Investigation of spatial trends and neurochemical impacts of mercury in herring gulls across the Laurentian Great Lakes

Legacy contaminant trends in the Great Lakes uncovered by the wildlife environmental quality index

Since the 1970s, wildlife managers have prioritized the recovery of Great Lakes ecosystems from contamination by Persistent Organic Pollutants (POPs). Monitoring and quantifying the region's recovery is challenged by the diversity of legacy contaminants in the environment and the lack of benchmarks for their potential biological effects. We address this gap by introducing the Wildlife Environmental Quality Index (WEQI) based on prior water and sediment quality indices. The tool summarizes, in a single score, the exposure of wildlife to harmful levels of multiple contaminants - with harmful levels set by published guidelines for protecting piscivorous wildlife from biological impacts. We applied the new index to a combined Canadian and American dataset of Herring Gull (Larus argentatus) egg data to elucidate trends in wildlife for eight legacy industrial pollutants and insecticides in the Great Lakes. Environmental quality of the Great Lakes region (as indexed by WEQI) improved by 18% between 2002 and 2017. Improvement came from reductions in both the scope of contamination (the number of guideline-exceeding contaminants) and its amplitude (the average size of guideline exceedances) at bird colonies. But recovery was unequal among lakes, with Lake Erie showing no improvement at one extreme. Weakly- or non-recovering lakes (Erie, Ontario, Huron) were marked by inconsistent improvement in scope and amplitude, likely due to ongoing loading, sediment resuspension and other stressors reported elsewhere. Fast-recovering lakes (Superior and Michigan), meanwhile, improved in both scope and amplitude. Contrasting trends and contaminant profiles (e.g., exceedances of PCBs versus DDTs) highlight the importance of lake-specific management for equalizing recoveries. Lower environmental quality at American than Canadian colonies, particularly in Lake Huron, further suggest uneven success in - and opportunities for - the binational management of wildlife exposure to legacy contaminants.

The effect of environmental pollution on gene expression of seabirds: A review

One of the biggest challenges for ecotoxicologists is to detect harmful effects of contaminants on individual organisms before they have caused significant harm to natural populations. One possible approach for discovering sub-lethal, negative health effects of pollutants is to study gene expression, to identify metabolic pathways and physiological processes affected by contaminants. Seabirds are essential components of ecosystems but highly threatened by environmental changes. Being at the top of the food chain and exhibiting a slow pace of life, they are highly exposed to contaminants and to their ultimate impacts on populations. Here we provide an overview of the currently available seabird-related gene expression studies in the context of environmental pollution. We show that studies conducted, so far, mainly focus on a small selection of xenobiotic metabolism genes, often using lethal sampling protocols, while the greater promise of gene expression studies for wild species may lie in non-invasive procedures focusing on a wider range of physiological processes. However, as whole genome approaches might still be too expensive for large-scale assessments, we also bring out the most promising candidate biomarker genes for future studies. Based on the biased geographical representativeness of the current literature, we suggest expanding studies to temperate and tropical latitudes and urban environments. Also, as links with fitness traits are very rare in the current literature, but would be highly relevant for regulatory purposes, we point to an urgent need for establishing long-term monitoring programs in seabirds that would link pollutant exposure and gene expression to fitness traits.

Mercury in wetlands over 60 years: Research progress and emerging trends

Wetlands are considered the hotspots for mercury (Hg) biogeochemistry, garnering global attention. Therefore, it is important to review the research progress in this field and predict future frontiers. To achieve that, we conducted a literature analysis by collecting 15,813 publications about Hg in wetlands from the Web of Science Core Collection. The focus of wetland Hg research has changed dramatically over time: 1) In the initial stage (i.e., 1959-1990), research mainly focused on investigating the sources and contents of Hg in wetland environments and fish. 2) For the next 20 years (i.e., 1991-2010), Hg transformation (e.g., Hg reduction and methylation) and environmental factors that affect Hg bioaccumulation have attracted extensive attention. 3) In the recent years of 2011-2022, hot topics in Hg study include microbial Hg methylators, Hg bioavailability, methylmercury (MeHg) demethylation, Hg stable isotope, and Hg cycling in paddy fields. Finally, we put forward future research priorities, i.e., 1) clarifying the primary factors controlling MeHg production, 2) uncovering the MeHg demethylation process, 3) elucidating MeHg bioaccumulation process to better predict its risk, and 4) recognizing the role of wetlands in Hg circulation. This research shows a comprehensive knowledge map for wetland Hg research and suggests avenues for future studies.

