Dimethylmercury as a Source of Monomethylmercury in a Highly Productive Upwelling System.

Monomethylmercury (MMHg) is a neurotoxicant that biomagnifies in marine food webs, reaching high concentrations in apex predators. To predict changes in oceanic MMHg concentrations, it is important to quantify the sources and sinks of MMHg. Here, we study mercury speciation in the California Current System through cruise sampling and modeling. Previous work in the California Current System has found that upwelling transports mercury-enriched deep waters to productive surface waters. These upwelled waters originate within the California Undercurrent water mass and are subsequently advected as a surface water parcel to the California Current. Between the two major water masses, we find that compared to the California Current, the California Undercurrent contains elevated dissolved total mercury (THg) and dimethylmercury (DMHg) concentrations by 59 and 69%, respectively. We explain that these differences result from losses during advection, specifically scavenging of THg and DMHg demethylation. We calculate a net DMHg demethylation rate of 2.0 ± 1.1% d-1 and build an empirically constrained mass budget model to demonstrate that net DMHg demethylation accounts for 61% of surface MMHg sources. These findings illustrate that DMHg is a significant source of MMHg in this region, challenging the current understanding of the major sources of marine MMHg.

Bluefin tuna reveal global patterns of mercury pollution and bioavailability in the world's oceans.

Bluefin tuna (BFT), highly prized among consumers, accumulate high levels of mercury (Hg) as neurotoxic methylmercury (MeHg). However, how Hg bioaccumulation varies among globally distributed BFT populations is not understood. Here, we show mercury accumulation rates (MARs) in BFT are highest in the Mediterranean Sea and decrease as North Pacific Ocean > Indian Ocean > North Atlantic Ocean. Moreover, MARs increase in proportion to the concentrations of MeHg in regional seawater and zooplankton, linking MeHg accumulation in BFT to MeHg bioavailability at the base of each subbasin's food web. Observed global patterns correspond to levels of Hg in each ocean subbasin; the Mediterranean, North Pacific, and Indian Oceans are subject to geogenic enrichment and anthropogenic contamination, while the North Atlantic Ocean is less so. MAR in BFT as a global pollution index reflects natural and human sources and global thermohaline circulation.

Description of a Dimethylmercury Automatic Analyzer for the High-Resolution Measurement of Dissolved Gaseous Mercury Species in Surface Ocean Waters.

Our understanding of the significance of dimethylmercury (DMHg) to the mercury (Hg) global ocean biogeochemical cycle is unclear because of the lack of detailed DMHg measurements in the water column. To our knowledge, 30 years of published studies have generated no more than 200 DMHg data points in the ocean surface waters and marine boundary layer (MBL). To improve the precision and reduce the uncertainty in determining DMHg in surface seawater, we developed a simple and robust DMHg automatic analyzer (DAA). This DAA system couples the main sampling and analytic steps, including a continuous flow chamber, with dual Carbotrap preconcentration, a gas chromatographic column, a cold vapor atomic fluorescence spectrometry, and a data logger for signal integration. We compared the operation, performance, and reproducibility between our DAA and the traditional manual analytic method. Its advantages include the ease of operation, the high time resolution and precision (30 min sampling and <5% relative variation), and long-term stability (2 weeks). The DAA can determine DMHg in both the MBL and surface seawater. The estimated detection limits for DMHg with the DAA in the atmosphere and in surface seawater are 10 pg/m3 and 0.2 fM, respectively. The successful DAA field measurement in coastal waters indicates that it can help detect the low DMHg concentration in surface seawater, and the time series DMHg data helped our understanding of the DMHg behavior (sources and sinks) and its flux into the MBL. The comparison of DMHg concentration in various oceans also suggests that the coastal region had the lowest averaged DMHg, up to an order of magnitude lower than other ecosystems.

A global ocean inventory of anthropogenic mercury based on water column measurements.

