Effects of mercury on growth, emergence, and behavior of Chironomus riparius Meigen (Diptera: Chironomidae)

Integrated transcriptomics and metabolomics reveal the toxic mechanisms of mercury exposure to an endangered species Tachypleus tridentatus

Mercury (Hg) pollution is threatening the health of endangered Tachypleus tridentatus whereas the toxic mechanism is still unclear. This study combined transcriptomic and metabolomics technology to reveal the toxic mechanisms of mercury (Hg 2+, 0.025 mg/L) exposing to T. tridentatus larvae for 15 days. Mercury induced cellular toxicity and cardiovascular dysfunction by dysregulating the genes related to endocrine system, such as polyubiquitin-A, cathepsin B, atrial natriuretic peptide, etc. Mercury induced lipid metabolic disorder with the abnormal increase of lysoPC, leukotriene D4, and prostaglandin E2. Cytochrome P450 pathway was activated to produce anti-inflammatory substances to reconstruct the homeostasis. Mercury also inhibited arginine generation, which may affect the development of T. tridentatus by disrupting the crucial signaling pathway. The mercury methylation caused enhancement of S-adenosylmethionine to meet the need of methyl donor. The mechanisms described in present study provide new insight into the risk assessment of mercury exposure to T. tridentatus.

Behavioral and biochemical patterns in the Andean highland macroinvertebrate Nectopsyche sp. after chronic mercury exposure

Environmental pollution by mercury (Hg) is of particular interest in Latin American countries like Ecuador, where artisanal and industrial gold mining has widely expanded. However, classic assessments, either based on water parameters or bioindicators, are not always effective in determining the effect of this type of pollutant on the ecological quality of the rivers. This study analyzed the behavioral and biochemical responses in benthic macroinvertebrate Nectopsyche sp. after sub-lethal Hg exposure emulating the maximum limits allowed by the current legislation in Ecuador (0.0002 mg L-1). Behavioral changes were significant, with an average decrease in the movement (measured as the number of strokes per 15 s of larvae) after 6 days of exposure to the maximum limit concentration of Hg. Furthermore, the antioxidant enzyme Glutathione S-Transferase activity significantly increased when exposed for 7 days to 0.0002 mg L-1 of Hg concentration. These results preliminarily proved that behavioral and biochemical patterns could function as biomarkers that efficiently identify ecological impairment caused by Hg, which would otherwise be neglected by a purely chemical-based or biomonitoring based on the presence/absence of macroinvertebrates type of assessment. Moreover, we demonstrate that the current limits allowed can impair aquatic life.

Magnetized vermiculite as a tool for the treatment of produced water generated by oil companies: Effects on aquatic organisms before and after treatment

Produced water (PW) generated by oil companies is a highly impacting waste that contains chemicals such as metals and organic and inorganic compounds. Given its polluting potential, PW requires effective treatment before being discharged into the environment. Conventional treatments have limited efficiency in removing PW toxicity, so alternative approaches must be developed and standardized. In this context, treatment with adsorbent materials like magnetized vermiculite (VMT-mag) is highlighted. This work aimed to evaluate the efficiency of treatment with VMT-mag in reducing PW toxicity to aquatic biota. For this purpose, three aquatic species (the midge Chironomus riparius, the planarian Girardia tigrina, and the crustacean Daphnia magna) were exposed to untreated PW and to PW treated with VMT-mag at laboratory conditions. The assessed endpoints included mortality, growth, emergence, and developmental time of C. riparius; mortality, locomotion, feeding, and head regeneration of G. tigrina; and intrinsic population growth rate (r) and reproductive output of D. magna. The results showed that all the species exposed to raw PW were impaired: C. riparius had delayed development, G. tigrina had reduced locomotor activity and delayed head regeneration, and D. magna had reduced reproduction and delayed intrinsic population growth rate (r). Most of the analyzed parameters showed that treatment with VMT-mag diminished PW toxicity. Therefore, using VMT-mag to treat PW may be the key to reducing the PW effects on aquatic organisms.

Subfossil Chironomid Assemblages as Indicators of Remedial Efficacy in the Historically Contaminated St. Lawrence River at Cornwall, Ontario

Long-term data are required to quantify the impacts of historic industrial pollution and subsequent remedial action on the nearshore benthic community in the St. Lawrence River Area of Concern at Cornwall, Ontario. Specifically, high-quality temporal records are needed to understand changes in benthic invertebrate assemblages in response to multiple possible drivers including industrial pollution, environmental heterogeneity, and climate warming. We compare long-term records of subfossil chironomid assemblages and geochemical variables among sediment cores from two Cornwall sites with differing pollution histories and a minimally disturbed downstream reference site. Chironomids were functionally absent from the Cornwall sediment cores when mercury and zinc concentrations were elevated. As metal concentrations decreased in more recent sediment intervals, chironomid abundance and the relative abundance of pollution-sensitive taxa increased. Recently deposited sediment in all three sediment cores display increased relative abundance of warm-water, macrophyte-associated taxa. We conclude that these temporal changes in chironomid assemblages provide evidence for ecological recovery for both of the impacted sites, consistent with the objectives of the current management strategy. These findings advance our understanding of industrial impacts on fluvial chironomid ecology, directly inform local management strategies, and further develop the application of chironomids as bioindicators for contaminated sediments.

