Subcellular metal partitioning in larvae of the insect Chaoborus collected along an environmental metal exposure gradient (Cd, Cu, Ni and Zn)

Response of Propsilocerus akamusi (Diptera: Chironomidae) to the leachates from AMD-contaminated sediments: Implications for metal bioremediation of AMD-polluted areas

Acid mine water (AMD) is a global environmental problem caused by coal mining with the characteristics of low pH and high concentrations of metals and sulfates. It is a pertinent topic to seek both economical and environmentally friendly approaches to minimize the harmful effects of AMD on the environment. Insect larvae are considered a promising solution for pollution treatment. Chironomidae is the most tolerant family to contaminants in pools and its larvae have a strong capacity for metal accumulation from sediment. This paper aimed to evaluate the larvae of Propsilocerus akamusi, a dominant species in the chironomid community, as a new species for entomoremediation in AMD-polluted areas. We detected the toxic effects of AMD on P. akamusi larvae based on their survival and the trace metals bioaccumulation capabilities of P. akamusi larvae. Moreover, we analyzed the expression patterns of four stress-response genes, HSP70, Eno1, HbV, and Hb VII in P. akamusi larvae. Our results revealed that AMD exposure did not significantly affect the survival of the P. akamusi larvae and individuals exposed to some AMD gradients even exhibited higher survival. We also observed the significantly accumulated concentrations of Fe, Ni, and Zn as well as higher bioaccumulation factors (BAFs) for Ni and Zn in the P. akamusi larvae exposure to AMD. Induced expression of Eno1 and Hb VII may play important roles in the AMD tolerance of P. akamusi larvae. This study indicated the potential application of P. akamusi larvae in the metal bioremediation of AMD-polluted areas. STATEMENT OF ENVIRONMENTAL IMPLICATION: Acid mine drainage (AMD) is a global environmental problem related to coal mining activities. AMD pollution has become a long-term, worldwide issue for its interactive and complex stress factors. Bioremediation is an effective method to remove the metals of AMD from wastewater to prevent downstream pollution. However, the disadvantages of the slow growth rate, susceptibility to seasonal changes, difficult post-harvest management, and small biomass of hyperaccumulating plants greatly limit the usefulness of phytoremediation. Insect larvae may be useful candidate organisms to overcome these shortcomings and have been considered a promising pollution solution. Propsilocerus akamusi is a dominant species in the chironomid community and is distributed widely in many lakes of eastern Asia. This species has extraordinary abilities to resist various stresses. This research is the first time to our knowledge to evaluate the application of P. akamusi as a new species in entomoremediation in AMD-contaminated areas.

Distribution of metals in the queen scallop Aequipecten opercularis during a transplant experiment: Metal rich granules as drivers of Pb bioaccumulation

The queen scallop Aequipecten opercularis accumulates high concentrations of lead (Pb) in its tissues, what has led to the interruption of this fishery in some extraction areas in Galicia (NW Spain). This study follows the dynamics of bioaccumulation of Pb and other metals in this species, the tissue distribution and the subcellular partitioning in selected organs, in order to understand the mechanisms that provoke the high Pb levels reached in its tissues and to increase our knowledge about metal bioaccumulation dynamics in this species. Scallops originating from a clean area were exposed in cages in two places in the Ría de Vigo (one shipyard and a less impacted location) and 10 individuals were collected every month over a three months period. Metal bioaccumulation and metal distribution in several organs, including gills, digestive gland, kidneys, muscle, gonad and remaining tissues, was studied. The results showed that scallops accumulated similar levels of Cd, Pb and Zn at both sites, while Cu and Ni showed an opposite pattern at the shipyard, with Cu concentrations increasing around 10 times and Ni decreasing during the 3 months of exposure. The preferential organs for metal accumulation were the kidneys for Pb and Zn, the digestive gland for Cd, both organs for Cu and Ni, and the muscle for As. Subcellular partitioning of kidney samples additionally showed an extraordinary ability to accumulate Pb and Zn at very high concentrations in kidney granules, a fraction that accounted for 30 to 60 % of Pb in soft-tissues. It is concluded that Pb bioaccumulation in kidney granules is the mechanism responsible for the high levels of Pb observed in this species.