Mercury is a toxic, bioaccumulating trace metal whose emissions to the environment have increased significantly as a result of anthropogenic activities such as mining and fossil fuel combustion. Several recent models have estimated that these emissions have increased the oceanic mercury inventory by 36-1,313 million moles since the 1500s. Such predictions have remained largely untested owing to a lack of appropriate historical data and natural archives. Here we report oceanographic measurements of total dissolved mercury and related parameters from several recent expeditions to the Atlantic, Pacific, Southern and Arctic oceans. We find that deep North Atlantic waters and most intermediate waters are anomalously enriched in mercury relative to the deep waters of the South Atlantic, Southern and Pacific oceans, probably as a result of the incorporation of anthropogenic mercury. We estimate the total amount of anthropogenic mercury present in the global ocean to be 290 ± 80 million moles, with almost two-thirds residing in water shallower than a thousand metres. Our findings suggest that anthropogenic perturbations to the global mercury cycle have led to an approximately 150 per cent increase in the amount of mercury in thermocline waters and have tripled the mercury content of surface waters compared to pre-anthropogenic conditions. This information may aid our understanding of the processes and the depths at which inorganic mercury species are converted into toxic methyl mercury and subsequently bioaccumulated in marine food webs.

The Mercury-Tolerant Microbiota of the Zooplankton Daphnia Aids in Host Survival and Maintains Fecundity under Mercury Stress.

Many aquatic organisms can thrive in polluted environments by having the genetic capability to withstand suboptimal conditions. However, the contributions of microbiomes under these stressful environments are poorly understood. We investigated whether a mercury-tolerant microbiota can extend its phenotype to its host by ameliorating host survival and fecundity under mercury-stress. We isolated microbiota members from various clones of Daphnia magna, screened for the mercury-biotransforming merA gene, and determined their mercury tolerance levels. We then introduced the mercury-tolerant microbiota, Pseudomonas-10, to axenic D. magna and quantified its merA gene expression, mercury reduction capability, and measured its impact on host survival and fecundity. The expression of the merA gene was up-regulated in Pseudomonas-10, both in isolation and in host-association with mercury exposure. Pseudomonas-10 is also capable of significantly reducing mercury concentration in the medium. Notably, mercury-exposed daphnids containing only Pseudomonas-10 exhibited higher survival and fecundity than mercury-exposed daphnids supplemented with parental microbiome. Our study showed that zooplankton, such as Daphnia, naturally harbor microbiome members that are eco-responsive and tolerant to mercury exposure and can aid in host survival and maintain host fecundity in a mercury-contaminated environment. This study further demonstrates that under stressful environmental conditions, the fitness of the host can depend on the genotype and the phenotype of its microbiome.

Global and Local Sources of Mercury Deposition in Coastal New England Reconstructed from a Multiproxy, High-Resolution, Estuarine Sediment Record.

Historical reconstruction of mercury (Hg) accumulation in natural archives, especially lake sediments, has been essential to understanding human perturbation of the global Hg cycle. Here we present a high-resolution chronology of Hg accumulation between 1727 and 1996 in a varved sediment core from the Pettaquamscutt River Estuary (PRE), Rhode Island. Mercury accumulation is examined relative to (1) historic deposition of polycyclic aromatic hydrocarbons (PAHs) and lead (Pb) and its isotopes (206Pb/207Pb) in the same core, and (2) other reconstructions of Hg deposition in urban and remote settings. Mercury deposition in PRE parallels the temporal patterns of PAHs, and both track industrialization and regional coal use between 1850 and 1950 as well as rising petroleum use after 1950. There is little indication of increased Hg deposition from late 19th-century silver and gold mining in the western U.S. A broad maximum of Hg deposition during 1930-1980, and not found in remote sites, is consistent with the predicted influence of additional industrial sources and commercial products. Our results imply that a significant portion of global anthropogenic Hg emissions during the 20th century was deposited locally, near urban and industrial centers of Hg use and release.

Extensive Dark Biological Production of Reactive Oxygen Species in Brackish and Freshwater Ponds.

Within natural waters, photodependent processes are generally considered the predominant source of reactive oxygen species (ROS), a suite of biogeochemically important molecules. However, recent discoveries of dark particle-associated ROS production in aquatic environments and extracellular ROS production by various microorganisms point to biological activity as a significant source of ROS in the absence of light. Thus, the objective of this study was to explore the occurrence of dark biological production of the ROS superoxide (O2(-)) and hydrogen peroxide (H2O2) in brackish and freshwater ponds. Here we show that the ROS superoxide and hydrogen peroxide were present in dark waters at comparable concentrations as in sunlit waters. This suggests that, at least for the short-lived superoxide species, light-independent processes were an important control on ROS levels in these natural waters. Indeed, we demonstrated that dark biological production of ROS extensively occurred in brackish and freshwater environments, with greater dark ROS production rates generally observed in the aphotic relative to the photic zone. Filtering and formaldehyde inhibition confirmed the biological nature of a majority of this dark ROS production, which likely involved phytoplankton, particle-associated heterotrophic bacteria, and NADH-oxidizing enzymes. We conclude that biological ROS production is widespread, including regions devoid of light, thereby expanding the relevance of these reactive molecules to all regions of our oxygenated global habit.