Mercury entomotoxicology

Mercury is an industrial pollutant of global concern. Currently entomofauna is disappearing and chemical pollution is one cause, however, it is unknown whether mercury is an additional threat. Therefore, it is necessary to know the entomotoxicology of mercury. The aim of the present work was to perform a comprehensive literature review on the entomotoxicology of mercury. The toxicokinetics and toxicity of mercury in insects, the participation of insects in the mercury cycle and the fact that this element is a threat to entomofauna are characterized. Insects can be exposed to mercury through ingestion, tracheal respiration, and gill respiration. Organic forms of mercury are better absorbed, bioaccumulated and distributed than inorganic forms. In addition, insects can biotransform mercury, for example, by methylating it. Metal elimination occurs through feces, eggs and exuvia. Toxicity molecular mechanisms include oxidative stress, enzymatic disruptions, alterations in the metabolism of neurotransmitters and proteins, genotoxicity, cell death and unbalances in the energetic state. Moreover, mercury affects lipid, germ, and gut cells, causes deformations, disturbs development, reproduction, behavior, and locomotion, besides to alters insect populations and communities. In terrestrial ecosystems, entomofauna participate in the mercury cycle by bioaccumulating mercury from soil and air, predating, being predated and decomposing organic matter. In aquatic ecosystems insects participate by accumulating mercury from water and sediment, predating, being predated and transporting it to terrestrial ecosystems when they emerge as winged adults. There are still information gaps that need to be addressed.

Increased Mercury and Reduced Insect Diversity in Linked Stream-Riparian Food Webs Downstream of a Historical Mercury Mine

Historical mining left a legacy of abandoned mines and waste rock in remote headwaters of major river systems in the western United States. Understanding the influence of these legacy mines on culturally and ecologically important downstream ecosystems is not always straightforward because of elevated natural levels of mineralization in mining-impacted watersheds. To test the ecological effects of historic mining in the headwaters of the upper Salmon River watershed in Idaho (USA), we measured multiple community and chemical endpoints in downstream linked aquatic-terrestrial food webs. Mining inputs impacted downstream food webs through increased mercury accumulation and decreased insect biodiversity. Total mercury (THg) in seston, aquatic insect larvae, adult aquatic insects, riparian spiders, and fish at sites up to 7.6 km downstream of mining was found at much higher concentrations (1.3-11.3-fold) and was isotopically distinct compared with sites immediately upstream of mining inputs. Methylmercury concentrations in bull trout and riparian spiders were sufficiently high (732-918 and 347-1140 ng MeHg g^-1  dry wt, respectively) to affect humans, birds, and piscivorous fish. Furthermore, the alpha-diversity of benthic insects was locally depressed by 12%-20% within 4.3-5.7 km downstream from the mine. However, because total insect biomass was not affected by mine inputs, the mass of mercury in benthic insects at a site (i.e., ng Hg m^-2 ) was extremely elevated downstream (10-1778-fold) compared with directly upstream of mining inputs. Downstream adult aquatic insect-mediated fluxes of THg were also high (~16 ng THg m^-2  day^-1 ). Abandoned mines can have ecologically important effects on downstream communities, including reduced biodiversity and increased mercury flux to higher order consumers, including fish, birds, and humans. Environ Toxicol Chem 2022;41:1696-1710. Published 2022. This article is a U.S. Government work and is in the public domain in the USA.

Single and mixed exposure to cadmium and mercury in Drosophila melanogaster: Molecular responses and impact on post-embryonic development

Heavy metals, like many other chemical elements, are naturally present in the environment; however, the concentrations of these metals in various environmental matrices have increased through their intensive use in many human activities (such as industry, mining and agriculture). Among the heavy metals, cadmium (Cd) and mercury (Hg) induce a wide variety of defects in animals. While the effects of these heavy metals have been widely documented, a single exposure paradigm is typically used. Few studies have focused on evaluating combined exposure to these metals. However, in the environment, animals are confronted with a plethora of substances simultaneously; thus, the presence and origin of such substances must be determined to reduce the sources of contamination. Using the model of the fruit fly Drosophila melanogaster, for which many tools are readily available, we investigated how different concentrations of Cd and Hg in single and combined exposures impact post-embryonic development. In parallel, we evaluated the extended expression pattern of 38 molecular targets used as potential biomarkers of exposure through qPCR. Our results showed that both metals caused developmental delays and mortality in dose-dependent responses. Both metals were able to deregulate genes involved in hormonal control, general stress, and oxidative stress. Importantly, we confirmed synergistic interactions between Cd and Hg. Our results indicate the importance of assessing several biomarkers and their kinetics in mixtures. Drosophila represents a useful model for monitoring the toxicity of substances in polluted environments.