Methylmercury effects on avian brains

Methylmercury (MeHg) is a concerning contaminant due to its ubiquity and harmful effects on organisms. Although birds are important models in the neurobiology of vocal learning and adult neuroplasticity, the neurotoxic effects of MeHg are less understood in birds than mammals. We surveyed the literature on MeHg effects on biochemical changes in the avian brain. Publication rates of papers related to neurology and/or birds and/or MeHg increased with time and can be linked with historical events, regulations, and increased understanding of MeHg cycling in the environment. However, publications on MeHg effects on the avian brain remain relatively low across time. The neural effects measured to evaluate MeHg neurotoxicity in birds changed with time and researcher interest. The measures most consistently affected by MeHg exposure in birds were markers of oxidative stress. NMDA, acetylcholinesterase, and Purkinje cells also seem sensitive to some extent. MeHg exposure has the potential to affect most neurotransmitter systems but more studies are needed for validation in birds. We also review the main mechanisms of MeHg-induced neurotoxicity in mammals and compare it to what is known in birds. The literature on MeHg effects on the avian brain is limited, preventing full construction of an adverse outcome pathway. We identify research gaps for taxonomic groups such as songbirds, and age- and life-stage groups such as immature fledgling stage and adult non-reproductive life stage. In addition, results are often inconsistent between experimental and field studies. We conclude that future neurotoxicological studies of MeHg impacts on birds need to better connect the numerous aspects of exposure from molecular physiological effects to behavioural outcomes that would be ecologically or biologically relevant for birds, especially under challenging conditions.

Mercury concentrations in blood, brain and muscle tissues of coastal and pelagic birds from northeastern Canada

Mercury (Hg) is a toxic element which has increased in marine environments for more than a century, due largely to anthropogenic activities, and biomagnifies in food chains to harmful levels in some top predators like waterfowl and seabirds. We analysed total mercury (THg) concentrations in blood, brain and muscle tissue from healthy specimens of 13 coastal and pelagic bird species from eastern and northern Canada to provide a baseline on current concentrations, especially for brain concentrations which are highly underrepresented in the literature. We also examined within and among tissues relationships of THg concentrations within individuals. THg concentrations were generally higher in pelagic species and scavenging gulls, when compared to coastal waterfowl. Brain and muscle tissue had similar concentrations of THg in the birds examined, but both of these tissues had lower concentrations that those found in blood. Our results, and that of a previous study, suggest that body condition has a large influence on blood THg concentrations and should be considered when using blood as a sampling medium. Many of the species we examined had tissue THg above levels known to cause deleterious, sublethal effects in some species.

Impacts of Sublethal Mercury Exposure on Birds: A Detailed Review

Mercury is a ubiquitous environmental contaminant known to accumulate in, and negatively affect, fish-eating and oceanic bird species, and recently demonstrated to impact some terrestrial songbirds to a comparable extent. It can bioaccumulate to concentrations of >1 μg/g in tissues of prey organisms such as fish and insects. At high enough concentrations, exposure to mercury is lethal to birds. However, environmental exposures are usually far below the lethal concentrations established by dosing studies.The objective of this review is to better understand the effects of sublethal exposure to mercury in birds. We restricted our survey of the literature to studies with at least some exposures >5 μg/g. The majority of sublethal effects were subtle and some studies of similar endpoints reached different conclusions. Strong support exists in the literature for the conclusion that mercury exposure reduces reproductive output, compromises immune function, and causes avoidance of high-energy behaviors. For some endpoints, notably certain measures of reproductive success, endocrine and neurological function, and body condition, there is weak or contradictory evidence of adverse effects and further study is required. There was no evidence that environmentally relevant mercury exposure affects longevity, but several of the sublethal effects identified likely do result in fitness reductions that could adversely impact populations. Overall, 72% of field studies and 91% of laboratory studies found evidence of deleterious effects of mercury on some endpoint, and thus we can conclude that mercury is harmful to birds, and the many effects on reproduction indicate that bird population declines may already be resulting from environmental mercury pollution.

