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Brett A. Poulin

Publications and source records attributed to Brett A. Poulin.

At least 19 recordsLinked to original sources
USGS2026

Abrupt onset and persistent downstream effects of acid rock drainage on rivers across northwest Arctic Alaska

Arctic streams and rivers are increasingly vulnerable to degradation from permafrost thaw that releases toxic elements via natural acid rock drainage. However, the spatial and temporal evolution of acid rock drainage on river chemistry across diverse mineral settings remain poorly constrained. Here, water chemistry in six watersheds across Alaska's Brooks Range (2022–2024) quantified the effects of acid rock drainage on river chemistry. Point-source acid rock drainage inputs (seeps and impaired tributaries) exhibited lower pH (median; pH: 3.2 and 6.9, respectively), higher sulfate (SO 4 2− : 3,025 and 390 mg L −1 , respectively), and elevated major and trace element concentrations (sum of 23 metals: 451 and 9.2 mg L −1 , respectively), compared to upstream unimpaired mainstem rivers (pH = 8.3, SO 4 2− = 68 mg L −1 ; metals = 0.1 mg L −1 ). Across watersheds, these inputs increased mainstem concentrations of major and trace elements (e.g., Fe, Al, Mn, Zn, Ni, Cd, and rare earth elements) and SO 4 2− ; however, mainstem river pH remained stable due to sufficient buffering. Within the Salmon River, concentrations of filter-passing and particulate fractions of constituents (SO 4 2− , Fe, Zn, Ni, and Cd) were elevated ≤70-fold at >100 km downstream of acidic inputs, documenting long-range transport. Temporal analyses of SO 4 2− and Zn concentration (2015–2024) revealed abrupt increases (≤263%) in two watersheds between 2019 and 2020, aligning with antecedent record warm and snowy winter conditions. Collectively, our results document the abrupt onset of acid rock drainage across Arctic rivers with potential for long-range downstream transport and implications for biogeochemical cycles and ecological and human health.

Alaska↗
USGS2026

Mercury mobilization and export from the Greenland Ice Sheet using an ice-to-ocean approach

The Greenland Ice Sheet (GrIS) is a poorly constrained source of mercury (Hg) to Arctic ecosystems. We measured Hg concentrations and stable isotopes along an ice-to-ocean continuum to identify controls on GrIS Hg export. Early-season permafrost melt and rainfall produced high filtered total mercury (fTHg, ~17 pM) and monomethylmercury (MMHg, ~2 pM). As subglacial drainage evolved, particulate Hg doubled (from ~8 to 17 pM) and MMHg production remained elevated, indicating Hg mobilization from subglacial environments. Shifts in Hg stable isotope ratios and Δ 199 Hg mass balance show supraglacial sources contribute 20–48% of exported Hg, suggesting subglacial inputs dominate the seasonal Hg flux. Fjord waters were enriched in fTHg ( ~ 10 pM) and MMHg ( ~ 2 pM) relative to rivers, consistent with particulate Hg transformations and terrestrial Hg inputs. The estimated GrIS Hg yield ( ~ 23 mmol km −2 yr −1 ) is similar to that of Arctic rivers and will likely increase with climate-driven mass loss.

Communications Earth & Environment↗
USGS2026

Comparing in vivo methylmercury detoxification in hunted duck: Implications for wildlife and human health