Effects of zinc and mercury on ROS-mediated oxidative stress-induced physiological impairments and antioxidant responses in the microalga Chlorella vulgaris

The rapid growth of industrialization and urbanization results in deterioration of freshwater systems around the world, rescinding the ecological balance. Among many factors that lead to adverse effects in aquatic ecology, metals are frequently discharged into aquatic ecosystems from natural and anthropogenic sources. Metals are highly persistent and toxic substances in trace amounts and can potentially induce severe oxidative stress in aquatic organisms. In this study, adverse effects of the two metal elements zinc (maximum concentration of 167.25 mg/L) and mercury (104.2 mg/L) were examined using Chlorella vulgaris under acute and chronic exposure period (48 h and 7 days, respectively). The metal-induced adverse effects have been analyzed through photosynthetic pigment content, total protein content, reactive oxygen species (ROS) generation, antioxidant enzymatic activities, namely catalase and superoxide dismutase (SOD) along with morphological changes in C. vulgaris. Photosynthetic pigments were gradually reduced (~32-100% reduction) in a dose-dependent manner. Protein content was initially increased during acute (~8-12%) and chronic (~57-80%) exposure and decreased (~44-56%) at higher concentration of the two metals (80%). Under the two metal exposures, 5- to 7-fold increase in ROS generation indicated the induction of oxidative stress and subsequent modulations in antioxidant activities. SOD activity was varied with an initial increase (58-129%) followed by a gradual reduction (~3.7-79%), while ~1- to 12-fold difference in CAT activity was observed in all experimental condition (~83 to 1605%). A significant difference was observed in combined toxic exposure (Zn+Hg), while comparing the toxic endpoint data of individual metal exposure (Zn and Hg alone). Through this work, lethal effects caused by single and combined toxicity of zinc and mercury were assessed, representing the significance of appropriate monitoring system to trim down the release of metal contaminants into the aquatic ecosystems.

Rearing methods and life cycle characteristics of Chironomus sp. Florida (Chironomidae: Diptera): A rapid-developing species for laboratory studies

The species Chironomus sp. “Florida” has several qualities that make it a potential aquatic laboratory model to be used in Puerto Rico. Its use as such, however, requires a rearing protocol and life cycle description not previously reported. The present study addresses this lack of information by first describing a rearing method obtained through three years of observations. Next we describe and discuss the life cycle and the effects of temperature and feeding on development. The species has a short life cycle (typically 11 days) and larval stages easily identified using body measurements. Temperature affects the duration of the life cycle, with warm temperatures producing faster development than cold temperatures. The effects of different food concentrations vary: in large water volumes, concentrations of 2 mg/larva/day produce faster developmental times, but at low water volumes, small food concentrations of 0.5 mg/larva/day produce faster developmental times. The rearing protocol and life cycle parameters presented in this study are intended to promote the use of this species as a laboratory model. The fast development of Chironomus sp. “Florida” makes it ideal for toxicological studies.

Mercury Bioavailability in Fluvial Sediments Estimated Using Chironomus riparius and Diffusive Gradients in Thin-Films (DGT)

Mercury bioavailability was assessed by exposing the dipteran Chironomus riparius for the whole life cycle to legacy-contaminated fluvial sediments (0.038–0.285 mg Hg kg−1 d.w.) and analyzing tissue concentrations in larvae at different exposure times (7, 11, and 16 days) and in adults. In the same experiment, diffusive gradients in thin-film passive samplers (DGTs), both piston- and probe-shaped, were co-deployed in the same sediments and retrieved at the same times as the organisms. To compare the two approaches, results showed a good agreement between accumulation kinetics of C. riparius and DGTs, both approximating an apparent steady-state. A strong correlation was found between values in tissues and in both types of DGTs (r between 0.74 and 0.99). Concentrations in mature larvae (19–140 µg kg−1 w.w.), which may represent a basal level of the aquatic food web, exceeded the European Environmental Quality Standard for biota (20 µg kg−1 w.w.), which aims at protecting the top predators from secondary poisoning. Body burdens in larvae and in adults were similar, showing negligible decontamination during metamorphosis and proving an efficient mercury transfer from sediments to terrestrial food webs.

