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The contamination of lakes by industrial emissions is an issue of international concern. Traditional paleolimnology examines sedimentary micro-fossils to infer the biological response to natural and anthropogenic stressors over time. Here, we calculate a theoretical biological effect for historic sediment sections using Probable Effect Concentration Quotient (PEC-Q) and arsenic specific quotient methods and develop novel time-constrained sediment toxicity test methods using a cultured Daphnia sp. combined with a whole cell microbial biosensor to assess the toxicity of past industrial contamination with modern testing methods. These methods were developed using sediments collected from Pocket Lake (Northwest Territories, Canada), a lake known to have exhibited a significant ecological shift following input from nearby gold smelter emissions during the mid 20th century. We then applied these methods to near-, mid-, and far-field sites to assess the response of Daphnia sp. to varying contaminant load. Daphnia sp. mortality exposed to dated sediments indicated a strong concordance with the timing of mining activities, and a strong concordance with PEC-Q and arsenic specific toxicity quotients. In contrast, a decrease in Daphnia mortality was observed during pre-, and post-mining periods when the contaminant burden was lower. Initial assessments of bioavailability using a microbial biosensor indicated that arsenic in porewater is 72-96% bioavailable, and limited evidence that oxidative stress may contribute to the Daphnia sp. toxic response. These results indicate that lake sediment archives can be used to infer missing biomonitoring data in sites of legacy anthropogenic influence, which will be useful for those seeking to conduct cost-effective and efficient preliminary environmental risk assessments., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Microbe-mediated transformations of arsenic (As) often require As to be taken up into cells prior to enzymatic reaction. Despite the importance of these microbial reactions for As speciation and toxicity, understanding of how As bioavailability and uptake are regulated by aspects of extracellular water chemistry, notably dissolved organic matter (DOM), remains limited. Whole-cell biosensors utilizing fluorescent proteins are increasingly used for high-throughput quantification of the bioavailable fraction of As in water. Here, we present a mathematical framework for interpreting the time series of biosensor fluorescence as a measure of As uptake kinetics, which we used to evaluate the effects of different forms of DOM on uptake of trivalent arsenite. We found that thiol-containing organic compounds significantly inhibited uptake of arsenite into cells, possibly through the formation of aqueous complexes between arsenite and thiol ligands. While there was no evidence for competitive interactions between arsenite and low-molecular-weight neutral molecules (urea, glycine, and glyceraldehyde) for uptake through the aquaglyceroporin channel GlpF, which mediates transport of arsenite across cell membranes, there was evidence that labile DOM fractions may inhibit arsenite uptake through a catabolite repression-like mechanism. The observation of significant inhibition of arsenite uptake at DOM/As ratios commonly encountered in wetland pore waters suggests that DOM may be an important control on the microbial uptake of arsenite in the environment, with aspects of DOM quality playing an important role in the extent of inhibition. IMPORTANCE The speciation and toxicity of arsenic in environments like rice paddy soils and groundwater aquifers are controlled by microbe-mediated reactions. These reactions often require As to be taken up into cells prior to enzymatic reaction, but there is limited understanding of how microbial arsenic uptake is affected by variations in water chemistry. In this study, we explored the effect of dissolved organic matter (DOM) quantity and quality on microbial As uptake, with a focus on the role of thiol functional groups that are well known to form aqueous complexes with arsenic. We developed a quantitative framework for interpreting fluorescence time series from whole-cell biosensors and used this technique to evaluate effects of DOM on the rates of microbial arsenic uptake. We show that thiol-containing compounds significantly decrease rates of As uptake into microbial cells at environmentally relevant DOM/As ratios, revealing the importance of DOM quality in regulating arsenic uptake, and subsequent biotransformation, in the environment.