Great Salt Lake (GSL) waterfowl have elevated mercury (Hg) concentrations, and in vivo detoxification of neurotoxic and bioaccumulative methylmercury (MeHg) can occur through Hg-selenium (Se) complexation, influencing the suitability of Hg consumption advisories. Here, we present Hg chemical speciation and Hg stable isotope measurements of brain, breast muscle, liver, and kidney samples from two GSL duck species─northern shoveler ( Spatula clypeata ) and cinnamon teal (Spatula cyanoptera). Chemical and stable isotope measurements, along with high energy resolution fluorescence detected X-ray absorption near-edge structure (HERFD-XANES) spectroscopy, indicate the occurrence of in vivo MeHg demethylation in both bird species. The percentage of total Hg (THg) as MeHg (% MeHg) varied with tissue type (brain > breast muscle > kidney > liver), and differences between δ 202 THg and δ 202 MeHg were linearly correlated with % MeHg. Demethylated inorganic Hg was a mixture of Hg-dithiolate (Hg(SR) 2 ) and Hg-tetraselenolate (Hg(Sec) 4 ) in both bird species. Notably, liver THg concentrations were elevated in northern shoveler liver tissues relative to those of cinnamon teal (7.54 ± 4.69 mg/kg versus 2.19 ± 1.27 mg/kg, dry weight, respectively) and % MeHg liver values were significantly lower (40% ± 19% versus 68% ± 14%), indicating taxonomic differences in Hg detoxification and depuration. THg concentrations in waterfowl species from this study were comparable to previously reported levels, indicating that elevated Hg concentrations in northern shoveler and cinnamon teal at GSL have persisted over the past two decades. Due to in vivo demethylation, we show that THg is not an effective proxy for MeHg within duck species, carrying implications for current GSL waterfowl consumption advisories and assessments exposure risk. This study highlights important differences in MeHg detoxification between waterbird taxa and provides further insights into Hg toxicity risk to GSL waterbirds.

Utah↗
USGS2025

Linking permafrost to the abundance, biomass, and energy density of fish in Arctic headwater streams

Permafrost thaw alters groundwater flow, river hydrology, stream-catchment interactions, and the availability of carbon and nutrients in headwater streams. The impact of permafrost on watershed hydrology and biogeochemistry of headwater streams has been demonstrated, but there is little understanding of how permafrost influences fish in these ecosystems. We examined relations among permafrost characteristics, the resulting changes in water temperature, stream hydrology (e.g., discharge flashiness), and macroinvertebrates, with the abundance, biomass, and energy density of juvenile Dolly Varden ( Salvelinus malma ) and Arctic Grayling ( Thymallus arcticus ) across 10 headwater streams in northwestern Alaska. Macroinvertebrate density was driven by concentrations of dissolved carbon and nutrients supporting stream food webs. Dolly Varden abundance was primarily related to water temperature with fewer fish in warmer streams, whereas Dolly Varden energy density decreased with the flashiness of the headwater streams. Dolly Varden biomass was related to both temperature and bottom-up food web effects. The energy density of Arctic Grayling decreased with warmer temperatures and discharge flashiness. These relations demonstrate the importance of terrestrial–aquatic connections in permafrost landscapes and indicate the complexity of landscape effects on fish. Because permafrost thaw is one of the most impactful changes occurring as the Arctic warms, an improved understanding of how stream temperature, hydrology, and bottom-up food web processes influence fish populations can aid forecasting of future conditions across the Arctic.

Alaska↗
USGS2025

A comprehensive sulfate and DOM framework to assess methylmercury formation and risk in subtropical wetlands

Wetlands play a vital role in contaminant cycling and uptake. Understanding how sulfate (SO 4 2‒ ) influences the conversion of inorganic mercury (Hg(II)) to toxic methylmercury (MeHg) is critical for predicting wetland responses to land use and climate change. Here, we sampled surface and pore waters across SO 4 2‒ gradients in three freshwater Everglades wetlands to assess linkages between SO 4 2‒ , MeHg, dissolved organic matter (DOM), and inorganic sulfide (S(‒II)). Increasing SO 4 2‒ concentrations increase S(‒II) and DOM concentrations and DOM aromaticity. MeHg concentration show a unimodal response to surface water SO 4 2‒ , which reflect high Hg(II) methylation at low-to-intermediate SO 4 2‒ concentration (2-12 mg/L) and low Hg(II) methylation at higher SO 4 2‒ concentrations ( > 12 mg/L). MeHg concentrations in surface waters correlate positively with MeHg concentrations in prey fish. The coherent biogeochemical relationships between SO 4 2‒ and MeHg concentrations and biologic uptake improve MeHg risk assessment for aquatic food webs and are globally relevant due to anthropogenic and climate-driven increases in SO 4 2‒ .