Exposure to heavy metal-contaminated sediments disrupts gene expression, lipid profile, and life history traits in the midge Chironomus riparius

Despite the concern about anthropogenic heavy metal accumulation, there remain few multi-level ecotoxicological studies to evaluate their effects in fluvial ecosystems. The toxicity of field-collected sediments exhibiting a gradient of heavy metal contamination (Cd, Pb, and Zn) was assessed in Chironomus riparius. For this purpose, larvae were exposed throughout their entire life cycle to these sediments, and toxic effects were measured at different levels of biological organization, from the molecular (lipidomic analysis and transcriptional profile) to the whole organism response (respiration rate, shape markers, and emergence rate). Alterations in the activity of relevant genes, as well as an increase of storage lipids and decrease in membrane fluidity, were detected in larvae exposed to the most contaminated sediments. Moreover, reduced larval and adult mass, decrease of larval respiration rate, and delayed emergence were observed, along with increased mentum and mandible size in larvae and decreased wing loading in adults. This study points out the deleterious effects of heavy metal exposure at various levels of biological organization and provides some clues regarding the mode of toxic action. This integrative approach provides new insights into the multi-level effects on aquatic insects exposed to heavy metal mixtures in field sediments, providing useful tools for ecological risk assessment in freshwater ecosystems.

Effects of methylmercury on mosquito oviposition behavior: Maladaptive response to non-toxic exposure

Animals can modulate their own exposure to environmental contaminants through behavioral plasticity such as diet and habitat choice. However, it remains unclear if behavior also has cascading effects on contaminant exposure across multiple generations. In insects, oviposition site selection is an important behavior females can use to modify offspring contaminant exposure risk. In this study, we use the yellow fever mosquito, Aedes aegypti, to test how methylmercury (MeHg) affects oviposition site selection. We found that mosquito larval development rate and survival were negatively affected at MeHg concentrations ≥100 ppb. Adult females not exposed to MeHg as larvae avoided oviposition sites with high MeHg concentrations (>50 ppb), but MeHg exposure at the larval stage significantly affected this oviposition site selection. Specifically, females raised from larvae exposed to non-toxic MeHg levels (i.e., five-50 ppb) showed a significant increase in preference for oviposition sites contaminated with toxic MeHg concentrations (≥500 ppb), compared to unexposed controls. This maladaptive behavioral response could be because, when conditioned with non-toxic MeHg concentrations, MeHg-associated olfactory cues act as a "supernormal" stimulus during oviposition site selection. Importantly, however, this maladaptive behavioral response is eliminated in female mosquitoes raised from larvae exposed to toxic MeHg concentrations (i.e. 100 ppb), and these mosquitoes showed a significant increase in preference for MeHg uncontaminated oviposition sites, compared to unexposed controls. Thus, in mosquitoes, the magnitude of MeHg exposure in one generation can impact MeHg exposure in subsequent generations by altering oviposition site selection behavior. Our results have broad implications for our understanding of how contaminant-mediated behavioral modifications can feedback on contaminant exposure risk across multiple generations, and consequently how behavior can affect the evolutionary trajectory of organisms inhabiting a heterogeneously contaminated environment.

Assessment of potential mercury toxicity to native invertebrates in a high-gradient stream

This study evaluated potential effects of mercury (Hg) on benthic macroinvertebrates in the South River, Virginia, USA. The study used a multiple lines of evidence approach, including spatially and temporally matched sediment chemistry, biological, and toxicological information (Sediment Quality Triad), exposure and effect analysis in bulk and interstitial sediment and interstitial water, and critical body residue analysis. Ten-day Chironomus dilutus and Hyalella azteca toxicity tests established site-specific no-effect concentrations (NOEC) at 18.9 μg/g total Hg (THg) and 102 ng/g methylmercury (MeHg). However, the benthic community at these locations was impaired, with lower mayfly and caddisfly composition. Few locations had concentrations of THg and MeHg that exceeded the NOEC in bulk or interstitial sediment. The THg concentrations in interstitial water were far below concentrations expected to reduce survival in benthic invertebrates, and only a low percentage of samples exceeded sublethal (growth) low-effect concentrations (LOEC) for THg or MeHg. The THg concentrations in invertebrate tissue did not exceed the NOEC or LOEC in the South River, and MeHg concentrations exceeded the LOEC at all locations, including those with no evidence of benthic impairment, illustrating the uncertainty associated with this line of evidence. Finally, statistical modeling that evaluated diversity of sensitive invertebrate species as a function of Hg exposure, geomorphological parameters, and physicochemical variables indicated that physicochemical and geomorphological parameters were most predictive of benthic community; where Hg was indicated, we were unable to distinguish between models with or without interstitial water Hg. Overall, the lines of evidence indicate that Hg, while clearly toxic to invertebrates at sufficiently high exposure concentrations, is not negatively impacting invertebrate communities in the South River. This study illustrates the difficulty of assessing risks to invertebrates using traditional tools of risk assessment and identifies critical gaps in knowledge that complicate the management of Hg risk. Integr Environ Assess Manag 2019;00:000-000. © 2019 SETAC.