Methylmercury is a potent neurotoxin that biomagnifies through food webs and which production depends on anaerobic microbial uptake of inorganic mercury (Hg) species. One outstanding knowledge gap in understanding Hg methylation is the nature of bioavailable Hg species. It has become increasingly obvious that Hg bioavailability is spatially diverse and temporally dynamic but current models are mostly built on single thiolated ligand systems, omitting ligand exchanges and interactions, or the inclusion of dissolved gaseous phases. In this study, we used a whole-cell anaerobic biosensor to determine the role of a mixture of thiolated ligands on Hg bioavailability. Serendipitously, we discovered how the diffusion of trace amounts of exogenous biogenic H 2 S, originating from anaerobic microbial ligand degradation, can alter Hg speciation - away from H 2 S production site - to form bioavailable species. Regardless of its origins, H 2 S stands as a mobile mediator of microbial Hg metabolism, connecting spatially separated microbial communities. At a larger scale, global planetary changes are expected to accelerate the production and mobilization of H 2 S and Hg, possibly leading to increased production of the potent neurotoxin; this work provides mechanistic insights into the importance of co-managing biogeochemical cycle disruptions., (© 2022 Society for Applied Microbiology and John Wiley & Sons Ltd.)

Smelting activities at Giant Mine (Yellowknife, NWT, Canada) have resulted in high sulfate and arsenic concentrations in nearby lakes. Here we tested whether historic smelting affects current mercury (Hg) cycling in 35 freshwater lakes over a 2800 km 2 area around the former gold mine. We sampled lake water and sediment over three consecutive years (2015-2017) using a factorial sampling design that accounted for different environmental variables known to affect the net methylmercury (MeHg) levels in water. Stable Hg(ii) and MeHg isotope tracers were used to quantify Hg methylation and demethylation rate constants in sediments, and 16S rRNA gene amplicon sequencing was used to characterize microbial community structure. This study reveals that the fraction of methylated total Hg (% MeHg) found in surface water is positively correlated to the sulfate gradient, while the rate at which Hg is methylated (K m ) in sediments is negatively correlated with total arsenic, and positively correlated with dissolved organic carbon, total phosphorous, and % MeHg in the water. Furthermore, 6 of the 28 lakes that had detectable demethylation rate constants (K d ) also had significantly lower DOC concentrations than lakes with non-detectable K d. Our results also show that legacy pollution from smelting activities is affecting the structure of microbial communities in lake sediments. This study reveals the complex dynamics of Hg cycling in this northern environment, highlighting the importance of large-scale studies in which the effect of multiple pollution gradients (e.g. arsenic and sulfate) must be taken into consideration.

In the latter half of the 1950s, a large amount of methylmercury (MeHg) was discharged directly into Minamata Bay, Japan by a chemical factory, resulting in the contamination of the fish and shellfish. Ultimately, an outbreak of MeHg intoxication, called Minamata disease, occurred. From 1977 to 1988, the Kumamoto Prefectural Government dredged and transferred sediments exceeding 25 μg/g of total mercury (THg, dry basis) into a strictly segregated area of the bay near the wastewater outlet, then this area was landfilled. We conducted analyses of the mercury speciation in preserved Minamata Bay sludge samples (collected from inside of the bay prior to the termination of the remediation project; n=4) and recent Minamata Bay sediments (collected outside the dredging area of the bay; n=5) to evaluate the potential risk of the sludge/sediment leakage from the reclaimed land to the Minamata Bay. Median THg (dry basis) concentrations were 241 μg/g for the preserved sludge, 6.1 μg/g for the recent Minamata Bay sediments, and 0.18 μg/g for a single control sample; median MeHg concentrations (percentage of MeHg in THg) were 108 ng/g (0.031%), 3.7 ng/g (0.12%), and 0.71 ng/g (0.41%), respectively. In all the samples, the MeHg% decreased exponentially with increasing THg concentration. The extractability of THg from each sample into seawater was shown to be much lower than that of MeHg. The extracted MeHg was 0.86% for the preserved sludge, 4.57% for the recent Minamata Bay sediments, and 7.89% for the control. The predominant chemical form of mercury in the preserved sludge containing the highest THg concentration was found to be stable β-mercury sulfide (HgS) based on transmission electron microscopy linked with energy-dispersive X-ray spectroscopy (TEM-EDX) and X-ray absorption fine structure (XAFS) analyses., (Copyright © 2019 Elsevier Inc. All rights reserved.)