Florida↗
USGS2025

Sulfate reduction drives elevated methylmercury formation in water column of eutrophic freshwater lake

Mercury (Hg) contamination of aquatic food webs is controlled in part by the formation and accumulation of toxic and bioaccumulative methylmercury (MeHg). MeHg production is mediated by metabolically diverse microorganisms carrying the hgcAB gene pair, while the demethylation reaction is mediated by several biotic and abiotic processes. However, the relative importance of these two processes on MeHg accumulation and the environmental factors that influence them are poorly characterized, especially in eutrophic environments. In this study, both Hg methylation and MeHg demethylation in a eutrophic freshwater lake were linked to ambient MeHg concentrations and hgcA abundance and expression. High methylation rate potentials indicated in situ MeHg formation was a key source of MeHg to the water column, driven by high hgcA abundance and transcription. Molybdate treatment decreased methylation rate potentials, highlighting the importance of sulfate reduction in driving MeHg formation. Sulfate-reducing bacteria accounted for over 50% of the hgcA gene transcription, despite representing less than 10% of the hgcA -carrying microbial community. An arsR -like transcriptional regulator preceded many hgcA sequences; these were transcriptionally active and linked to lower hgcA expression. Overall, this study elucidates the microbial and biogeochemical processes that influence the in situ formation of MeHg in understudied eutrophic freshwater environments.

Wisconsin↗
USGS2025

Contrasting magnitude and timing of pulsed aqueous methylmercury bioaccumulation across a reservoir food web

Water column hypoxia is a key process influencing methylmercury (MeHg) production and availability in waterbodies worldwide. During seasonal destratification, large, short-lived pulses of aqueous MeHg may be released into the subsequently mixed water column, but little is known about the fate of these pulses, particularly whether there are concomitant increases in MeHg uptake into aquatic food webs. We examined the magnitude and timing of MeHg uptake across several trophic guilds relative to the reservoir stratification status using biweekly mercury data from water, zooplankton, and fish (Bluegill, Lepomis macrochirus and Smallmouth Bass, Micropterus dolomieu ). Zooplankton MeHg concentrations increased by up to 250% during destratification, concurrent with increases in aqueous MeHg concentrations. Zooplankton and filter-passing MeHg concentrations were positively correlated when the reservoir was mixed ( R 2 = 0.95) and destratifying ( R 2 = 0.57) but not while the reservoir was stratified ( R 2 = 0.21). Mercury concentrations in adult bluegill and juveniles of both fish species increased 20–70% following destratification, with responses lagging 4–8 weeks behind those in water and zooplankton MeHg. Mercury concentrations in piscivorous adult bass varied little over the course of the study. Our findings demonstrate the responsiveness of reservoir food webs to pulses in MeHg availability, suggesting that these pulses could play an important role in biotic MeHg exposure within and downstream of reservoirs.

Idaho, Oregon↗
USGS2025

Mercury speciation and stable isotopes in emperor penguins: First evidence for biochemical demethylation of methylmercury to mercury-dithiolate and mercury-tetraselenolate complexes

Apex marine predators, such as toothed whales and large petrels and albatrosses, ingest mercury (Hg) primarily in the form of methylmercury (MeHg) via prey consumption, which they detoxify as tiemannite (HgSe). One of the most intriguing current questions in Hg research is how more abundant lower trophic level predators detoxify MeHg, particularly in marine environments where tissue Hg burdens can be elevated. To address this need, we used high energy-resolution X-ray absorption near edge structure spectroscopy paired with nitrogen (N) and Hg stable isotopes to identify the chemical forms of Hg, Hg source, and species-specific δ 202 Hg isotopic values in emperor penguin, a mesopredator feeding primarily on Antarctic silverfish. The penguin liver contains variable proportions of MeHg and two inorganic Hg species (IHg), Hg-dithiolate (Hg(SR) 2 ) and Hg-tetraselenolate (Hg(Sec) 4 ) complexes, each characterized by a specific isotopic value (δ 202 MeHg = 0.3 ± 0.2‰, δ 202 Hg(SR) 2 = −1.6 ± 0.2‰, δ 202 Hg(Sec) 4 = −2.0 ± 0.1‰). Using δ 15 N as tracer of food source, we show that Hg(SR) 2 is not dietary but a biochemical demethylation product of MeHg metabolism. Penguin females transfer Hg to the egg as MeHg in the egg albumen, 89% MeHg and 11% IHg in the membrane, and 32% MeHg and 68% Hg(Sec) 4 in the yolk, on average (n = 15). Despite IHg species in eggs, MeHg is the main species quantitatively transferred by the mother to the chick because of the disproportionate mass of the MeHg-rich albumen compared to the yolk (n = 18). Further research is needed to elucidate the MeHg to Hg(SR) 2 demethylation pathway firmly documented here for the first time in multicellular organisms, and to understand why the thiolate ligands are not exchanged for Se ligands to form Hg(Sec) 4 , as the liver does not suffer from Se deficiency.