Next-Generation Complex Metal Oxide Nanomaterials Negatively Impact Growth and Development in the Benthic Invertebrate Chironomus riparius upon Settling

Most studies of nanomaterial environmental impacts have focused on relatively simple first-generation nanomaterials, including metals or metal oxides (e.g., Ag, ZnO) for which dissolution largely accounts for toxicity. Few studies have considered nanomaterials with more complex compositions, such as complex metal oxides, which represent an emerging class of next-generation nanomaterials used in commercial products at large scales. Importantly, many nanomaterials are not colloidally stable in aqueous environments and will aggregate and settle, yet most studies use pelagic rather than benthic-dwelling organisms. Here we show that exposure of the model benthic species Chironomus riparius to lithium cobalt oxide (Li _xCo_{1- x}O₂, LCO) and lithium nickel manganese cobalt oxide (Li _xNi _yMn _zCo_{1- y- z}O₂, NMC) at 10 and 100 mg·L^-1 caused 30-60% declines in larval growth and a delay of 7-25 d in adult emergence. A correlated 41-48% decline in larval hemoglobin concentration and related gene expression changes suggest a potential adverse outcome pathway. Metal ions released from nanoparticles do not cause equivalent impacts, indicating a nanospecific effect. Nanomaterials settled within 2 days and indicate higher cumulative exposures to sediment organisms than those in the water column, making this a potentially realistic environmental exposure. Differences in toxicity between NMC and LCO indicate compositional tuning may reduce material impact.

Determination of Hg in Farmed and Wild Atlantic Bluefin Tuna (Thunnus thynnus L.) Muscle

Mercury (Hg) is a well-known toxic element, diffused in the environment, especially in the Mediterranean Sea which is rich in cinnabar deposits. Mercury bioaccumulation in fish is of great concern, especially for top-level aquatic predators (e.g., shark, tuna, swordfish) and above all for species of large human consumption and high nutritional value. This work aimed to determine Hg concentrations in farmed and wild Atlantic Bluefin tuna (Thunnus thynnus) caught in the Mediterranean area in order to evaluate the level of Hg bioaccumulation. selenium (Se) content was also determined, since this element is an antagonist of mercury toxicity. Mercury and Se were analysed by atomic absorption spectrometry after microwave digestion of the samples. Hg content in farmed tuna was below the legal limit (1 mg/kg, wet weight, w.w.) for all specimens (0.6 ± 0.2 mg/kg), whereas the wild ones had a content over the limit (1.7 ± 0.6 mg/kg); Se concentration was higher in farmed specimens (1.1 ± 0.9 mg/kg) compared to wild ones (0.6 ± 0.3 mg/kg). A safe seafood could show a Se/Hg ratio >1 and a health benefit value (HBV_Se) > 0: farmed tuna had higher values than the wild specimens (Se/Hg 5.48 vs. 1.32; HBV_Se 11.16 vs. 0.29). These results demonstrate that for Hg, there is a better risk/benefit ratio in farmed T. thynnus. making it safer than wild tuna.

Amitraz toxicity to the midge Chironomus riparius: Life-history and biochemical responses

Acute and chronic toxicity of the formamidine pesticide amitraz to the midge Chironomus riparius was assessed using conventional ecotoxicological tests and biochemical approaches (biomarkers). Amitraz is mainly used as an ectoparasiticide in veterinary medicine, but also in agriculture and apiculture. However, information of amitraz toxicity to non-target invertebrates is limited. Besides the impairment of developmental and emergence rates (reduced larval growth, emergence, and delayed development time) caused by chronic exposure to amitraz, acute exposures induced alterations in the antioxidant enzymes glutathione peroxidase (GPx) and catalase (CAT), and in energetic metabolism biomarkers, lactate dehydrogenase (LDH) and electron transport system (ETS) activities. Moreover, lipid peroxidation (LPO) increased by amitraz exposure. Our results reveal potential secondary effects of amitraz to invertebrates and biomarkers that may aid in the interpretation of sub-lethal toxic responses to amitraz. These results add information concerning the potential outcomes of amitraz exposure to freshwater invertebrates underlining the importance of risk assessment studies of formamidine pesticides.

Effects of phenanthrene on different levels of biological organization in larvae of the sediment-dwelling invertebrate Chironomus sancticaroli (Diptera: Chironomidae)

The hydrocarbon phenanthrene is an organic compound commonly found in the environment. In aquatic ecosystems, it is highly toxic to organisms, although little is known about its effects on sediment-dwelling organisms. The purpose of this study was to evaluate phenanthrene effects on biochemical, histological, and ontogenetic levels in larvae of the sediment-dwelling invertebrate Chironomus sancticaroli at acute and chronic exposure. Lethal concentrations were estimated and toxicity (acute-96 h and chronic- 8 d) tests were performed at phenanthrene concentrations from 0.12 to 1.2 mg L^-1. At acute and chronic exposure, we evaluated acetylcholinesterase (AChE), alpha esterase (EST-α), and beta esterase (EST-β) activities as well as histological alterations. In the assays with chronic exposure, effects on larval development were estimated using antennae length (instar estimative) and body length (growth estimative). The EST-α showed a significantly increased activity after 48 h at acute exposure to high concentrations of phenanthrene, while EST-β activity was increased after 48 and 72 h at acute exposure at higher concentrations and at 0.12 mg L^-1 at chronic exposure. At acute exposure, the midgut showed alterations such as brush border disruption, gastric caeca regression, and lumen area reduction; the fat body showed nuclear alteration in the trophocytes, while the Malpighian tubules showed brush border reduction and the salivary glands were subject to cytoplasm vacuolation. At chronic exposure, the same alterations were observed, in addition to vacuolar coalescence in the trophocytes of the fat body. Regarding larval development, a reduction of body length was observed with increasing phenanthrene concentrations. Similarly, molting was delayed; in the control group, all larvae were in the fourth instar, while at higher phenanthrene concentrations, larvae were predominantly in the third instar. Phenanthrene had toxic effects on this chironomid, indicating risks for natural populations.