Role of Toxicokinetic and Toxicodynamic Parameters in Explaining the Sensitivity of Zebrafish Larvae to Four Metals

Given the persistence and toxic potencies of metal contaminants in ecosystems, animals, and human beings, they are considered to be hazardous global pollutants. While the lethality of metal toxicities (e.g., LC₅₀) can significantly vary, even within the same species, the underlying mechanisms are less well-understood. In this study, we developed a subcellular two-compartment toxicokinetic-toxicodynamic (TK-TD) model for zebrafish larvae when exposed to four metals (cadmium, lead, copper, and zinc) to reveal whether differences in metal toxicity (LC₅₀ values) were dominated by the TK or TD processes. Results showed that the subcellular TK and TD parameters of the four metals were significantly different, and the bioconcentration factor (BCF) value of copper was higher than those of the other metals. We also found that the TD parameter internal threshold concentration (C_IT) was significantly positively correlated to the LC₅₀ values (R² = 0.7), suggesting a dominant role of TD processes in metal toxicity. Furthermore, the combined parameter C_IT/BCF for a metal-sensitive fraction (BCF_MSF), which linked exposure to effects through the TK-TD approach, explained up to 89% of the variation in toxicity to the four metals. The present study suggests that the observed variation in toxicity of these four metals was mainly determined by TD processes but that TK processes should not be ignored, especially for copper.

Breaking down insect stoichiometry into chitin-based and internal elemental traits: Patterns and correlates of continent-wide intraspecific variation in the largest European saproxylic beetle

Stoichiometric, trophic and ecotoxicological data have traditionally been acquired from patterns of variation in elemental traits of whole invertebrate bodies, whereas the critical issue of the extracellular origin of some portion of elements, such as those present in ingested food and internal organs, has been ignored. Here we investigated an unexplored, yet crucial, question relating to whether, and to what degree, metals from two major body fractions: exoskeleton (elytra) and internal (body organs with gut material present in abdomens), are correlated with each other in wild populations of the largest European saproxylic insect, the Stag Beetle Lucanus cervus, and how metals from these two fractions vary with insect size and local habitat conditions. We examined the continent-wide variation in the concentrations of 12 chemical elements (Ca, Mg, K, Na, Mn, Fe, Zn, Cu, As, Cd, Pb and Ni) measured in the elytra and abdomen of specimens from 28 populations inhabiting an urban-woodland habitat gradient across the species' entire distributional range from Spain to Russia. Across populations, elemental concentrations (except Ni and Pb) were 2-13 times higher in abdominal samples than in elytra, and the magnitude of these differences was related to both insect size and local habitat conditions. Smaller individuals from both woodland and urban habitat tended to have higher concentrations of trace elements (Zn, As, Cd, Pb and Ni). The concentration of only six elements (Mg, K, Na, Mn, Cd and Ni) was correlated in the elytra and abdomen at the individual and population levels, implying a limitation to the broader applicability of elytra as a surrogate for internal elemental pools. We highlight that in non-feeding adult saproxylic beetles, minerals, acquired during the larval stage, may be concentrated in the large quantities of residual body fat.

Toxicity and Subcellular Fractionation of Yttrium in Three Freshwater Organisms: Daphnia magna, Chironomus riparius, and Oncorhynchus mykiss

The demand for rare earth elements (REEs) has increased since the 1990s leading to the development of many mining projects worldwide. However, less is known about how organisms can handle these metals in natural aquatic systems. Through laboratory experiments, we assessed the chronic toxicity and subcellular fractionation of yttrium (Y), one of the four most abundant REEs, in three freshwater organisms commonly used in aquatic toxicology: Daphnia magna, Chironomus riparius, and Oncorhynchus mykiss. In bioassays using growth as an end point, C. riparius was the only organism showing toxicity at Y exposure concentrations close to environmental ones. The lowest observable effect concentrations (LOECs) of Y assessed for D. magna and O. mykiss were at least 100 times higher than the Y concentration in natural freshwater. A negative correlation between Y toxicity and water hardness was observed for D. magna. When exposed to their respective estimated LOECs, D. magna bioaccumulated 15-45 times more Y than the other two organisms exposed to their own LOECs. This former species sequestered up to 75% of Y in the NaOH-resistant fraction, a putative metal-detoxified subcellular fraction. To a lesser extent, C. riparius bioaccumulated 20-30% of Y in this detoxified fraction. In contrast, the Y subcellular distribution in O. mykiss liver did not highlight any notable detoxification strategy; Y was accumulated primarily in mitochondria (ca. 32%), a putative metal-sensitive fraction. This fraction was also the main sensitive fraction where Y accumulated in C. riparius and D. magna. Hence, the interaction of Y with mitochondria could explain its toxicity. In conclusion, there is a wide range of subcellular handling strategies for Y, with D. magna accumulating high quantities but sequestering most of it in detoxified fractions, whereas O. mykiss tending to accumulate less Y but in highly sensitive fractions.