Investigating endocrine and physiological parameters of captive American kestrels exposed by diet to selected organophosphate flame retardants

Organophosphate triesters are high production volume additive flame retardants (OPFRs) and plasticizers. Shown to accumulate in abiotic and biotic environmental compartments, little is known about the risks they pose. Captive adult male American kestrels (Falco sparverius) were fed the same dose (22 ng OPFR/g kestrel/d) daily (21 d) of tris(2-butoxyethyl) phosphate (TBOEP), tris(2-chloroethyl) phosphate (TCEP), tris(2-chloroisopropyl) phosphate (TCIPP), or tris(1,2-dichloro-2-propyl) phosphate (TDCIPP). Concentrations were undetected in tissues (renal, hepatic), suggesting rapid metabolism. There were no changes in glutathione status, indicators of hepatic oxidative status, or the cholinergic system (i.e., cerebrum, plasma cholinesterases; cerebrum muscarinic, nicotinic receptors). Modest changes occurred in hepatocyte integrity and function (clinical chemistry). Significant effects on plasma free triiodothyronine (FT3) concentrations occurred with exposure to TBOEP, TCEP, TCIPP, and TDCIPP; TBOEP and TCEP had additional overall effects on free thyroxine (FT4), whereas TDCIPP also influenced total thyroxine (TT4). Relative increases (32%-96%) in circulating FT3, TT3, FT4, and/or TT4 were variable with each OPFR at 7 d exposure, but limited thereafter, which was likely maintained through decreased thyroid gland activity and increased hepatic deiodinase activity. The observed physiological and endocrine effects occurred at environmentally relevant concentrations and suggest parent OPFRs or metabolites may have been present despite rapid degradation.

Applications and implications of neurochemical biomarkers in environmental toxicology

Thousands of environmental contaminants have neurotoxic properties, but their ecological risk is poorly characterized. Contaminant-associated disruptions to animal behavior and reproduction, both of which are regulated by the nervous system, provide decision makers with compelling evidence of harm, but such apical endpoints are of limited predictive or harm-preventative value. Neurochemical biomarkers, which may be used to indicate subtle changes at the subcellular level, may help overcome these limitations. Neurochemical biomarkers have been used for decades in the human health sciences and are now gaining increased attention in the environmental realm. In the present review, the applications and implications of neurochemical biomarkers to the field of ecotoxicology are discussed. The review provides a brief introduction to neurochemistry, covers neurochemical-based adverse outcome pathways, discusses pertinent strengths and limitations of neurochemical biomarkers, and provides selected examples across invertebrate and vertebrate taxa (worms, bivalves, fish, terrestrial and marine mammals, and birds) to document contaminant-associated neurochemical disruption. With continued research and development, neurochemical biomarkers may increase understanding of the mechanisms that underlie injury to ecological organisms, complement other measures of neurological health, and be integrated into risk assessment practices.