Trophic fate of inorganic and methyl-mercury in a macrophyte-chironomid food chain

Dietary transfer of mercury (Hg) is central for its effects on higher trophic animals, nonetheless, its driving parameters and characteristics are not well understood. Here we measured Hg species transfer (uptake) from the macrophyte Elodea nuttallii -mimicking tissues incorporation in sediments after decay- to Chironomus riparius. Methyl-Hg (MMHg) was more transferable than inorganic Hg (IHg) from plant's intracellular and cell wall compartments. After 10-d-long exposure, MMHg was predominantly found in MMHg form in the cytosolic compartment (S) of chironomids, while IHg showed similar concentrations in S and insoluble debris (P) compartments. After cessation of Hg species exposure (depuration), only MMHg resulted in a bioaccumulation factor >1. Toxicokinetics modelling indicated a demethylation of MMHg in the S fraction and its concomitant storage in the P fraction as IHg during both uptake and depuration, revealing an elimination and detoxification mechanism. Our data support that MMHg is more transferable than IHg to sensitive subcellular targets as well as bioavailable fraction in chironomids, in line with field studies showing higher MMHg transfer than IHg in food webs. Hence our data point out macrophytes as a potential Hg source to benthic food webs to be considered for enhancing aquatic environment protection during phytoremediation programs.

Biological responses of midge (Chironomus riparius) and lamprey (Lampetra fluviatilis) larvae in ecotoxicity assessment of PCDD/F-, PCB- and Hg-contaminated river sediments

We evaluated the utility of chironomid and lamprey larval responses in ecotoxicity assessment of polychlorinated dibenzo-p-dioxins, dibenzofurans (PCDD/F)-, polychlorinated biphenyls (PCB)- and mercury (Hg)-contaminated river sediments. Sediment samples were collected from the River Kymijoki with a known industrial pollution gradient. Sediment for the controls and lamprey larvae were obtained from an uncontaminated river nearby. Contamination levels were verified with sediment and tissue PCDD/F, PCB and Hg analyses. Behaviour of sediment-exposed chironomid and lamprey larvae were measured with Multispecies Freshwater Biomonitor© utilizing quadrupole impedance conversion technique. In addition, mortality, growth and head capsule deformity incidence of chironomids were used as ecotoxicity indicators. WHOPCDD/F+PCB-TEQ in the R. Kymijoki sediments ranged from the highest upstream 22.36 ng g(-1) dw to the lowest 1.50 ng g(-1) near the river mouth. The sum of PCDD/Fs and PCBs correlated strongly with Hg sediment concentrations, which ranged from <0.01 to 1.15 μg g(-1). Lamprey tissue concentrations of PCDD/Fs were two orders and PCBs one order of magnitude higher in the R. Kymijoki compared to the reference. Chironomid growth decreased in contaminated sediments and was negatively related to sediment ∑PCDD/Fs, WHOPCDD/F+PCB-TEQ and Hg. There were no significant differences in larval mortality or chironomid mentum deformity incidence between the sediment exposures. The distinct behavioural patterns of both species indicate overall applicability of behavioural MFB measurements of these species in sediment toxicity bioassays. Chironomids spent less and lampreys more time in locomotion in the most contaminated sediment compared to the reference, albeit statistically significant differences were not detected. Lamprey larvae had also a greater activity range in some of the contaminated sediments than in the reference. High pollutant levels in lamprey indicate risks for biomagnification in the food webs, with potential health risks to humans consuming fish.

Mercury tissue residue approach in Chironomus riparius: Involvement of toxicokinetics and comparison of subcellular fractionation methods