The biological mobilization of mercury (Hg) into microbes capable of Hg methylation is one of the limiting steps in the formation of the neurotoxin methylmercury (MeHg). Although algal dissolved organic matter (DOM) has been associated with increased MeHg production, the relationship between bacterial Hg uptake and algal DOM remains unexplored. In this study, we aimed to address how the quantity and quality of DOM, freshly harvested from several algae, affected the bacterial uptake of Hg with the use of a biosensor capable of functioning both aerobically and anaerobically. We combined biosensor measurements with high-resolution mass spectrometry and field-flow fractionation to elucidate how DOM composition and molecular weight influenced microbial Hg uptake. We showed that freshly harvested DOM from Chlorophyte and Euglena mutabilis strongly inhibited aerobic and anaerobic Hg uptake, whereas DOM harvested from Euglena gracilis did not exhibit this same pronounced effect. Once fractionated, we found that amino acids and polyamines, most abundant in Euglena gracilis DOM, were positively correlated to increase Hg uptake, suggesting that these molecules are potentially underappreciated ligands affecting Hg bioavailability. As water quality is affected by eutrophication, algal community assemblages will change, leading to variations in the nature of autochthonous DOM released in aquatic systems. Our results highlight that variations in the emergent properties of DOM originating from varying algal species can have a profound effect on bacterial Hg uptake and thus methylation.

We investigated monomethylmercury (MMHg) bioaccumulation in lakes across a 30° latitudinal gradient in eastern Canada to test the hypothesis that climate-related environmental conditions affect the sensitivity of Arctic lakes to atmospheric mercury contamination. Aquatic invertebrates (chironomid larvae, zooplankton) provided indicators of MMHg bioaccumulation near the base of benthic and planktonic food chains. In step with published data showing latitudinal declines in atmospheric mercury deposition in Canada, we observed lower total mercury concentrations in water and sediment of higher latitude lakes. Despite latitudinal declines of inorganic mercury exposure, MMHg bioaccumulation in aquatic invertebrates did not concomitantly decline. Arctic lakes with greater MMHg in aquatic invertebrates either had (1) higher water MMHg concentrations (reflecting ecosystem MMHg production) or (2) low water concentrations of MMHg, dissolved organic carbon (DOC), chlorophyll, and total nitrogen (reflecting lake sensitivity). The MMHg:DOC ratio of surface water was a strong predictor of lake sensitivity to mercury contamination. Bioaccumulation factors for biofilms and seston in Arctic lakes showed more efficient uptake of MMHg in low DOC systems. Environmental conditions associated with low biological production in Arctic lakes and their watersheds increased the sensitivity of lakes to MMHg.

The Canadian Arctic has vast freshwater resources, and fish are important in the diet of many Northerners. Mercury is a contaminant of concern because of its potential toxicity and elevated bioaccumulation in some fish populations. Over the last decade, significant advances have been made in characterizing the cycling and fate of mercury in these freshwater environments. Large amounts of new data on concentrations, speciation and fluxes of Hg are provided and summarized for water and sediment, which were virtually absent for the Canadian Arctic a decade ago. The biogeochemical processes that control the speciation of mercury remain poorly resolved, including the sites and controls of methylmercury production. Food web studies have examined the roles of Hg uptake, trophic transfer, and diet for Hg bioaccumulation in fish, and, in particular, advances have been made in identifying determinants of mercury levels in lake-dwelling and sea-run forms of Arctic char. In a comparison of common freshwater fish species that were sampled across the Canadian Arctic between 2002 and 2009, no geographic patterns or regional hotspots were evident. Over the last two to four decades, Hg concentrations have increased in some monitored populations of fish in the Mackenzie River Basin while other populations from the Yukon and Nunavut showed no change or a slight decline. The different Hg trends indicate that the drivers of temporal change may be regional or habitat-specific. The Canadian Arctic is undergoing profound environmental change, and preliminary evidence suggests that it may be impacting the cycling and bioaccumulation of mercury. Further research is needed to investigate climate change impacts on the Hg cycle as well as biogeochemical controls of methylmercury production and the processes leading to increasing Hg levels in some fish populations in the Canadian Arctic., (Crown Copyright © 2014. Published by Elsevier B.V. All rights reserved.)