The fish or the egg: Maternal transfer and subcellular partitioning of mercury and selenium in Yellow Perch (Perca flavescens)

Mercury (Hg) is a trace element of particular concern since it is ubiquitous in the environment and because its methylated form (MeHg) readily bioaccumulates and biomagnifies in food webs. This latter process leads to elevated Hg concentrations in fish and may thus induce toxicity. Maternal transfer of bioaccumulated contaminants to offspring is a suggested mechanism of impaired reproductive success in fish. The purpose of this study was to assess the toxicity potential of Hg during maternal transfer in Yellow Perch from Lake Saint-Pierre (Quebec, Canada) using a subcellular partitioning approach. We also evaluated potential protective effects of selenium, as this element has been shown to alleviate Hg toxicity through sequestration. A customized subcellular partitioning protocol was used to separate liver and gonad of Yellow Perch into various subcellular fractions. Results show that, in the liver, MeHg was primarily (51%) associated to the subcellular fraction containing cytosolic enzymes. Furthermore, 23% and 15% of MeHg was found in hepatic and gonadal mitochondria, respectively, suggesting that Yellow Perch is not effectively detoxifying this metal. There was also a strong relationship (R² = 0.73) between MeHg bioaccumulation in the liver and MeHg concentrations in gonadal mitochondria, which corroborates the potential risk linked to MeHg maternal transfer. On the other hand, we also found that selenium might have a protective effect on Hg toxicity at a subcellular level. In fact, Se:Hg molar ratios in subcellular fractions were systematically above 1 in all tissues and fractions examined, which corresponds to the suggested protective threshold. This study provides the first assessment of subcellular Se:Hg molar ratios in fish. Since early developmental stages in aquatic biota are particularly sensitive to Hg, this study represents a step forward in understanding the likelihood for toxic effects in wild fish through maternal transfer.

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.

Subcellular distributions of trace elements (Cd, Pb, As, Hg, Se) in the livers of Alaskan yelloweye rockfish (Sebastes ruberrimus)

Yelloweye rockfish (Sebastes ruberrimus) is an extremely long-lived species (up to ∼120 years) of fish, which inhabits the coastal waters of Alaska. Due to their long lifespans, yelloweye are known to accumulate high levels of mercury, and potentially other trace elements, in their tissues. Relatively little is known about the subcellular distribution of trace elements in the tissues of yelloweye rockfish; such information can provide important insights into detoxification/toxicity mechanisms at the subcellular level. To address this, we collected yelloweye rockfish (n = 8) from the eastern coast of Prince of Wales Island, Alaska in 2014. We determined the subcellular partitioning of trace elements (cadmium (Cd), lead (Pb), arsenic (As), total mercury (Hg), and selenium (Se)) in yelloweye livers with a partitioning procedure designed to separate liver cells into putative metal-sensitive fractions (cytosolic enzymes, organelles) and detoxified metal fractions (metallothionein or metallothionein-like proteins and peptides, granule-like structures) using differential centrifugation, NaOH digestion, and heat denaturation steps. The resulting fractions were then analyzed for total Hg with a direct Hg analyzer and for trace element concentrations by inductively coupled plasma-mass spectrometry (ICP-MS). For Cd, Pb, and As, the greatest contributions were found in the detoxified fractions, whereas the majority of total Hg was found in sensitive fractions. Selenium, an essential trace element, was distributed to a similar degree between the sensitive and detoxified compartments. Results indicate that although yelloweye sequestered and immobilized potentially toxic elements in detoxified fractions, the extent of binding differed among elements and followed the order: Cd > As > Pb > Hg. In yelloweye rockfish livers, the accumulation of non-essential elements at sensitive sites could lead to deleterious effects at the subcellular level, which should be evaluated in future studies.

In vivo isotopic fractionation of zinc and biodynamic modeling yield insights into detoxification mechanisms in the mayfly Neocloeon triangulifer