Along with the growing body of evidence that total internal concentration is not a good indicator of toxicity, the Critical Body Residue (CBR) approach recently evolved into the Tissue Residue Approach (TRA) which considers the biologically active portion of metal that is available to contribute to the toxicity at sites of toxic action. For that purpose, we examined total mercury (Hg) bioaccumulation and subcellular fractionation kinetics in fourth stage larvae of the midge Chironomus riparius during a four-day laboratory exposure to Hg-spiked sediments and water. The debris (including exoskeleton, gut contents and cellular debris), granule and organelle fractions accounted only for about 10% of the Hg taken up, whereas Hg concentrations in the entire cytosolic fraction rapidly increased to approach steady-state. Within this fraction, Hg compartmentalization to metallothionein-like proteins (MTLP) and heat-sensitive proteins (HSP), consisting mostly of enzymes, was assessed in a comparative manner by two methodologies based on heat-treatment and centrifugation (HT&C method) or size exclusion chromatography separation (SECS method). The low Hg recoveries obtained with the HT&C method prevented accurate analysis of the cytosolic Hg fractionation by this approach. According to the SECS methodology, the Hg-bound MTLP fraction increased linearly over the exposure duration and sequestered a third of the Hg flux entering the cytosol. In contrast, the HSP fraction progressively saturated leading to Hg excretion and physiological impairments. This work highlights several methodological and biological aspects to improve our understanding of Hg toxicological bioavailability in aquatic invertebrates.

Behavioural responses of freshwater planarians after short-term exposure to the insecticide chlorantraniliprole

Recent advances in video tracking technologies provide the tools for a sensitive and reproducible analysis of invertebrate activity under stressful conditions nurturing the field of behavioural ecotoxicology. This study aimed to evaluate behavioural responses of the freshwater planarian Dugesia subtentaculata exposed to a model compound, chlorantraniliprole (CAP). This compound is an anthranilic diamide insecticide and due to its neurotoxic action can, at low concentrations, impair behaviour of exposed organisms. Behavioural endpoints measured included feeding and locomotor activities. Feeding responses were based on planarian predatory behaviour using Chironomus riparius larvae as prey. Locomotion was measured by the traditional planarian locomotor velocity (pLMV) assay and additionally using an automated video tracking system using a Zebrabox(®) (Viewpoint, France) device. While feeding and pLMV were significantly impaired at 131.7μg/L CAP, the video tracking system showed that total distance covered by planarians was significantly reduced at concentrations as low as 26.2μg/L CAP. Our results show that more advanced automated video recording systems can be used in the development of sensitive bioassays allowing a reliable, time- and cost-effective quantification of behaviour in aquatic invertebrates. Due to their ecological relevance, behavioural responses should not be disregarded in risk assessment strategies and we advocate the suitability of planarians as suitable organisms for behavioural ecotoxicological studies.

Endocrine disruption and oxidative stress in larvae of Chironomus dilutus following short-term exposure to fresh or aged oil sands process-affected water

Understanding the toxicity of oil sands process-affected water (OSPW) is a significant issue associated with the production of oil from the Alberta oil sands. OSPW is acutely and chronically toxic to organisms, including larvae of Chironomus dilutus. In this study, fresh OSPW ('WIP-OSPW') was collected from the West In-Pit settling pond and aged OSPW ('FE5-OSPW') was collected from the FE5 experimental reclamation pond, both of which are located on the Syncrude Canada Ltd. lease site near Fort McMurray, Alberta, Canada. Larvae of C. dilutus were exposed to a freshwater control, WIP-OSPW, or FE5-OSPW for 4 or 7 days and survival, growth, and markers of oxidative stress and endocrine disruption were assessed. Survival was not significantly different among treatment groups. Compared to masses of larvae exposed to freshwater, masses of larvae exposed to WIP-OSPW were 49% lesser on day 4 and 62% lesser on day 7. However, organisms exposed to FE5-OSPW did not have significantly lesser masses than controls. Abundances of transcripts of glutathione-s-transferase (gst), catalase (cat), and glutathione peroxidase (gpx), which are important for the response to oxidative stress, were significantly altered in larvae exposed to WIP-OSPW, but not FE5-OSPW, relative to controls. Peroxidation of lipids was greater in larvae exposed to WIP-OSPW, but not FE5-OSPW. Exposure to fresh OSPW might have caused endocrine disruption because abundances of transcripts of the steroid hormone receptors, ultraspiricle protein (usp), ecysteroid receptor (esr), and estrogen related receptor (err) were greater in larvae exposed to WIP-OSPW for 7 days, but not FE5-OSPW. These results suggest that lesser growth of larvae of C. dilutus exposed to fresh OSPW might be due to oxidative stress and disruption of endocrine processes, and that aging of OSPW attenuates these adverse effects.

Mercury toxicity to freshwater organisms: extrapolation using species sensitivity distribution

Mercury toxicity to aquatic organisms was evaluated in different taxonomic groups showing the following species sensitivity gradient: Daphnia magna > Daphnia longispina > Pseudokirchneriella subcapitata > Chlorella vulgaris > Lemna minor > Chironomus riparius. Toxicity values ranged from 3.49 μg/L (48 h-EC₅₀ of D. magna) to 1.58 mg/L (48 h-EC₅₀ of C. riparius). A species sensitivity distribution was used to estimate hazardous mercury concentration at 5 % level (HC5) and the predicted no effect concentration (PNEC). The HC5 was 3.18 μg Hg/L and the PNEC varied between 0.636 and 3.18 μg Hg/L, suggesting no risk of acute toxicity to algae, plants, crustaceans and insects in most freshwaters.

