to biomass under all of the pH conditions studied. Hg XAS results show that the presence of FA does not alter the binding environment of Hg adsorbed onto the biomass regardless of pH or FA concentration, indicating that ternary bacteria-Hg-FA complexes do not form to an appreciable extent under the experimental conditions, and that Hg binding on the bacteria is dominated by sulfhydryl binding. We used the experimental results to calculate apparent partition coefficients, K d , for Hg under each experimental condition. The calculations yield similar coefficients for Hg onto each of the bacterial species studies, suggesting there is no significant difference in Hg partitioning between the three bacterial species. The calculations also indicate similar coefficients for Hg-bacteria and Hg-FA complexes. S XAS measurements confirm the presence of sulfhydryl sites on both the FA and bacterial cells, and demonstrate the presence of a wide range of S moieties on the FA in contrast to the bacterial biomass, whose S sites are dominated by thiols. Our results suggest that

High‐Energy Resolution Fluorescence Detected X‐Ray Absorption Spectroscopy: A Powerful New Structural Tool in Environmental Biogeochemistry Sciences

“…Anaerobic microorganisms are responsible for producing the neurotoxin methylmercury (MeHg) in the environment, requiring inorganic Hg II as a substrate (Parks et al, 2013). Many studies have employed XAS to study Hg II speciation in environmental and biological samples to better understand which Hg II species are bioavailable for MeHg production (Andrews, 2006; Skyllberg et al, 2006; Nagy et al, 2011; Dunham‐Cheatham et al, 2014, 2015; Thomas et al, 2014; Manceau et al, 2015). In the anoxic environments where MeHg is produced, Hg II is primarily bound to thiol functional groups in organic matter or sulfide due to its high affinity for reduced sulfur.…”

Thiol-Based Selective Extraction Assay to Comparatively Assess Bioavailable Mercury in Sediments

“…Moreover, the reactivity of dissolved and particulate Hg sulfides with thiols correlated with differences in net methylation potential by methylating microorganisms (Zhang et al, 2012). Likewise, a number of recent studies point to thiols as the site for mercury binding to the cell envelope of methylating bacteria ( Joe-Wong et al, 2012;Hu et al, 2013;Dunham-Cheatham et al, 2014, 2015. While the interactions between Hg and thiols in these studies and the mechanism of Hg uptake by the cell remain unknown, we hypothesize that a simple thiolbased selective extraction could be used to approximate the bioavailable fraction of Hg in anaerobic settings.…”

An Anaerobic Biosensor Assay for the Detection of Mercury and Cadmium

“…Whereas Cd II is hypothesized to be accidentally transported into the cytosol through divalent metal transport pathways (particularly Mn II or Zn II ), mechanisms of Cd II transport inside the cells remain speculative, and no Cd-specific transport pathway has been identified 13,18 . Regardless of the nature of the transporters involved, three mechanistic factors ultimately determine the ability of metals to enter a cell: i) the metal speciation in solution 2,6,15,16,17,19,20,21,22,23 , ii) the biophysiochemical properties of the cell membrane 17,24,25,26,27,28,29,30,31 , and iii) the ability for the metal to access a transport site 7,32 . Cd and Hg are unlikely to exists as free ions under microbial physiologically relevant conditions due to their high affinity for Dissolved Organic Matter (DOM), chelating contaminants (e.g., EDTA), or reduced sulfur moieties 33,34,35 (Cd II can exist as a free ion or form ion-pairs in the absence of these ligands).…”

“…One current limitation of the approach presented here is that our limit of detection has not yet reached pM levels, contrary to existing aerobic systems 4,19,37 . It is however important to note that to achieve these low detection limits several steps need to be taken 44 : i) ligand addition is required to ensure that Hg remains in solution and does not adsorb onto the microbial cell wall (Hg will be irreversible bound to cell surface thiols preventing its bioavailability 25,27 ; see the threshold response for Hg in Figure 3A), ii) modifications to cell density, or iii) modify the genetic construct to include transport proteins of the mer-operon (namely merT and merP), increasing Hg flux inside the cell 50,51 . These modifications would be beneficial in detecting low concentrations of Hg, but not necessarily…”