Methylmercury egg injections: part 2--pathology, neurochemistry, and behavior in the avian embryo and hatchling

Methylmercury (MeHg) is a toxic metal that has been frequently linked to neurochemical alterations, brain lesions, neurobehavioral changes, and reproductive impairments in wild and captive birds. Much less is known about the effects of MeHg on the developing avian brain and resulting effects on hatchling behavior. The objective of this work was to use air cell injection studies to investigate the effect of in ovo MeHg exposure on brain pathology and four neurochemical biomarkers (N-methyl-d-aspartate (NMDA) receptor, γ-aminobutyric acid (GABA) receptor, glutamine synthetase (GS) and glutamic acid decarboxylase (GAD)) that have previously been studied in wild birds, and on hatchling righting response, balance, and startle response. In a series of six studies, we exposed white leghorn chicken and Japanese quail embryos to methylmercury chloride (MeHgCl) (range: 0-6.4μg/g egg) via egg injection on embryonic day (ED) 0 and measured receptor levels and enzyme activity at different stages of embryonic (days 11, 14, and 19 in chicken; day 15 in quail) and hatchling (day 1 and day 7) development, and in whole brain or discrete brain regions (cerebrum, cerebellum, optic lobe). We assessed neurobehaviors on post hatch (PH) days 1 and 7. Despite accumulating relatively high levels of Hg in the brain, embryos and hatchlings did not consistently display neurochemical changes consistent with those seen in wild birds and laboratory mammals. Hatchlings also did not demonstrate behavioral alterations. Pathology did not indicate a difference in occurrence and types of lesions between control and dosed birds. These findings suggest that in ovo MeHg exposure alone may not be responsible for neurological impacts in bird. This work draws attention to factors, such as age and species, that may influence responses to MeHg in birds.

Toxicity of methylmercury injected into eggs of thick-billed murres and arctic terns

Mercury (Hg) has been increasing in some marine birds in the Canadian Arctic over the past several decades. To evaluate the potential reproductive impact of Hg exposure, eggs of two species of arctic-breeding seabirds, the thick-billed murre and arctic tern, were dosed with graded concentrations of methylmercury (MeHg) and artificially incubated in the laboratory to determine species differences in sensitivity. Based on the dose-response curves, the median lethal concentrations (LC(50)) for thick-billed murre and arctic tern embryos were 0.48 and 0.95 μg g(-1) Hg on a wet-weight (ww) basis, respectively. Compared with published LC(50) values for other avian species, the murres and terns had a medium sensitivity to MeHg exposure. LC(50) values were also calculated for the actual Hg concentration measured in the embryos, that is, the maternally-deposited Hg plus the injected MeHg dose. This increased the LC(50) values to 0.56 μg g(-1) Hg ww in the thick-billed murre and to 1.10 μg g(-1) Hg ww in the arctic tern. Although muscarinic acetylcholine and N-methyl-D-aspartic acid glutamate receptor levels have been correlated with increasing Hg concentrations in brains of adult birds, no significant associations were found in brain tissue of the murre or tern embryos. The incidence of gross external anatomical deformities was 4.3 % in the murre embryos and 3.6 % in the tern embryos. However, given that the eggs were taken from wild populations, it is unlikely that the deformities observed in this study were due to MeHg exposure alone.

Postmortem stability of brain GABAergic and glutamatergic receptors and enzymes under ecological conditions

Neurochemical biomarkers have emerged as useful tools for assessing the subclinical neurological impacts of environmental toxicants in birds and other wildlife. Careful consideration of biomarker stability is necessary before implementing their use on tissues from ecological studies, as receptors and enzymes in the brain may be affected by postmortem conditions. The goal of this study was to evaluate the postmortem stability of key GABAergic and glutamatergic receptors (N-methyl-D-aspartate (NMDA), gamma-aminobutyric acid (GABAA-benzodiazepine)) and enzymes (glutamine synthetase (GS), glutamic acid decarboxylase (GAD)) under environmentally relevant field and storage conditions to determine their suitability as biomarkers. We exposed chicken embryo brains to postmortem environmental and storage conditions typical for ecological studies (12, 24, and 48 h at 7 °C or 25 °C; 1, 4, and 8 weeks at -80 °C or -20 °C; 1 or 2 freeze thaw cycles), and measured [3H] MK-801 binding to the NMDA receptor, [3H] flunitrazipam binding to the GABAA-benzodiazepine receptor, GS activity, and GAD activity. We found that [3H] MK-801 binding is stable under all conditions studied. GAD activity was fairly stable under each storage and environmental temperatures for all durations, but was significantly less stable when stored at -20 °C than at -80 °C. [3H] flunitrazipam binding and GS activity were both impacted by environmental and storage temperature and duration, and might best be utilized in studies of samples with similar histories. Our findings here demonstrate that caution is warranted when comparing samples with different collection and storage histories, but that some biomarkers are fairly stable under various conditions.