Bioaccumulation and elimination of waterborne mercury in the midge larvae, Chironomus riparius Meigen (Diptera: Chironomidae)

Here, mercury kinetics and behavioural effects in the midge larvae under a water-only exposure were assessed. Uptake and elimination of waterborne mercury were described by using a one-compartment kinetic model. Results show that midges were able to readily accumulate the heavy metal (BCF = 450), presenting a fast uptake, up to 13.1 μg Hg g of animal(-1) at the end of the exposure period. Elimination was slow, with c.a. 39 % of the mercury in larvae being depurated after 48 h in clean medium. Behaviour did not present differences upon exposure or elimination, but a trend to increase ventilation was noticed during the exposure period.

Effects of exposure to oil sands process-affected water from experimental reclamation ponds on Chironomus dilutus

Effective detoxification of oil sands process-affected water (OSPW) is one issue associated with bitumen extraction in the Alberta oil sands. It has been suggested that reclamation ponds can be used to passively treat OSPW, potentially allowing for its safe return to the environment. In this study, OSPW was sampled in two batches (A and B) from the Syncrude Canada Ltd. West In-Pit (WIP) settling pond and from three experimental reclamation ponds - Big Pit, FE5, and TPW. Acute (10 d) and chronic (until adult emergence) exposures of Chironomus dilutus larvae to OSPW were conducted and survival, growth, development, and behavior were assessed. Masses of larvae exposed to WIP-OSPW were 64-77% less than the freshwater control (p < 0.001). Similarly, chronic exposure to WIP-OSPW resulted in significantly (p < 0.05) less pupation than in the freshwater control, with 31% (A) and 71% (B) less pupation of larvae exposed to WIP-OSPW. Rates of emergence were significantly less for larvae exposed to WIP-OSPW, with only 13% (A) and 8% (B) of larvae emerging as adults when exposed to WIP-OSPW, compared to 81% in the freshwater control (p < 0.0001). Pupation and emergence rates were significantly less in TPW than freshwater control (p < 0.05), but there were no differences observed in Big Pit or FE5. Lesser toxicity was observed in reclaimed OSPW compared to fresh OSPW and this coincided with lesser concentrations of NAs. The results presented are consistent with the hypothesis that an organic fraction is the cause of the toxicity of OSPW toward C. dilutus and that OSPW aged in reclamation ponds retains toxicity and therefore, more aggressive, targeted treatment of OSPW is required to accelerate decreases.

Effectiveness of ozonation treatment in eliminating toxicity of oil sands process-affected water to Chironomus dilutus

Water soluble organic compounds (OCs), including naphthenic acids (NAs), are potentially toxic constituents of oil sands process-affected water (OSPW) that is generated during extraction of bitumen from Alberta oil sands. Ozonation can decrease concentrations of OCs in OSPW. However, effects of ozonated-OSPW on multicellular organisms are unknown. A 10-day and a chronic exposure of Chironomus dilutus to OSPW were conducted to assess effects on survival, growth, development, and behavior. Two separate batches of OSPW were treated with 30 or 80 mg ozone (O(3))/L. Wet body masses of larvae exposed to OSPW were 64 to 77% less than their respective controls (p < 0.001). However, both levels of ozonation significantly attenuated effects of OSPW on growth. Similarly, chronic exposure to untreated OSPW resulted in significantly less pupation than in the controls, with 31% and 71% less pupation of larvae exposed to the two batches of OSPW (p < 0.05). Emergence was significantly less for larvae exposed to OSPW, with 13% and 8% of larvae emerging, compared to 81% in controls (p < 0.0001). Both levels of ozonation of OSPW attenuated effects on emergence. These results suggest that OCs degraded by ozonation causes toxicity of OSPW toward C. dilutus, and that ozonation attenuates toxicity of OSPW.

Effects of imidacloprid exposure on Chironomus riparius Meigen larvae: linking acetylcholinesterase activity to behaviour

Imidacloprid (IMI) is an insecticide that interferes with the transmission of stimuli in the nervous system of insects. It is neurotoxic by mimicking nicotine through its binding to the nicotinic acetylcholine receptor. In this work, experiments comprising 96 h exposure followed by 48 h in clean medium were conducted to evaluate the toxicity of IMI to Chironomus riparius and its potential recovery. Behavioural parameters and AChE activity were assessed. After 96 h exposure to IMI, AChE activity, and the behaviour parameters ventilation and locomotion were reduced. There were no signs of recovery after removal to clean water for 48 h. Ventilation behaviour was the most sensitive parameter and the one with the highest correlation to AChE activity. Despite the possibility that IMI might be having an indirect effect on AChE activity, the behavioural endpoint showed a higher sensitivity than the biochemical response itself. This work highlights the importance of linking parameters with ecological relevance at individual level (behavioural parameters) with biochemical responses, to unravel xenobiotics mode of action.