Aging and field representation of species-specific Hg(II) methylation rates in sediments: how FeS-bound and nanoparticulate versus microparticulate HgS phases control Hg(II) availability over environmental timescales

In anoxic sediments, the rate at which microorganisms convert inorganic divalent mercury (Hg(II)) to bioaccumulative monomethylmercury depends strongly on the chemical form of Hg(II): species-specific isotope-tracer experiments showed methylation rate constants spanning roughly two orders of magnitude among geochemically relevant solid and adsorbed phases, with the traditional dissolved tracer yielding the highest rate constants. What remains unresolved is the time dimension and the field translation of these results: the aging pathways and timescales by which Hg associated with iron monosulfide (FeS) and with nanoparticulate versus microparticulate HgS shifts between dissolution-prone and crystalline low-solubility forms, how these transformations modulate Hg(II) availability for methylation over environmental timescales, and whether slurry-derived species-specific rate constants hold in intact sediment cores at in situ temperatures. Later work has clarified the mechanisms (particle surface structure, FeS aging, crystalline phase) in laboratory and mesocosm settings, but quantitative aging kinetics in natural sediments and field-representative species-resolved methylation rates remain open.

Background

Monomethylmercury (MeHg) is the mercury form that biomagnifies in aquatic food webs and poses the main health hazard of environmental mercury; it is produced primarily by anaerobic microorganisms (sulfate-reducing bacteria and other anaerobes carrying hgcAB, the gene pair whose protein products are required for mercury methylation) that methylate inorganic divalent mercury, Hg(II). In anoxic sediments, Hg(II) occurs overwhelmingly not as free dissolved ions but as surface-adsorbed and solid phases: Hg(II) bound to thiol groups of natural organic matter (NOM), Hg(II) adsorbed on iron monosulfide (FeS, the mineral mackinawite), and crystalline mercury sulfides — cinnabar ($\alpha$-HgS) and metacinnabar ($\beta$-HgS). Because methylating microbes take up Hg(II) from solution, methylation depends on how fast these solid and adsorbed pools release dissolved Hg(II), which differs greatly among phases.

For decades, methylation rates were measured by adding an enriched mercury isotope as a dissolved salt (e.g., Hg(NO$_3$)$_2$) to homogenized sediment slurries — sediment mixed into a slurry and incubated — and fitting a first-order methylation rate constant $k_m$. This implicitly assumes the dissolved tracer equilibrates rapidly with the ambient solid phases. Jonsson et al. (2012) tested that assumption by synthesizing isotopically enriched tracers as geochemically relevant solid or adsorbed phases ($\alpha$-HgS, $\beta$-HgS, Hg on FeS, Hg bound to NOM) and adding them to estuarine sediment slurries. They found $k_m$ spanning over two orders of magnitude, ordered $\beta$-HgS < $\alpha$-HgS < FeS-bound Hg(II) < NOM-bound Hg(II) < dissolved tracer. Combining species-specific rate constants with the ambient solid-phase speciation determined by Hg extended X-ray absorption fine structure spectroscopy (EXAFS, a synchrotron technique that identifies the atomic coordination environment of Hg) reproduced the independently fitted ambient rate — validating the species-specific approach. A side observation pointed to aging: the availability of FeS-bound Hg(II) depended on the age of the FeS phase, with aged FeS-bound Hg(II) methylating more slowly than freshly prepared FeS-bound Hg(II), though without a systematic aging time series.

Several scientifically linked limitations of this slurry-based approach define the open problem; they are identified as open needs by subsequent open-access work from the same research group and others (Jonsson et al. 2014; Wadle et al. 2024; Rivera et al. 2019). The $k_m$ values were measured in slurries at room temperature, so translating them to true field-representative rates in intact sediment cores at controlled in situ temperature was not done. The long-term evolution (aging) pathways and timescales by which FeS phases and nanoparticulate versus microparticulate HgS transition between dissolution-prone, disordered forms and crystalline, low-solubility forms require systematic study to quantify how mineralogical transformation modulates Hg(II) availability over environmental timescales. And because the method requires knowledge of the ambient solid-phase speciation — while EXAFS, sequential extractions (selective chemical leaches that operationally partition solid-phase Hg among forms), and thermodynamic modeling each have constraints — the representativeness of tracer-prepared nanoparticulate HgS relative to natural nanoparticulates across diverse sediments is unresolved.

Problem Statement

Determine, quantitatively and under field-representative conditions, how the chemical form and aging of solid and adsorbed Hg(II) phases control the availability of Hg(II) for microbial methylation in sediments. This objective comprises three linked components:

1. Quantify the aging pathways and timescales by which Hg(II) associated with FeS phases and with nanoparticulate versus microparticulate HgS(s) transforms between dissolution-prone, disordered forms and crystalline, low-solubility forms, and how these mineralogical transformations modulate Hg(II) availability for methylation over environmental (months-to-decades) timescales in natural sediments — extending short-term laboratory aging observations into a systematic account.
2. Establish whether slurry-derived species-specific methylation rate constants $k_m$ translate to intact sediment cores under controlled in situ temperature conditions, i.e., produce field-representative species-resolved methylation rates rather than room-temperature slurry values.
3. Establish the representativeness of the tracer phases themselves: whether nanoparticulate HgS formed during tracer preparation or in situ differs in solubility and bioavailability from natural nanoparticulates across the sediment types examined, judged by solid-phase speciation evidence.

Together these three components serve one objective: making species-specific methylation rates accurate enough to predict ambient MeHg formation and its response to changing mercury emissions. No single component alone resolves the problem, although each can be addressed independently. This objective is grounded in the verified conclusion of Jonsson et al. (2012) that the selection of relevant solid-phase and surface-adsorbed Hg(II) tracers is crucial for biogeochemically accurate estimates of ambient methylation rates, and in the subsequent open-access literature that identifies the individual gaps: ecosystem-relevant quantitative availability data require experimental systems representing natural conditions with intact sediment cores, and the quantitative importance of nanoparticulate HgS in natural sediment remains unclear (Jonsson et al. 2014); prior species-specific tracer experiments were limited in timescale to weeks-to-two-months, with longer-term reactivity insights lacking (Wadle et al. 2024); and aging of synthetic FeS over 1 hour to 1 month alters the Hg coordination environment and bioavailability of sorbed and coprecipitated Hg without establishing in-sediment kinetics (Rivera et al. 2019).

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