An examination of the role of particles in oceanic mercury cycling.

Recent models of global mercury (Hg) cycling have identified the downward flux of sinking particles in the ocean as a prominent Hg removal process from the ocean. At least one of these models estimates the amount of anthropogenic Hg in the ocean to be about 400 Mmol, with deep water formation and sinking fluxes representing the largest vectors by which pollutant Hg is able to penetrate the ocean interior. Using data from recent cruises to the Atlantic, we examined the dissolved and particulate partitioning of Hg in the oceanic water column as a cross-check on the hypothesis that sinking particle fluxes are important. Interestingly, these new data suggest particle-dissolved partitioning (Kd) that is approximately 20× greater than previous estimates, which thereby challenges certain assumptions about the scavenging and active partitioning of Hg in the ocean used in earlier models. For example, the new particle data suggest that regenerative scavenging is the most likely mechanism by which the association of Hg and particles occurs.This article is part of the themed issue 'Biological and climatic impacts of ocean trace element chemistry'.

Atmospheric Hg emissions from preindustrial gold and silver extraction in the Americas: a reevaluation from lake-sediment archives.

Human activities over the last several centuries have transferred vast quantities of mercury (Hg) from deep geologic stores to actively cycling earth-surface reservoirs, increasing atmospheric Hg deposition worldwide. Understanding the magnitude and fate of these releases is critical to predicting how rates of atmospheric Hg deposition will respond to future emission reductions. The most recently compiled global inventories of integrated (all-time) anthropogenic Hg releases are dominated by atmospheric emissions from preindustrial gold/silver mining in the Americas. However, the geophysical evidence for such large early emissions is equivocal, because most reconstructions of past Hg-deposition have been based on lake-sediment records that cover only the industrial period (1850-present). Here we evaluate historical changes in atmospheric Hg deposition over the last millennium from a suite of lake-sediment cores collected from remote regions of the globe. Along with recent measurements of Hg in the deep ocean, these archives indicate that atmospheric Hg emissions from early mining were modest as compared to more recent industrial-era emissions. Although large quantities of Hg were used to extract New World gold and silver beginning in the 16th century, a reevaluation of historical metallurgical methods indicates that most of the Hg employed was not volatilized, but rather was immobilized in mining waste.

Mercury speciation and mobilization in a wastewater-contaminated groundwater plume.

We measured the concentration and speciation of mercury (Hg) in groundwater down-gradient from the site of wastewater infiltration beds operated by the Massachusetts Military Reservation, western Cape Cod, Massachusetts. Total mercury concentrations in oxic, mildly acidic, uncontaminated groundwater are 0.5-1 pM, and aquifer sediments have 0.5-1 ppb mercury. The plume of impacted groundwater created by the wastewater disposal is still evident, although inputs ceased in 1995, as indicated by anoxia extending at least 3 km down-gradient from the disposal site. Solutes indicative of a progression of anaerobic metabolisms are observed vertically and horizontally within the plume, with elevated nitrate concentrations and nitrate reduction surrounding a region with elevated iron concentrations indicating iron reduction. Mercury concentrations up to 800 pM were observed in shallow groundwater directly under the former infiltration beds, but concentrations decreased with depth and with distance down-gradient. Mercury speciation showed significant connections to the redox and metabolic state of the groundwater, with relatively little methylated Hg within the iron reducing sector of the plume, and dominance of this form within the higher nitrate/ammonium zone. Furthermore, substantial reduction of Hg(II) to Hg(0) within the core of the anoxic zone was observed when iron reduction was evident. These trends not only provide insight into the biogeochemical factors controlling the interplay of Hg species in natural waters, but also support hypotheses that anoxia and eutrophication in groundwater facilitate the mobilization of natural and anthropogenic Hg from watersheds/aquifers, which can be transported down-gradient to freshwaters and the coastal zone.

Mercury biogeochemical cycling in the ocean and policy implications.