Adelie Land↗
USGS2024

Longitudinal and seasonal changes of organic matter sources through a semi-arid river-reservoir system

The quality and quantity of organic matter (OM) in a river system directly affects ecosystem health; thus, managers benefit from an in-depth understanding of the drivers and sources of OM. In the Snake River, a highly altered river-reservoir system in the semi-arid western United States, OM production and loading are key drivers of reservoir anoxia, which leads to several deleterious processes such as mercury methylation. However, sources and quantities of OM to the Snake River, and the effects of impoundment on OM moving through the river-reservoir system, are not well understood. Particulate organic carbon (POC), dissolved organic carbon (DOC), particulate nitrogen (PN), chlorophyll a (chl- a ), and δ 15 N–PN and δ 13 C–POC isotopic ratios were measured bi-weekly for over 2 years at four locations through the Snake River Hells Canyon Reservoir Complex to determine spatial and temporal patterns of OM quantities and sources. POC concentrations increased through the riverine zone upstream of the reservoirs, likely due to in situ primary production and/or inputs from tributaries and agricultural drains; then decreased through the most upstream reservoir likely due to particle settling. Isotopic ratios and other OM source indicators (δ 15 N–PN, δ 13 C–POC, POC:PN, chl- a :POC) show that the dominant source of particulate OM was phytoplankton with seasonal terrestrial/macrophytic inputs. Results highlight the effects of major tributary and agricultural drain inputs, primary production, and impoundment on OM composition and concentration through a large river-reservoir system and may inform water quality management efforts in this and similar systems.

Idaho, Oregon↗
USGS2024

Riparian methylmercury production increases riverine mercury flux and food web concentrations

The production and uptake of toxic methylmercury (MeHg) impacts aquatic ecosystems globally. Rivers can be dynamic and difficult systems to study for MeHg production and bioaccumulation, hence identifying sources of MeHg to these systems is both challenging and important for resource management within rivers and main-stem reservoirs. Riparian zones, which are known biogeochemical hotspots for MeHg production, are understudied as potential sources of MeHg to rivers. Here, we present a comprehensive quantification of the hydrologic and biogeochemical processes governing MeHg concentrations, loads, and bioaccumulation at 16 locations along 164 km of the agriculturally intensive Snake River (Idaho, Oregon USA) during summer baseflow conditions, with emphasis on riparian production of MeHg. Approximately one-third of the MeHg load of the Snake River could not be attributed to inflowing waters (upgradient, tributaries, or irrigation drains). Across the study reach, increases in MeHg loads in surface waters were significantly correlated with MeHg concentrations in riparian porewaters, suggesting riparian zones were likely an important source of MeHg to the Snake River. Across all locations, MeHg concentrations in surface waters positively correlated with MeHg concentrations in benthic snails and clams, supporting that riparian produced MeHg was assimilated into local aquatic food webs. This study contributes new insights into riparian MeHg production within rivers which can inform mitigation efforts to reduce MeHg bioaccumulation in fish.