Diversity and biomass of aquatic insects decline in metal-rich aquatic environments, but the mechanisms by which insects from such environments cope with potentially toxic metal concentrations to survive through adulthood are less well understood. In this study, we measured Zn concentrations and isotopes in laboratory-reared diatoms and mayflies (Neocloeon triangulifer) from larval through adult stages. The larvae were fed Zn-enriched diatoms, and bio-concentrated Zn by a factor of 2.5-5 relative to the diatoms but maintained the same Zn-isotopic ratio. These results reflect the importance of dietary uptake and the greater rate of uptake relative to excretion or growth. Upon metamorphosis to subimago, Zn concentrations declined by >70%, but isotopically heavy Zn remained in the subimago bodies. We surmised that the loss of isotopically light Zn during metamorphosis was due to the loss of detoxified Zn and retention of metabolically useful Zn. Through the transition from subimago to imago, Zn concentrations and isotope ratios were virtually unchanged. Because the decrease in Zn body concentration and increase in heavier Zn are seen in the subimagos relative to the larvae, the compartmentalization of Zn must be occurring within the larvae. A biodynamic model was constructed, allowing for isotopic fractionation and partitioning of Zn between metabolically essential and detoxified Zn reservoirs within larvae. The model provides a consistent set of rate and fractionation constants that successfully describe the experimental observations. Specifically, metabolically essential Zn is isotopically heavier and is tightly held once assimilated, and excess, isotopically light Zn is sequestered, detoxified, and ultimately lost during the metamorphosis of larvae to subimagos. To our knowledge, this is the first documentation of in vivo isotopic fractionation in insects, offering an improved understanding of the mechanisms and rates by which the N. triangulifer larvae regulate excess Zn in their bodies.

Evaluating Benthic Recovery Decades after a Major Oil Spill in the Laurentian Great Lakes

The long-term effects of oil spills on freshwater organisms have been little studied. In 1950, a large oil spill (10 million L) covered the harbor area of Parry Sound, Ontario, the deepest port in the Laurentian Great Lakes. Ecological impacts were not studied at the time, but 25 years later three-quarters of the Chironomus cucini larvae (Insecta, Diptera, Chironomidae) living in the harbor area were reported to be deformed. We returned six decades after the spill and found that the frequency of deformities had returned to background levels and that the community of burrowing invertebrates has largely recovered. By dating sediment cores and measuring the depth distribution of oils, we conclude that, although the oil persists six decades after the spill, sufficient uncontaminated sediment has covered the oil thereby putting it out of reach of most burrowing animals. Provided that the sediment remains undisturbed, the buried oil is unlikely to exert further negative effects on the biota in spite of the fact that it will likely persist for centuries.

Evaluation on subcellular partitioning and biodynamics of pulse copper toxicity in tilapia reveals impacts of a major environmental disturbance

Fluctuation exposure of trace metal copper (Cu) is ubiquitous in aquatic environments. The purpose of this study was to investigate the impacts of chronically pulsed exposure on biodynamics and subcellular partitioning of Cu in freshwater tilapia (Oreochromis mossambicus). Long-term 28-day pulsed Cu exposure experiments were performed to explore subcellular partitioning and toxicokinetics/toxicodynamics of Cu in tilapia. Subcellular partitioning linking with a metal influx scheme was used to estimate detoxification and elimination rates. A biotic ligand model-based damage assessment model was used to take into account environmental effects and biological mechanisms of Cu toxicity. We demonstrated that the probability causing 50% of susceptibility risk in response to pulse Cu exposure in generic Taiwan aquaculture ponds was ~33% of Cu in adverse physiologically associated, metabolically active pool, implicating no significant susceptibility risk for tilapia. We suggest that our integrated ecotoxicological models linking chronic exposure measurements with subcellular partitioning can facilitate a risk assessment framework that provides a predictive tool for preventive susceptibility reduction strategies for freshwater fish exposed to pulse metal stressors.

Bioaccumulation and subcellular partitioning of Cr(III) and Cr(VI) in the freshwater green alga Chlamydomonas reinhardtii

Chromium occurs in aquatic environments under two main redox forms, namely Cr(III) and Cr(VI), with different geochemical and biochemical properties. Cr(VI) readily crosses biological membranes of living organisms and once inside the cells it undergoes a rapid reduction to Cr(III). The route of entry for the latter form is, however, poorly known. Using the radioactive tracer ⁵¹Cr we compared the accumulation (absorption and adsorption) of the two Cr forms by the green unicellular alga Chlamydomonas reinhardii after 1h and 72h of exposure to 100nM of either Cr(III) or Cr(VI) at pH 7. Both Cr forms had similar accumulation, with a major part in the extracellular (adsorbed) fraction after 1h and a major part of total accumulated Cr in the intracellular (absorbed) fraction after 72h. We also investigated the intracellular partitioning of Cr using an operational fractionation scheme and found that both Cr forms had similar distributions among fractions: Cr was mostly associated with organelles (23±12% after 1h and 37±7% after 72h) and cytosolic heat-stable proteins and peptides (39±18% after 1h and 35±3% after 72h) fractions. Further investigations using a metallomic approach (SEC-ICP-MS) were performed with the heat-stable proteins and peptides fraction to compare the distribution of the two Cr forms among various biomolecules of this fraction. One Cr-binding biomolecule (∼28kDa) appeared after 1h of exposure for both Cr species. After 72h another biomolecule of lower molecular weight (∼0.7kDa) was involved in binding Cr and higher signal intensities were observed for Cr(VI) than for Cr(III). We show, for the first time, that both Cr(III) and Cr(VI) have similar fate within algal cells, supporting the tenet that a unique redox form occurs within cells.