Anthropogenic activities have enriched mercury in the biosphere by at least a factor of three, leading to increases in total mercury (Hg) in the surface ocean. However, the impacts on ocean fish and associated trends in human exposure as a result of such changes are less clear. Here we review our understanding of global mass budgets for both inorganic and methylated Hg species in ocean seawater. We consider external inputs from atmospheric deposition and rivers as well as internal production of monomethylmercury (CH3Hg) and dimethylmercury ((CH3)2Hg). Impacts of large-scale ocean circulation and vertical transport processes on Hg distribution throughout the water column and how this influences bioaccumulation into ocean food chains are also discussed. Our analysis suggests that while atmospheric deposition is the main source of inorganic Hg to open ocean systems, most of the CH3Hg accumulating in ocean fish is derived from in situ production within the upper waters (<1000 m). An analysis of the available data suggests that concentrations in the various ocean basins are changing at different rates due to differences in atmospheric loading and that the deeper waters of the oceans are responding slowly to changes in atmospheric Hg inputs. Most biological exposures occur in the upper ocean and therefore should respond over years to decades to changes in atmospheric mercury inputs achieved by regulatory control strategies. Migratory pelagic fish such as tuna and swordfish are an important component of CH3Hg exposure for many human populations and therefore any reduction in anthropogenic releases of Hg and associated deposition to the ocean will result in a decline in human exposure and risk.

Nutrient supply and mercury dynamics in marine ecosystems: a conceptual model.

There is increasing interest and concern over the impacts of mercury (Hg) inputs to marine ecosystems. One of the challenges in assessing these effects is that the cycling and trophic transfer of Hg are strongly linked to other contaminants and disturbances. In addition to Hg, a major problem facing coastal waters is the impacts of elevated nutrient, particularly nitrogen (N), inputs. Increases in nutrient loading alter coastal ecosystems in ways that should change the transport, transformations and fate of Hg, including increases in fixation of organic carbon and deposition to sediments, decreases in the redox status of sediments and changes in fish habitat. In this paper we present a conceptual model which suggests that increases in loading of reactive N to marine ecosystems might alter Hg dynamics, decreasing bioavailabilty and trophic transfer. This conceptual model is most applicable to coastal waters, but may also be relevant to the pelagic ocean. We present information from case studies that both support and challenge this conceptual model, including marine observations across a nutrient gradient; results of a nutrient-trophic transfer Hg model for pelagic and coastal ecosystems; observations of Hg species, and nutrients from coastal sediments in the northeastern U.S.; and an analysis of fish Hg concentrations in estuaries under different nutrient loadings. These case studies suggest that changes in nutrient loading can impact Hg dynamics in coastal and open ocean ecosystems. Unfortunately none of the case studies is comprehensive; each only addresses a portion of the conceptual model and has limitations. Nevertheless, our conceptual model has important management implications. Many estuaries near developed areas are impaired due to elevated nutrient inputs. Widespread efforts are underway to control N loading and restore coastal ecosystem function. An unintended consequence of nutrient control measures could be to exacerbate problems associated with Hg contamination. Additional focused research and monitoring are needed to critically examine the link between nutrient supply and Hg contamination of marine waters.

The role of plastic debris in the biogeochemical cycle of mercury in Lake Erie and San Francisco Bay.

The accumulation of plastic debris that concentrates hydrophobic compounds and microbial communities creates the potential for altered aquatic biogeochemical cycles. This study investigated the role of plastic debris in the biogeochemical cycling of mercury in surface waters of the San Francisco Bay, Sacramento River, Lake Erie, and in coastal seawater. Total mercury and monomethylmercury were measured on plastic debris from all study sites. Plastic-bound microbial communities from Lake Erie and San Francisco Bay contained several lineages of known mercury methylating microbes, however the hgcAB gene cluster was not detected using polymerase chain reaction. These plastic-bound microbial communities also contained species that possess the mer operon, and merA genes were detected using polymerase chain reaction. In coastal seawater incubations, rapid mercury methylation percentages were greater in the presence of microplastics and demethylation percentages decreased as monomethylmercury additions adsorbed to microplastics. These findings suggest that plastic pollution has the potential to alter the biogeochemical cycling of mercury in aquatic ecosystems.

Mercury methylation by metabolically versatile and cosmopolitan marine bacteria.