This review summarizes data and information which have been generated on mercury (Hg) in the marine environment of the Canadian Arctic since the previous Canadian Arctic Contaminants Assessment Report (CACAR) was released in 2003. Much new information has been collected on Hg concentrations in marine water, snow and ice in the Canadian Arctic. The first measurements of methylation rates in Arctic seawater indicate that the water column is an important site for Hg methylation. Arctic marine waters were also found to be a substantial source of gaseous Hg to the atmosphere during the ice-free season. High Hg concentrations have been found in marine snow as a result of deposition following atmospheric mercury depletion events, although much of this Hg is photoreduced and re-emitted back to the atmosphere. The most extensive sampling of marine sediments in the Canadian Arctic was carried out in Hudson Bay where sediment total Hg (THg) concentrations were low compared with other marine regions in the circumpolar Arctic. Mass balance models have been developed to provide quantitative estimates of THg fluxes into and out of the Arctic Ocean and Hudson Bay. Several recent studies on Hg biomagnification have improved our understanding of trophic transfer of Hg through marine food webs. Over the past several decades, Hg concentrations have increased in some marine biota, while other populations showed no temporal change. Marine biota also exhibited considerable geographic variation in Hg concentrations with ringed seals, beluga and polar bears from the Beaufort Sea region having higher Hg concentrations compared with other parts of the Canadian Arctic. The drivers of these variable patterns of Hg bioaccumulation, both regionally and temporally, within the Canadian Arctic remain unclear. Further research is needed to identify the underlying processes including the interplay between biogeochemical and food web processes and climate change., (Crown Copyright © 2014. Published by Elsevier B.V. All rights reserved.)

A dynamic mass balance model was developed for the St. Lawrence River near Cornwall, Ontario that predicts and hindcasts mercury concentrations and fluxes in three forms, elemental Hg (Hg(0)), divalent mercury (Hg(2+)), and methyl mercury (MeHg), in a six compartment environment (air, water, porewater, sediment, periphyton, and benthic invertebrates). Our objective was to construct a dynamic mass balance model for mercury in the St. Lawrence River near Cornwall, Ontario based on the framework and results of a steady-state mass balance model developed previously for this site. The second objective was to estimate industrial mercury emissions based on mercury residues deposited in sediments prior to 1970, the year when regulations were implemented to reduce mercury pollution in the environment. We compiled mercury concentrations, fluxes, and transformation rates from previous studies completed in this section of the river (area of approximately 100km(2)) to develop the model. Estimated mercury concentrations in all media were similar to measured data (R(2)=0.99), with only minor exceptions, providing a satisfactory overall description of the mercury loadings and transformation rates of the different mercury species. The estimated historical emissions prior to 1970 from local industries along the Cornwall waterfront were approximately 400kgyear(-1). A storm sewer discharge of 5000m(3)/day resulted in a significant increase in mercury concentrations, particularly in sediment (617ngg(-1) to 624ngg(-1); p=0.004). Model results suggest that discharges of mercury from sources such as local industries and storm sewers have an impact on mercury in media such as sediment and water. This model should provide a basis for predicting and hindcasting mercury concentrations in other river environments as well, because it considers three distinct forms of mercury, and contains environmental media common to all rivers, including some (e.g. periphyton) not typically included in previous mercury models., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Predicting the bioavailability of inorganic mercury (Hg) to bacteria that produce the potent bioaccumulative neurotoxin monomethylmercury remains one of the greatest challenges in predicting the environmental fate and transport of Hg. Dissolved organic matter (DOM) affects mercury methylation due to its influence on cell physiology (as a potential nutrient) and its influence on Hg(II) speciation in solution (as a complexing agent), therefore controlling Hg bioavailability. We assessed the role of DOM on Hg(II) bioavailability to a gram-negative bacterium bioreporter under oxic pseudo- and nonequilibrium conditions, using defined media and field samples spanning a wide range of DOM levels. Our results showed that Hg(II) was considerably more bioavailable under nonequilibrium conditions than when DOM was absent or when Hg(II) and DOM had reached pseudoequilibrium (24 h) prior to cell exposure. Under these enhanced uptake conditions, Hg(II) bioavailability followed a bell shaped curve as DOM concentrations increased, both for defined media and natural water samples, consistent with bioaccumulation results in a companion paper (this issue) observed for amphipods. Experiments also suggest that DOM may not only provide shuttle molecules facilitating Hg uptake, but also alter cell wall properties to facilitate the first steps toward Hg(II) internalization. We propose the existence of a short-lived yet critical time window (<24 h) during which DOM facilitates the entry of newly deposited Hg(II) into aquatic food webs, suggesting that the bulk of mercury incorporation in aquatic food webs would occur within hours following its deposition from the atmosphere.