Mercury, selenium and neurochemical biomarkers in different brain regions of migrating common loons from Lake Erie, Canada

Common loons (Gavia immer) can be exposed to relatively high levels of dietary methylmercury (MeHg) through fish consumption, and several studies have documented MeHg-associated health effects in this species. To further study the neurological risks of MeHg accumulation, migrating loons dying of Type E botulism were collected opportunistically from the Lake Erie shore at Long Point (Ontario, Canada) and relationships between total mercury (THg), selenium (Se), and selected neurochemical receptors and brain enzymes were investigated. THg concentrations were 1-78 μg/g in liver; and 0.3-4 μg/g in the brain (all concentrations reported on a dry weight basis). A significant (p < 0.05) positive correlation was found between THg in liver and THg in 3 subregions of the brain (cerebral cortex: r = 0.433; cerebellum: r = 0.293; brain stem: r = 0.405). THg varied significantly among different brain regions, with the cortex having the highest concentrations. Se levels in the cortex and cerebellum were 1-29 and 1-10 μg/g, respectively, with no significant differences between regions. Se was not measured in brain stem due to insufficient tissue mass. There were molar excesses of Se over mercury (Hg) in both cortex and cerebellum at all Hg concentrations, and a significant positive relationship between THg and the Hg:Se molar ratio (cortex: r = 0.63; cerebellum: r = 0.47). No significant associations were observed between brain THg and the N-methyl-D-aspartic acid (NMDA) receptor concentration, nor between THg and muscarinic cholinergic (mACh) receptor concentration; however, brain THg levels were lower than in previous studies that reported significant Hg-associated changes in neuroreceptor densities. Together with previous studies, the current findings add to our understanding of Hg distribution in the brain of common loons, and the associations between Hg and sub-lethal neurochemical changes in fish-eating wildlife.

Retrospective analysis of mercury content in feathers of birds collected from the state of Michigan (1895-2007)

Museum specimens were used to analyze temporal trends in feather mercury (Hg) concentrations in birds collected from the state of Michigan between the years 1895 and 2007. Hg was measured in flank and secondary feathers from three species of birds that breed in the Great Lakes region; common terns (n = 32), great blue herons (n = 35), and herring gulls (n = 35). More than 90% of the Hg in feathers should be organic, but some of the heron and gull feathers collected prior to 1936 showed evidence of contamination with inorganic Hg, likely from museum preservatives. The data presented here therefore consist of organic Hg in pre-1936 samples and total Hg in post-1936 samples. Insufficient tissue was available from terns to assess organic Hg content. Mean Hg concentrations ranged from 2.9 ± 2.5 μg/g Hg in tern flank feathers to 12.4 ± 10.6 μg/g Hg in gull flank feathers. No linear trend of Hg contamination over time was detected in herons and gulls. Though a significant decrease was noted for terns, these data are presented with caution given the strong likelihood that earlier samples were preserved with inorganic mercury. When data were separated into 30-year intervals, Hg content in heron and gull feathers collected from birds sampled between 1920 and 1949 were consistently highest but not to a level of statistical significance. For example, Hg concentrations in gull secondary feathers collected in the second time interval (1920-1949) were 11.5 ± 7.8. This value was 67% higher than the first time interval (1890-1919), 44% higher than the third interval (1950-1979), and 187% higher than the fourth interval (1980-2009). Studies on Great Lakes sediments also showed greatest Hg accumulations in the mid-twentieth century. Through the use of museum specimens, these results present a unique snapshot of Hg concentrations in Great Lakes biota in the early part of the twentieth century.