Subcellular partitioning of cadmium and zinc in mealworm beetle (Tenebrio molitor) larvae exposed to metal-contaminated flour

By studying the internal compartmentalization of metals in different subcellular fractions we are able to better understand the mechanisms of metal accumulation in organisms and the transfer of metals through trophic chains. We investigated the internal compartmentalization of cadmium (Cd) and zinc (Zn) in mealworm beetle (Tenebrio molitor) larvae by breeding them in flour contaminated with either Cd at 100, 300 and 600mgkg(-1), or Zn at 1000 and 2000mgkg(-1). We separated the cellular components of the larvae into 3 fractions: the S1 or cytosolic fraction containing organelles, heat-sensitive and heat-stable proteins, the S2 or cellular debris fraction and the G or metal-rich granule fraction. The concentration of Cd and Zn in each fraction was measured at 0, 7, 14 and 21 days of being fed the flour. The concentration of Cd in the flour affected the concentration of Cd measured in each larval subcellular fraction (p≤0.0001), while the concentration of Zn in the flour only affected the Zn concentration in the S2 and G fractions (p≤0.02). Both Cd and Zn concentrations in mealworms remained relatively constant during the exposure (days 7, 14 and 21) in all three fractions, but the Cd concentrations were much higher than those found in larvae before the exposure (day 0). The concentration of Cd in the flour, however, did not affect the percentage of Cd in the S1 fraction. The contribution of Cd in the G fraction to the total Cd amount was similar (30-40%) in all Cd treatments. The percentage of Zn in all three fractions was not affected by the concentration of Zn in the flour and the relative contributions of each subcellular fraction to the total burden of Zn remained generally constant for both control and treated larvae. In general, larvae sequestered approximately 30% of Cd and Zn in the S1 fraction, which is important for the transport of metals to higher trophic levels in a food web.

Subcellular partitioning profiles and metallothionein levels in indigenous clams Moerella iridescens from a metal-impacted coastal bay

In this study, the effect of environmental metal exposure on the accumulation and subcellular distribution of metals in the digestive gland of clams with special emphasis on metallothioneins (MTs) was investigated. Specimens of indigenous Moerella iridescens were collected from different natural habitats in Maluan Bay (China), characterized by varying levels of metal contamination. The digestive glands were excised, homogenized and six subcellular fractions were separated by differential centrifugation procedures and analyzed for their Cu, Zn, Cd and Pb contents. MTs were quantified independently by spectrophotometric measurements of thiols. Site-specific differences were observed in total metal concentrations in the tissues, correlating well with variable environmental metal concentrations and reflecting the gradient trends in metal contamination. Concentrations of the non-essential Cd and Pb were more responsive to environmental exposure gradients than were tissue concentrations of the essential metals, Cu and Zn. Subcellular partitioning profiles for Cu, Zn and Cd were relatively similar, with the heat-stable protein (HSP) fraction as the dominant metal-binding compartment, whereas for Pb this fraction was much less important. The variations in proportions and concentrations of metals in this fraction along with the metal bioaccumulation gradients suggested that the induced MTs play an important role in metal homeostasis and detoxification for M. iridescens in the metal-contaminated bay. Nevertheless, progressive accumulation of non-essential metals (Cd, and especially Pb) resulting from "spillover" was observed in putative metal- sensitive (e.g., mitochondria and heat-denaturable protein (HDP)) or lysosome/microsome fractions, demonstrating that metal detoxification was incomplete and increased the toxicological risk to M. iridescens inhabiting the metal-impacted environments. Through multiple stepwise regression analysis, the induction of MTs was statistically correlated with the HSP concentrations of Cu, and to a lesser extent with Zn, and ultimately to the Cd concentrations, exhibiting significant dose-dependent relationships. Overall, these findings not only revealed the fates of accumulated metals, but scientifically favored an improved understanding of the detoxification at the subcellular level in response to metal accumulation, supporting the focus of metabolic availability assessment on the intracellular processes or events occurring within organisms.