Microbes transform aqueous mercury (Hg) into methylmercury (MeHg), a potent neurotoxin that accumulates in terrestrial and marine food webs, with potential impacts on human health. This process requires the gene pair hgcAB, which encodes for proteins that actuate Hg methylation, and has been well described for anoxic environments. However, recent studies report potential MeHg formation in suboxic seawater, although the microorganisms involved remain poorly understood. In this study, we conducted large-scale multi-omic analyses to search for putative microbial Hg methylators along defined redox gradients in Saanich Inlet, British Columbia, a model natural ecosystem with previously measured Hg and MeHg concentration profiles. Analysis of gene expression profiles along the redoxcline identified several putative Hg methylating microbial groups, including Calditrichaeota, SAR324 and Marinimicrobia, with the last the most active based on hgc transcription levels. Marinimicrobia hgc genes were identified from multiple publicly available marine metagenomes, consistent with a potential key role in marine Hg methylation. Computational homology modelling predicts that Marinimicrobia HgcAB proteins contain the highly conserved amino acid sites and folding structures required for functional Hg methylation. Furthermore, a number of terminal oxidases from aerobic respiratory chains were associated with several putative novel Hg methylators. Our findings thus reveal potential novel marine Hg-methylating microorganisms with a greater oxygen tolerance and broader habitat range than previously recognized.

Characterization of the metalloproteome of Pseudoalteromonas (BB2-AT2): biogeochemical underpinnings for zinc, manganese, cobalt, and nickel cycling in a ubiquitous marine heterotroph.

Pseudoalteromonas (BB2-AT2) is a ubiquitous marine heterotroph, often associated with labile organic carbon sources in the ocean (e.g. phytoplankton blooms and sinking particles). Heterotrophs hydrolyze exported photosynthetic materials, components of the biological carbon pump, with the use of diverse metalloenzymes containing zinc (Zn), manganese (Mn), cobalt (Co), and nickel (Ni). Studies on the metal requirements and cytosolic utilization of metals for marine heterotrophs are scarce, despite their relevance to global carbon cycling. Here, we characterized the Zn, Mn, Co, and Ni metallome of BB2-AT2. We found that the Zn metallome is complex and cytosolic Zn is associated with numerous proteins for transcription (47.2% of the metallome, obtained from singular value decomposition of the metalloproteomic data), translation (33.5%), proteolysis (12.8%), and alkaline phosphatase activity (6.4%). Numerous proteolytic enzymes also appear to be putatively associated with Mn, and to a lesser extent, Co. Putative identification of the Ni-associated proteins, phosphoglucomutase and a protein in the cupin superfamily, provides new insights for Ni utilization in marine heterotrophs. BB2-AT2 relies on numerous transition metals for proteolytic and phosphatase activities, inferring an adaptative potential to metal limitation. Our field observations of increased alkaline phosphatase activity upon addition of Zn in field incubations suggest that such metal limitation operates in sinking particulate material collected from sediment traps. Taken together, this study improves our understanding of the Zn, Mn, Co, and Ni metallome of marine heterotrophic bacteria and provides novel and mechanistic frameworks for understanding the influence of nutrient limitation on biogeochemical cycling.

Increased accumulation of sulfur in lake sediments of the high arctic.

We report a synchronous increase in accumulation of reduced inorganic sulfur since c. 1980 in sediment cores from eight of nine lakes studied in the Canadian Arctic and Svalbard (Norway). Sediment incubations and detailed analyses of sediment profiles from two of the lakes indicate that increases in sulfur accumulation may be due ultimately to a changing climate. Warming-induced lengthening of the ice-free season is resulting in well-documented increases in algal production and sedimentation of the resulting detrital matter. Algal detritus is a rich source of labile carbon, which in these sediments stimulates dissimilatory sulfate reduction. The sulfide produced is stored in sediment (as acid volatile sulfide), converted to other forms of sulfur, or reoxidized to sulfate and lost to the water column. An acceleration of the sulfur cycle in Arctic lakes could have profound effects on important biogeochemical processes, such as carbon burial and mercury methylation.

A global perspective on mercury cycling in the ocean.

Mercury (Hg) is a ubiquitous metal in the ocean that undergoes in situ chemical transformations in seawater and marine sediment. Most relevant to public health is the production of monomethyl-Hg, a neurotoxin to humans that accumulates in marine fish and mammals. Here we synthesize 30 years of Hg measurements in the ocean to discuss sources, sinks, and internal cycling of this toxic metal. Global-scale oceanographic survey programs (i.e. CLIVAR and GEOTRACES), refined protocols for clean sampling, and analytical advancements have produced over 200 high-resolution, full-depth profiles of total Hg, methylated Hg, and gaseous elemental Hg throughout the Atlantic, Pacific, Arctic, and Southern Oceans. Vertical maxima of methylated Hg were found in surface waters, near the subsurface chlorophyll maximum, and in low-oxygen thermocline waters. The greatest concentration of Hg in deep water was measured in Antarctic Bottom Water, and in newly formed Labrador Sea Water, Hg showed a decreasing trend over the past 20 years. Distribution of Hg in polar oceans was unique relative to lower latitudes with higher concentrations of total Hg near the surface and vertical trends of Hg speciation driven by water column stratification and seasonal ice cover. Global models of Hg in the ocean require a better understanding of biogeochemical controls on Hg speciation and improved accuracy of methylated Hg measurements within the international community.