Idaho, Oregon↗
USGS2024

Simulated sea level rise in coastal peat oils stimulates mercury methylation

Coastal wetlands are vulnerable to sea level rise with unknown consequences for mercury (Hg) cycling, particularly the potential for exacerbating neurotoxic methylmercury (MeHg) production and bioaccumulation in food webs. Here, the effect of sea level rise on MeHg formation in the Florida Everglades was evaluated by incubating peat cores from a freshwater wetland for 0–20 days in the laboratory at five salinity conditions (0.16–6.0 parts-per-thousand; 0.20–454 mg L –1 sulfate (SO 4 2– )) to simulate the onset of sea level rise within coastal margins. Isotopically enriched inorganic mercury ( 201 Hg(II)) was used to track MeHg formation and peat-porewater partitioning. In all five salinity treatments, porewaters became anoxic within 1 day and became progressively enriched in dissolved organic matter (DOM) of greater aromatic composition over the 20 days compared to ambient conditions. In the four highest salinity treatments, SO 4 2– concentrations decreased and sulfide concentrations increased over time due to microbial dissimilatory SO 4 2– reduction that was concurrent with 201 Hg(II) methylation. Importantly, elevated salinity resulted in a greater proportion of produced Me 201 Hg observed in porewaters as opposed to bound to peat, interpreted to be due to the complexation of MeHg with aromatic DOM released from peat. The findings highlight the potential for enhanced production and mobilization of MeHg in coastal wetlands of the Florida Everglades due to the onset of saltwater intrusion.

Florida↗
USGS2024

Metal mobilization from thawing permafrost to aquatic ecosystems is driving rusting of Arctic streams

Climate change in the Arctic is altering watershed hydrologic processes and biogeochemistry. Here, we present an emergent threat to Arctic watersheds based on observations from 75 streams in Alaska’s Brooks Range that recently turned orange, reflecting increased loading of iron and toxic metals. Using remote sensing, we constrain the timing of stream discoloration to the last 10 years, a period of rapid warming and snowfall, suggesting impairment is likely due to permafrost thaw. Thawing permafrost can foster chemical weathering of minerals, microbial reduction of soil iron, and groundwater transport of metals to streams. Compared to clear reference streams, orange streams have lower pH, higher turbidity, and higher sulfate, iron, and trace metal concentrations, supporting sulfide mineral weathering as a primary mobilization process. Stream discoloration was associated with dramatic declines in macroinvertebrate diversity and fish abundance. These findings have considerable implications for drinking water supplies and subsistence fisheries in rural Alaska.

Alaska↗
USGS2024

Linking dissolved organic matter composition to landscape properties in wetlands across the United States of America

Wetlands are integral to the global carbon cycle, serving as both a source and a sink for organic carbon. Their potential for carbon storage will likely change in the coming decades in response to higher temperatures and variable precipitation patterns. We characterized the dissolved organic carbon (DOC) and dissolved organic matter (DOM) composition from 12 different wetland sites across the USA spanning gradients in climate, landcover, sampling depth, and hydroperiod for comparison to DOM in other inland waters. Using absorption spectroscopy, parallel factor analysis modeling, and ultra-high resolution mass spectroscopy, we identified differences in DOM sourcing and processing by geographic site. Wetland DOM composition was driven primarily by differences in landcover where forested sites contained greater aromatic and oxygenated DOM content compared to grassland/herbaceous sites which were more aliphatic and enriched in N and S molecular formulae. Furthermore, surface and porewater DOM was also influenced by properties such as soil type, organic matter content, and precipitation. Surface water DOM was relatively enriched in oxygenated higher molecular weight formulae representing HUP High O/C compounds than porewaters, whose DOM composition suggests abiotic sulfurization from dissolved inorganic sulfide. Finally, we identified a group of persistent molecular formulae (3,489) present across all sites and sampling depths (i.e., the signature of wetland DOM) that are likely important for riverine-to-coastal DOM transport. As anthropogenic disturbances continue to impact temperate wetlands, this study highlights drivers of DOM composition fundamental for understanding how wetland organic carbon will change, and thus its role in biogeochemical cycling.

Global Biogeochemical Cycles↗
USGS2023

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 Δ 199 Hg 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 Δ 199 Hg/Δ 201 Hg ratio was uniform across experiments (∼1.4), Δ 199 Hg/δ 202 Hg 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.