Subcellular distribution of trace elements and liver histology of landlocked Arctic char (Salvelinus alpinus) sampled along a mercury contamination gradient

We sampled landlocked Arctic char (Salvelinus alpinus) from four lakes (Small, 9-Mile, North, Amituk) in the Canadian High Arctic that span a gradient of mercury contamination. Metals (Hg, Se, Tl, and Fe) were measured in char tissues to determine their relationships with health indices (relative condition factor and hepatosomatic index), stable nitrogen isotope ratios, and liver histology. A subcellular partitioning procedure was employed to determine how metals were distributed between potentially sensitive and detoxified compartments of Arctic char livers from a low- and high-mercury lake (Small Lake and Amituk Lake, respectively). Differences in health indices and metal concentrations among char populations were likely related to differences in feeding ecology. Concentrations of Hg, Se, and Tl were highest in the livers of Amituk char, whereas concentrations of Fe were highest in Small and 9-Mile char. At the subcellular level we found that although Amituk char had higher concentrations of Tl in whole liver than Small Lake char, they maintained a greater proportion of this metal in detoxified fractions, suggesting an attempt at detoxification. Mercury was found mainly in potentially sensitive fractions of both Small and Amituk Lake char, indicating that Arctic char are not effectively detoxifying this metal. Histological changes in char livers, mainly in the form of melano-macrophage aggregates and hepatic fibrosis, could be linked to the concentrations and subcellular distributions of essential or non-essential metals.

Metal (Ag, Cd, Cu, Ni, Tl, and Zn) Binding to Cytosolic Biomolecules in Field-Collected Larvae of the Insect Chaoborus

We characterized the biomolecules involved in handling cytosolic metals in larvae of the phantom midge (Chaoborus) collected from five mining-impacted lakes by determining the distribution of Ag, Cd, Cu, Ni, Tl, and Zn among pools of various molecular weights (HMW: high molecular weight, >670-40 kDa; MMW: medium molecular weight, 40-<1.3 kDa; LMW: low molecular weight, <1.3 kDa). Appreciable concentrations of nonessential metals were found in the potentially metal-sensitive HMW (Ag and Ni) and LMW (Tl) pools, whereas the MMW pool, which includes metallothioneins (MTs) and metallothionein-like proteins and peptides (MTLPs), appears to be involved in Ag and Cd detoxification. Higher-resolution fractionation of the heat-stable protein (HSP) fraction revealed further differences in the partitioning of nonessential metals (i.e., Ag = Cd ≠ Ni ≠ Tl). These results provide unprecedented details about the metal-handling strategies employed by a metal-tolerant, freshwater animal in a field situation.

Analyzing the effectiveness of using branchial NKA activity as a biomarker for assessing waterborne copper toxicity in tilapia (Oreochromis mossambicus): A damage-based modeling approach

Branchial Na(＋)-K(＋)-ATPase (NKA) activity has been suggested as a promising biomarker for assessing metal stress in aquatic organisms. However, studies that systematically show the effectiveness of using NKA activity to detect metal exposure and toxicity at the individual level are limited. In this study, we aimed to determine whether branchial NKA activity mechanistically responds to the accumulation of waterborne copper (Cu) and accounts for observed toxicity over time under environmentally-relevant and aquafarming Cu exposure levels (0.2, 1 and 2 mg L(-1)). Temporal trends in Cu accumulation and the corresponding responses of branchial NKA activity resulting from Cu exposure were investigated in laboratory experiments conducted on juvenile tilapia (Oreochromis mossambicus), a freshwater teleost that shows potential as a bioindicator of real-time and historical metal pollution. We used the process-based damage assessment model (DAM) to inspect the time course of Cu toxicity by integrating the compensation process between Cu-induced inhibition and repair of branchial NKA activity. NKA activity acted as a sensitive biomarker for Cu exposure and accumulation in tilapia, which showed induced impairment of osmoregulation and lethality when they were exposed to environmentally relevant levels (0.2 mg L(-1)), but not to higher exposure levels (1 and 2 mg L(-1)) in aquaculture farms or contaminated aquatic ecosystems. This study highlights the benefits and limitations of using branchial NKA activity as a sensitive biomarker to assess the health status of a fish population and its ecosystem.

Metal accumulation and toxicity: the critical accumulated concentration of metabolically available zinc in an oyster model

Invertebrates typically carry out detoxification of accumulated metals. There is, therefore, no threshold total body concentration of accumulated metal initiating toxicity, the onset of toxic effects rather being related to a critical concentration of metabolically available (MA) accumulated metal. The challenge remains as to whether any particular combination of subcellular fractions of accumulated metal can be identified to represent this theoretical MA component. One candidate combined fraction is the so-termed metal sensitive fraction (MSF), consisting of metal bound to organelles and non-detoxificatory soluble proteins. In this study, we used laboratory zinc accumulation and toxicity data for four populations of the oyster Crassostrea hongkongensis with different histories of zinc exposure in the field to address the challenge. We conclude that in a 'control' population of the oyster, the MSF does approximate to the theoretical metabolically available zinc concentration. In populations with a history of field exposure to raised zinc bioavailabilities, however, the MSF would include more zinc detoxified in the lysosome component of organelle-bound metal, and the MSF in such populations would deviate more from the theoretical MA metal concentration.