Spatial Heterogeneity in Particle-Associated, Light-Independent Superoxide Production Within Productive Coastal Waters.

In the marine environment, the reactive oxygen species (ROS) superoxide is produced through a diverse array of light-dependent and light-independent reactions, the latter of which is thought to be primarily controlled by microorganisms. Marine superoxide production influences organic matter remineralization, metal redox cycling, and dissolved oxygen concentrations, yet the relative contributions of different sources to total superoxide production remain poorly constrained. Here we investigate the production, steady-state concentration, and particle-associated nature of light-independent superoxide in productive waters off the northeast coast of North America. We find exceptionally high levels of light-independent superoxide in the marine water column, with concentrations ranging from 10 pM to in excess of 2,000 pM. The highest superoxide concentrations were particle associated in surface seawater and in aphotic seawater collected meters off the seafloor. Filtration of seawater overlying the continental shelf lowered the light-independent, steady-state superoxide concentration by an average of 84%. We identify eukaryotic phytoplankton as the dominant particle-associated source of superoxide to these coastal waters. We contrast these measurements with those collected at an off-shelf station, where superoxide concentrations did not exceed 100 pM, and particles account for an average of 40% of the steady-state superoxide concentration. This study demonstrates the primary role of particles in the production of superoxide in seawater overlying the continental shelf and highlights the importance of light-independent, dissolved-phase reactions in marine ROS production.

Tuna Consumption, Mercury Exposure, and Knowledge about Mercury Exposure Risk from Tuna Consumption in University Students.

We examined the relationships among tuna consumption, hair mercury levels, and knowledge of mercury exposure risk from tuna consumption in university students that were offered tuna daily at university-run dining halls. Hair total mercury levels in tuna consumers were higher than those in non-tuna consumers (average = 0.466 µg/g ± 0.328 standard deviation [SD], n = 20 vs 0.110 µg/g ± 0.105 SD, n = 33, respectively; p < 0.0001, Mann-Whitney U test), with tuna eaters exhibiting a positive relationship between self-reported tuna consumption at dining halls and hair mercury levels (R2 = 0.868, p < 0.0001, n = 17, linear regression). For all tuna eaters surveyed, more than half (54%) self-reported eating ≥3 tuna meals/wk, potentially exceeding the US Environmental Protection Agency's reference dose for methylmercury of 0.1 µg/kg body weight/d. Seven percent of study participants reported they consumed >20 tuna meals/wk, which was related to hair mercury levels >1 µg/g, a level of concern. Study participants had an overall lack of knowledge and confidence in their knowledge about mercury exposure risk from tuna consumption, with >99% of participants reporting low knowledge and low confidence in survey answers. Our study highlights the importance of education about the risks of tuna consumption, particularly in institutional settings where individuals have unlimited access to tuna products. Environ Toxicol Chem 2019;38:1988-1994. © 2019 SETAC.

Presence of artisanal gold mining predicts mercury bioaccumulation in five genera of bats (Chiroptera).

Mercury, a toxic trace metal, has been used extensively as an inexpensive and readily available method of extracting gold from fine-grained sediment. Worldwide, artisanal mining is responsible for one third of all mercury released into the environment. By testing bat hair from museum specimens and field collected samples from areas both impacted and unimpacted by artisanal gold mining in Perú, we show monomethylmercury (MMHg) has increased in the last 100 years. MMHg concentrations were also greatest in the highest bat trophic level (insectivores), and in areas experiencing extractive artisanal mining. Reproductive female bats had higher MMHg concentrations, and both juvenile and adult bats from mercury contaminated sites had more MMHg than those from uncontaminated sites. Bats have important ecological functions, providing vital ecosystem services such as pollination, seed dispersal, and insect control. Natural populations can act as environmental sentinels and offer the chance to expand our understanding of, and responses to, environmental and human health concerns.