Kinetics of Methylmercury Production Revisited

Abstract

Laboratory measurements of the biologically mediated methylation of mercury (Hg) to the neurotoxin monomethylmercury (MMHg) often exhibit kinetics that are inconsistent with first-order kinetic models. Using time-resolved measurements of filter passing Hg and MMHg during methylation/demethylation assays, a multisite kinetic sorption model, and reanalyses of previous assays, we show that competing kinetic sorption reactions can lead to time-varying availability and apparent non-first-order kinetics in Hg methylation and MMHg demethylation. The new model employing a multisite kinetic sorption model for Hg and MMHg can describe the range of behaviors for time-resolved methylation/demethylation data reported in the literature including those that exhibit non-first-order kinetics. Additionally, we show that neglecting competing sorption processes can confound analyses of methylation/demethylation assays, resulting in rate constant estimates that are systematically biased low. Simulations of MMHg production and transport in a hypothetical periphyton biofilm bed illustrate the implications of our new model and demonstrate that methylmercury production may be significantly different than projected by single-rate first-order models.

Microbial Diversity and Mercury Methylation Activity in Periphytic Biofilms at a Run-of-River Hydroelectric Dam and Constructed Wetlands

“…Recent studies suggested a transient availability rate potentials calculation, a modification of the original Hg methylation kinetic model from H. Hintelmann et al ( 43 ) through the determination and addition of sorption reaction kinetics, leading to k m 15 times higher than the full availability rate potentials ( 29 , 44 ). However, the remoteness of our sites and field laboratory has prevented the determination of such additional sorption constants to fit the equations.…”

From legacy contamination to watershed systems science: a review of scientific insights and technologies developed through DOE-supported research in water and energy security

“…Hg and MeHg in contact with periphyton are subject to other reactions in addition to methylation and demethylation, including sorption, desorption, and, in the case of Hg II , reduction to Hg 0 and its reoxidation. These ancillary reactions were measured, and a novel model was created-the transient availability model (TAM)-to more accurately describe the periphyton methylation-demethylation results (Olsen et al 2018). Consistent with lab incubations data of field-derived samples from EFPC, the TAM reproduced concentration versus time behavior that had previously been thought to be anomalous.…”

Ecosystem Controls on Methylmercury Production by Periphyton Biofilms in a Contaminated Stream: Implications for Predictive Modeling

“…Methylation and demethylation rate potentials were calculated using the transient availability kinetic model described by Olsen et al (). The transient availability model incorporates kinetic expressions for multisite sorption of Hg and MMHg and Hg reduction to Hg 0 with methylation/demethylation kinetics to account for these competing processes when estimating methylation/demethylation potentials (Olsen et al ). Transient availability kinetics are described by Equations 1 and 2, where k m is the methylation rate constant, k d is the demethylation rate constant, k 1 is the Hg fast site sorption rate constant, k 2 is the Hg fast site desorption rate constant, k 3 is the Hg slow site adsorption rate constant, k 4 is the Hg slow site desorption rate constant, k 5 is the rate constant for the conversion of Hg to Hg 0 , k 6 is the rate constant for the conversion of Hg 0 to Hg, Hg f is the amount of Hg sorbed to fast sorption sites, Hg s is the amount of Hg sorbed to slow sorption sites, k 7 is the MMHg fast site adsorption rate constant, k 8 is the MMHg fast desorption rate constant, k 9 is the MMHg slow adsorption rate constant, k 10 is the MMHg slow desorption rate constant, MMHg f is the amount of MMHg sorbed to fast sites, and MMHg s is the amount of MMHg sorbed to slow sites.…”

“…However, the estimates for MMHg production from the Olsen et al (2016) study were generated with single-time point first-order kinetic models that assumed full bioavailability of Hg and MMHg. Recently, a new model for MMHg production kinetics has been developed, which accounts for the transient availability of Hg and MMHg for methylation and degradation reactions over time (Olsen et al 2018). The transient availability kinetic model provided estimates for periphyton MMHg generation and degradation rate potentials that were 25 times and 5 times higher, respectively, than full availability, single-time point estimates (Olsen et al 2018).…”

“…Recently, a new model for MMHg production kinetics has been developed, which accounts for the transient availability of Hg and MMHg for methylation and degradation reactions over time (Olsen et al 2018). The transient availability kinetic model provided estimates for periphyton MMHg generation and degradation rate potentials that were 25 times and 5 times higher, respectively, than full availability, single-time point estimates (Olsen et al 2018). It is unknown whether previously observed correlations between MMHg production and temperature, light exposure, and location in East Fork Poplar Creek hold when methylation and demethylation rate potentials are calculated from time series data using transient availability kinetics.…”