Dissolved organic carbon (DOC) is known to affect the Hg cycle in aquatic environments due to its overriding influence on complexation, photochemical, and microbial processes, but its role as a mediating factor in the bioaccumulation of Hg in aquatic biota has remained enigmatic. Here, we examined 26 tundra lakes in Canada's western Arctic that span a large gradient of DOC concentrations to show that total Hg (HgT) and methyl mercury (MeHg) accumulation by aquatic invertebrates is defined by a threshold response to Hg-DOC binding. Our results showed that DOC promotes HgT and MeHg bioaccumulation in tundra lakes having low DOC (<8.6 - 8.8 mg C L(-1); DOC threshold concentration, TC) whereas DOC inhibits HgT and MeHg bioaccumulation in lakes having high DOC (>DOC TC), consistent with bioaccumulation results in a companion paper (this issue) using a microbial bioreporter. Chemical equilibrium modeling showed that Hg bioaccumulation factors were elevated when Hg was associated mainly to fulvic acids, but became dramatically reduced when DOC was >8.5 mg C L(-1), at which point Hg was associated primarily with strong binding sites on larger, less bioaccessible humic acids. This study demonstrates that the biological uptake of Hg in lakes is determined by binding thresholds on DOC, a water quality variable predicted to change markedly with future environmental change.

We have developed a local mass balance model for the St. Lawrence River near Cornwall, Ontario that describes the fate and transport of mercury in three forms, elemental, divalent, and methylated, in a five compartment environment (air, water, sediments, periphyton, and benthos). Our objective was to construct a steady-state mass balance model to determine the dominant sources and sinks of mercury in this environment. We compiled mercury concentrations, fluxes, and transformation rates from previous studies completed in this section of the river to develop the model. The inflow of mercury was the major source to this system, accounting for 0.42 mol month(-1), or 95.5% of all mercury inputs, whereas outflow was 0.28 mol month(-1), or 63.6% of all losses, and sediment deposition was 0.12 mol month(-1), or 27.3% of all losses. Uncertainty estimates were greatest for advective fluxes in surface water, porewater, periphyton, and benthic invertebrates., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Spatial and temporal changes in mercury (Hg) concentrations and organic carbon in lake sediments were examined from the Hudson Bay Lowlands to investigate whether Hg deposition to sediments is related to indicators of autochthonous production. Total organic carbon, "S2" carbon (mainly algal-derived OC), C:N and ∂(13)C indicators suggest an increase in autochthonous productivity in recent decades. Up-core profiles of S2 concentrations and fluxes were significantly correlated with Hg suggesting that varying algal matter scavenging of Hg from the water column may play an important role in the temporal profiles of Hg throughout the sediment cores. Absence of significant relationship between total Hg and methyl Hg (MeHg) in surficial sediments suggested that inorganic Hg supply does not limit MeHg production. MeHg and OC were highly correlated across lakes in surface and deep sediment layers, indicating that sediment organic matter content explains part of the spatial variation in MeHg concentrations between lakes., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