Subcellular partitioning of non-essential trace metals (Ag, As, Cd, Ni, Pb, and Tl) in livers of American (Anguilla rostrata) and European (Anguilla anguilla) yellow eels

We determined the intracellular compartmentalization of the trace metals Ag, As, Cd, Ni, Pb, and Tl in the livers of yellow eels collected from the Saint Lawrence River system in Canada (Anguilla rostrata) and in the area of the Gironde estuary in France (Anguilla anguilla). Differential centrifugation, NaOH digestion and thermal shock were used to separate eel livers into putative "sensitive" fractions (heat-denatured proteins, mitochondria and microsomes+lysosomes) and detoxified metal fractions (heat-stable peptides/proteins and granules). The cytosolic heat-stable fraction (HSP) was consistently involved in the detoxification of all trace metals. In addition, granule-like structures played a complementary role in the detoxification of Ni, Pb, and Tl in both eel species. However, these detoxification mechanisms were not completely effective because increasing trace metal concentrations in whole livers were accompanied by significant increases in the concentrations of most trace metals in "sensitive" subcellular fractions, that is, mitochondria, heat-denatured cytosolic proteins and microsomes+lysosomes. Among these "sensitive" fractions, mitochondria were the major binding sites for As, Cd, Pb, and Tl. This accumulation of non-essential metals in "sensitive" fractions likely represents a health risk for eels inhabiting the Saint Lawrence and Gironde environments.

Subcellular distribution of Cd and Zn and MT mRNA expression in the hepatopancreas of Sinopotamon henanense after single and co-exposure to Cd and Zn

The freshwater crab Sinopotamon henanense was exposed to Cd and Zn either on their own or in combination. At 14 and 28 days the hepatopancreas was taken and total metal and metallothionein MT) in transcript abundance were assessed. In addition, the subcellular contents of Cd and Zn also were examined following a differential centrifugation. The results showed that concentration of Cd was more responsive to waterborne metal exposures than the concentration of Zn; Zn was better regulated and exhibited only 2-3 fold increases relative to the control. Of the subcellular fractions, the heat stable protein (HSP) fraction was the predominant metal-binding compartment for Cd. The proportion and accumulation of Cd in this fraction increased with the single Cd exposures, which suggest that metallothionein-like proteins play a key role in metal detoxification in the hepatopancreas of S. henanense. Despite the increases in Cd in the HSP fraction during single Cd exposures, some accumulation of Cd was observed in metal sensitive fraction (MSF), which showed that metal detoxification was incomplete. The results demonstrated that the Cd content decreased in metal mixture groups especially when mixed with higher Zn, while the Cd accumulation in MSF was also reduced when Zn was added. MT mRNA expression was also determined in our report, the results showed that both Cd and Zn had the ability of inducing MT mRNA expression. Additionally, the MT mRNA transcription level was enhanced when Cd was mixed with Zn.

Uptake and subcellular distributions of cadmium and selenium in transplanted aquatic insect larvae

We transplanted larvae of the phantom midge Chaoborus punctipennis from a lake having lower concentrations of Cd and Se (Lake Dasserat) to a more contaminated lake (Lake Dufault) located near a metal smelter in Rouyn-Noranda, Quebec. Transplanted individuals were held in mesh mesocosms for up to 16 days where they were fed with indigenous contaminated zooplankton. Larval Cd and Se burdens increased over time, and came to equal those measured in indigenous C. punctipennis from contaminated Lake Dufault. Larval Se burdens increased steadily, whereas those of Cd showed an initial lag phase that we explain by a change in the efficiency with which this insect assimilated Cd from its prey. We measured Cd and Se in subcellular fractions and found that larvae sequestered the majority (60%) of the incoming Cd in a detoxified fraction containing metal-binding proteins, whereas a minority of this nonessential metal was in sensitive fractions (20%). In contrast, a much higher proportion of the essential element Se (40%) was apportioned to metabolically active sensitive fractions. Larvae took up equimolar quantities of these elements over the course of the experiment. Likewise, Cd and Se concentrations in wild larvae were equimolar, which suggests that they are exposed to equimolar bioavailable concentrations of these elements in our study lakes.