ACS Earth and Space Chemistry↗
USGS2023

Comparing sediment microbial communities of Arctic beaver ponds to tundra lakes and streams

In recent decades the habitat of North American beaver ( Castor canadensis ) has expanded from boreal forests into Arctic tundra ecosystems. Beaver ponds in Arctic watersheds are known to alter stream biogeochemistry, which is likely coupled with changes in the activity and composition of microbial communities inhabiting beaver pond sediments. We investigated bacterial, archaeal, and fungal communities in beaver pond sediments along tundra streams in northwestern Alaska (AK), USA and compared them to those of tundra lakes and streams in north-central Alaska that are unimpacted by beavers. β -glucosidase activity assays indicated higher cellulose degradation potential in beaver ponds than in unimpacted streams and lakes within a watershed absent of beavers. Beta diversity analyses showed that dominant lineages of bacteria and archaea in beaver ponds differed from those in tundra lakes and streams, but dominant fungal lineages did not differ between these sample types. Beaver pond sediments displayed lower relative abundances of Crenarchaeota and Euryarchaeota archaea and of bacteria from typically anaerobic taxonomic groups, suggesting differences in rates of fermentative organic matter (OM) breakdown, syntrophy, and methane generation. Beaver ponds also displayed low relative abundances of Chytridiomycota (putative non-symbiotic) fungi and high relative abundances of ectomycorrhizal (plant symbionts) Basidiomycota fungi, suggesting differences in the occurrence of plant and fungi mutualistic interactions. Beaver ponds also featured microbes with taxonomic identities typically associated with the cycling of nitrogen and sulfur compounds in higher relative abundances than tundra lakes and streams. These findings help clarify the microbiological implications of beavers expanding into high latitude regions.

Alaska↗
USGS2023

Metabolically diverse microorganisms mediate methylmercury formation under nitrate-reducing conditions in a dynamic hydroelectric reservoir

Brownlee Reservoir is a mercury (Hg)-impaired hydroelectric reservoir that exhibits dynamic hydrological and geochemical conditions and is located within the Hells Canyon Complex in Idaho, USA. Methylmercury (MeHg) contamination in fish is a concern in the reservoir. While MeHg production has historically been attributed to sulfate-reducing bacteria and methanogenic archaea, microorganisms carrying the hgcA gene are taxonomically and metabolically diverse and the major biogeochemical cycles driving mercury (Hg) methylation are not well understood. In this study, Hg speciation and redox-active compounds were measured throughout Brownlee Reservoir across the stratified period in four consecutive years (2016–2019) to identify the location where and redox conditions under which MeHg is produced. Metagenomic sequencing was performed on a subset of samples to characterize the microbial community with hgcA and identify possible links between biogeochemical cycles and MeHg production. Biogeochemical profiles suggested in situ water column Hg methylation was the major source of MeHg. These profiles, combined with genome-resolved metagenomics focused on hgcA -carrying microbes, indicated that MeHg production occurs in this system under nitrate- or manganese-reducing conditions, which were previously thought to preclude Hg-methylation. Using this multidisciplinary approach, we identified the cascading effects of interannual variability in hydrology on the redox status, microbial metabolic strategies, abundance and metabolic diversity of Hg methylators, and ultimately MeHg concentrations throughout the reservoir. This work expands the known conditions conducive to producing MeHg and suggests that the Hg-methylation mitigation efforts by nitrate or manganese amendment may be unsuccessful in some locations.

Idaho↗
USGS2023

Environmental formation of methylmercury is controlled by synergy of inorganic mercury bioavailability and microbial mercury-methylation capacity

Methylmercury (MeHg) production is controlled by the bioavailability of inorganic divalent mercury (Hg(II) i ) and Hg-methylation capacity of the microbial community (conferred by the hgcAB gene cluster). However, the relative importance of these factors and their interaction in the environment remain poorly understood. Here, metagenomic sequencing and a full-factorial MeHg formation experiment were conducted across a wetland sulfate gradient with different microbial communities and pore water chemistries. From this experiment, the relative importance of each factor on MeHg formation was isolated. Hg(II) i bioavailability correlated with the dissolved organic matter composition, while the microbial Hg-methylation capacity correlated with the abundance of hgcA genes. MeHg formation responded synergistically to both factors. Notably, hgcA sequences were from diverse taxonomic groups, none of which contained genes for dissimilatory sulfate reduction. This work expands our understanding of the geochemical and microbial constraints on MeHg formation in situ and provides an experimental framework for further mechanistic studies.

Florida↗