Stable Isotope Fractionation Reveals Similar Atomic-Level Controls during Aerobic and Anaerobic Microbial Hg Transformation Pathways

Competition between Dissolved Organic Matter and Freshwater Plankton Control Methylmercury Isotope Fractionation during Uptake and Photochemical Demethylation

Isotope fractionation related to photochemical reactions and planktonic uptake at the base of the food web is a major uncertainty in the biological application of mercury (Hg) stable isotopes. In freshwater systems, it is unclear how competitive interactions among methylmercury (MeHg), dissolved organic matter (DOM), and phytoplankton govern the magnitude of mass-dependent and mass-independent fractionation. This study investigated how DOM alters rates of planktonic MeHg uptake and photodegradation and corresponding Hg isotope fractionation in the presence of freshwater phytoplankton species, Raphidocelis subcapitata. Outdoor sunlight exposure experiments utilizing R. subcapitata were performed in the presence of different DOM samples using environmentally relevant ratios of MeHg-DOM thiol groups. The extent of Δ199Hg in phytoplankton incubations (2.99‰ St. Louis River HPOA, 1.88‰ Lake Erie HPOA) was lower compared to paired abiotic control experiments (4.29 and 2.86‰, respectively) after ∼30 h of irradiation, resulting from cell shading or other limiting factors reducing the extent of photodemethylation. Although the Δ199Hg/Δ201Hg ratio was uniform across experiments (∼1.4), Δ199Hg/δ202Hg slopes varied dramatically (from -0.96 to 15.4) across incubations with R. subcapitata and DOM. In addition, no evidence of Hg isotope fractionation was observed within R. subcapitata cells. This study provides a refined examination of Hg isotope fractionation markers for key processes occurring in the lower food web prior to bioaccumulation, critical for accurately accounting for the photochemical processing of Hg isotopes across a wide spectrum of freshwater systems.

Mercury Isotope Fractionation during Dark Abiotic Reduction of Hg(II) by Dissolved, Surface-Bound, and Structural Fe(II)

Stable mercury (Hg) isotope ratios are an emerging tracer for biogeochemical transformations in environmental systems, but their application requires knowledge of isotopic enrichment factors for individual processes. We investigated Hg isotope fractionation during dark, abiotic reduction of Hg(II) by dissolved iron(Fe)(II), magnetite, and Fe(II) sorbed to boehmite or goethite by analyzing both the reactants and products of laboratory experiments. For homogeneous reduction of Hg(II) by dissolved Fe(II) in continuously purged reactors, the results followed a Rayleigh distillation model with enrichment factors of -2.20 ± 0.16‰ (ε202Hg) and 0.21 ± 0.02‰ (E199Hg). In closed system experiments, allowing reequilibration, the initial kinetic fractionation was overprinted by isotope exchange and followed a linear equilibrium model with -2.44 ± 0.17‰ (ε202Hg) and 0.34 ± 0.02‰ (E199Hg). Heterogeneous Hg(II) reduction by magnetite caused a smaller isotopic fractionation (-1.38 ± 0.07 and 0.13 ± 0.01‰), whereas the extent of isotopic fractionation of the sorbed Fe(II) experiments was similar to the kinetic homogeneous case. Small mass-independent fractionation of even-mass Hg isotopes with 0.02 ± 0.003‰ (E200Hg) and ≈ -0.02 ± 0.01‰ (E204Hg) was consistent with theoretical predictions for the nuclear volume effect. This study contributes significantly to the database of Hg isotope enrichment factors for specific processes. Our findings show that Hg(II) reduction by dissolved Fe(II) in open systems results in a kinetic MDF with a larger ε compared to other abiotic reduction pathways, and combining MDF with the observed MIF allows the distinction from photochemical or microbial Hg(II) reduction pathways.

Mining-impacted rice paddies select for Archaeal methylators and reveal a putative (Archaeal) regulator of mercury methylation

Methylmercury (MeHg) is a microbially produced neurotoxin derived from inorganic mercury (Hg), which accumulation in rice represents a major health concern to humans. However, the microbial control of MeHg dynamics in the environment remains elusive. Here, leveraging three rice paddy fields with distinct concentrations of Hg (Total Hg (THg): 0.21-513 mg kg-1 dry wt. soil; MeHg: 1.21-6.82 ng g-1 dry wt. soil), we resorted to metagenomics to determine the microbial determinants involved in MeHg production under contrasted contamination settings. We show that Hg methylating Archaea, along with methane-cycling genes, were enriched in severely contaminated paddy soils. Metagenome-resolved Genomes of novel putative Hg methylators belonging to Nitrospinota (UBA7883), with poorly resolved taxonomy despite high completeness, showed evidence of facultative anaerobic metabolism and adaptations to fluctuating redox potential. Furthermore, we found evidence of environmental filtering effects that influenced the phylogenies of not only hgcA genes under different THg concentrations, but also of two housekeeping genes, rpoB and glnA, highlighting the need for further experimental validation of whether THg drives the evolution of hgcAB. Finally, assessment of the genomic environment surrounding hgcAB suggests that this gene pair may be regulated by an archaeal toxin-antitoxin (TA) system, instead of the more frequently found arsR-like genes in bacterial methylators. This suggests the presence of distinct hgcAB regulation systems in bacteria and archaea. Our results support the emerging role of Archaea in MeHg cycling under mining-impacted environments and shed light on the differential control of the expression of genes involved in MeHg formation between Archaea and Bacteria.