Bioavailability and toxicity of zinc from contaminated freshwater sediments: linking exposure-dose-response relationships of the freshwater bivalve Hyridella australis to zinc-spiked sediments

To evaluate the use of the freshwater bivalve Hyridella australis as a potential biomonitor for zinc contamination in freshwater sediments, the bioavailability and toxicity of zinc contaminated sediments (low 44 ± 5, medium 526 ± 41, high 961 ± 38 μg/g dry mass) were investigated in laboratory microcosms for 28 days by examining H. australis exposure-dose-response relationships. Zinc concentrations in sediments and surface waters were measured as zinc exposure. Zinc in whole organism soft body tissues and five individual tissues were measured as organism zinc dose. Sub-cellular localisation of zinc in hepatopancreas tissues was investigated to further understand the zinc handling strategies and tolerance of H. australis. Total antioxidant capacity, lipid peroxidation and lysosomal membrane stability were measured in hepatopancreas tissues as zinc induced biomarker responses. Accumulated zinc concentrations in whole body tissues of H. australis reflected the zinc exposure and exhibited exposure dependent zinc accumulation at day 28. Gills accumulated significantly higher zinc concentrations than other tissues, however, no significant differences in zinc accumulation between treatments were detected for any of the individual tissues analysed. Analysis of individual tissue zinc concentrations, therefore, may not offer any advantages for monitoring bioavailable zinc in freshwater environments with this organism. Relationships between tissue zinc and calcium concentration suggest accumulation of zinc by H. australis may have occurred as an analogue of calcium which is a major constituent in shell and granules of unionid bivalves. A high percentage of accumulated zinc in the hepatopancreas tissues was detoxified and stored in metallothionein like proteins and metal rich granules. Of the zinc accumulated in the biologically active metal pool, 59-70% was stored in the lysosome+microsome fraction. At the concentrations tested, increasing zinc exposure resulted in decreasing total antioxidant capacity and measurable increases in the sublethal effects, lipid peroxidation and lysosomal membrane destabilisation, were observed. Based on exposure-dose analysis, H. australis partially regulates zinc uptake and weakly exhibits bioavailability of zinc in freshwater environments, however, exposure-response analysis shows zinc induced toxicological effects, suggesting the potential of this organism as a biomonitor for zinc in heavily contaminated freshwater environments.

Accumulation and detoxification of cadmium by larvae of Prodenia litura (Lepidoptera: Noctuidae) feeding on Cd-enriched amaranth leaves

Cadmium is a potentially toxic and carcinogenic nonessential heavy metal. This study investigated Cd accumulation along the soil-plant (Amaranthus hypochondriacus L.)-insect (Prodenia litura) food chain and the detoxification strategies at different trophic levels. A. hypochondriacus leaves could accumulate high levels of Cd from polluted soil. The Cd concentration in P. litura larvae increased with increasing Cd concentrations in the food plant. Transfer coefficients of Cd were high from soil to leaf and from larvae to feces. The leaves of A. hypochondriacus had the highest value of Cd accumulation in pectates and protein-integrated forms (extracted by 1M NaCl). Among all the subcellular fractions in larvae of P. litura, the heat-stable protein fraction was the dominant metal-binding compartment for Cd. The Cd subcellular level played an important role in Cd sequestration and excretion by P. liura larva feeding on Cd contaminated amaranth leaves. This is the first attempt to account for subcellular distribution associated with Cd in P. litura when interpreting Cd detoxification and transfer along insect food chain.

Facilitated bioaccumulation of cadmium and copper in the oyster Crassostrea hongkongensis solely exposed to zinc

Exposure to one metal might have significant effects on the bioaccumulation of other metals. In the present study, we examined the possible effects of Zn exposure on the bioaccumulation of Cd and Cu in three populations of the oyster Crassostrea hongkongensis. We found that Zn exposure significantly enhanced the tissue concentrations of Cd and Cu in all populations, and the tissue concentrations of Cd and Cu were highly and positively related to the tissue Zn concentration. Furthermore, the enhanced bioaccumulation of Cd and Cu resulted mainly from their increasing accumulation and distribution in two subcellular fractions (i.e., metallothionein-like proteins and metal-rich granules). Tissue concentrations of Cd and Cu in the natural Zn-contaminated oysters also covaried with tissue Zn concentration, and prediction analyses revealed that Zn exposure was a significant contributor to tissue Cd and Cu concentrations. Therefore, we concluded that the increased Zn bioavailability in ambient waters not only increased the tissue Zn concentration but also enhanced the overall bioaccumulation of Cd and Cu. This study strongly demonstrates that contamination of metals in oysters may result from concurrent exposure to other metals. Thus, environmental managers should consider the possible exposure to other metals such as Zn in order to interpret/predict the tissue concentrations of toxic metals in oysters.