We examined dated sediment cores from 14 thermokarst affected lakes in the Mackenzie Delta uplands, NT, Arctic Canada, using a case-control analysis to determine how retrogressive thaw slump development from degrading permafrost affected the delivery of mercury (Hg) and organic carbon (OC) to lakes. We show that sediments from the lakes with retrogressive thaw slump development on their shorelines (slump-affected lakes) had higher sedimentation rates and lower total Hg (THg), methyl mercury (MeHg), and lower organic carbon concentrations compared to lakes where thaw slumps were absent (reference lakes). There was no difference in focus-corrected Hg flux to sediments between reference lakes and slump-affected lakes, indicating that the lower sediment Hg concentration in slump-affected lakes was due to dilution by rapid inorganic sedimentation in the slump-affected lakes. Sedimentation rates were inversely correlated with THg concentrations in sediments among the 14 lakes considered, and explained 68% of the variance in THg concentration in surface sediment, further supporting the dilution hypothesis. We observed higher S2 (algal-derived carbon) and particulate organic carbon (POC) concentrations in sediment profiles from reference lakes than in slump lakes, likely because of dilution by inorganic siliciclastic matter in cores from slump-affected lakes. We conclude that retrogressive thaw slump development increases inorganic sedimentation in lakes, and decreases concentrations of organic carbon and associated Hg and MeHg in sediments.

The contamination of polar regions due to the global distribution of anthropogenic pollutants is of great concern because it leads to the bioaccumulation of toxic substances, methylmercury among them, in Arctic food chains. Here we present the first evidence that microbes in the high Arctic possess and express diverse merA genes, which specify the reduction of ionic mercury [Hg(II)] to the volatile elemental form [Hg(0)]. The sampled microbial biomass, collected from microbial mats in a coastal lagoon and from the surface of marine macroalgae, was comprised of bacteria that were most closely related to psychrophiles that had previously been described in polar environments. We used a kinetic redox model, taking into consideration photoredox reactions as well as mer-mediated reduction, to assess if the potential for Hg(II) reduction by Arctic microbes can affect the toxicity and environmental mobility of mercury in the high Arctic. Results suggested that mer-mediated Hg(II) reduction could account for most of the Hg(0) that is produced in high Arctic waters. At the surface, with only 5% metabolically active cells, up to 68% of the mercury pool was resolved by the model as biogenic Hg(0). At a greater depth, because of incident light attenuation, the significance of photoredox transformations declined and merA-mediated activity could account for up to 90% of Hg(0) production. These findings highlight the importance of microbial redox transformations in the biogeochemical cycling, and thus the toxicity and mobility, of mercury in polar regions.

The purpose of our study was to test the hypothesis that dissolved gaseous mercury (DGM) production and evasion is directly proportional to the loading rate of inorganic mercury [Hg(II)] to aquatic ecosystems. We simulated different rates of atmospheric mercury deposition in 10-m diameter mesocosms in a boreal lake by adding multiple additions of Hg(II) enriched with a stable mercury isotope ((202)Hg). We measured DGM concentrations in surface waters and estimated evasion rates using the thin-film gas exchange model and mass transfer coefficients derived from sulfur hexafluoride (SF(6)) additions. The additions of Hg(II) stimulated DGM production, indicating that newly added Hg(II) was highly reactive. Concentrations of DGM derived from the experimental Hg(II) additions ("spike DGM") were directly proportional to the rate of Hg(II) loading to the mesocosms. Spike DGM concentrations averaged 0.15, 0.48 and 0.94 ng l(-1) in mesocosms loaded at 7.1, 14.2, and 35.5 microg Hg m(-2) yr(-1), respectively. The evasion rates of spike DGM from these mesocosms averaged 4.2, 17.2, and 22.3 ng m(-2)h(-1), respectively. The percentage of Hg(II) added to the mesocosms that was lost to the atmosphere was substantial (33-59% over 8 weeks) and was unrelated to the rate of Hg(II) loading. We conclude that changes in atmospheric mercury deposition to aquatic ecosystems will not change the relative proportion of mercury recycled to the atmosphere.