Accumulated metal speciation in earthworm populations with multigenerational exposure to metalliferous soils: cell fractionation and high-energy synchrotron analyses

Metallomic mapping of gut and brain in heavy metal exposed earthworms: A novel paradigm in ecotoxicology

This study explored the uptake of lead in the epigeic earthworm Dendrobaena veneta exposed to 0, 1000, and 2500 μg Pb/g soil. The soil metal content was extracted using strong acid digestion and water leaching, and analysed by means of Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to estimate absolute and bioavailable concentrations of metals in the soil. The guts and heads of lead-exposed earthworms were processed into formalin-fixed and paraffin embedded sections for high-resolution multi-element metallomic imaging via Laser Ablation ICP-MS (LA-ICP-MS). Metallomic maps of phosphorus, zinc, and lead were produced at 15-μm resolution in the head and gut of D. veneta. Additional 4-μm resolution metallomic maps of the earthworm brains were taken, revealing the detailed localisation of metals in the brain. The Pb bioaccumulated in the chloragogenous tissues of the earthworm in a dose-dependent manner, making it possible to track the extent of soil contamination. The bioaccumulation of P and Zn in earthworm tissues was independent of Pb exposure concentration. This approach demonstrates the utility of LA-ICP-MS as a powerful approach for ecotoxicology and environmental risk assessments.

Subcellular localization and compartment-specific toxicokinetics of cadmium, arsenic, and zinc in brandling worm Eisenia fetida

Awareness of toxicokinetics at the subcellular level is crucial to deciphering the underlying intoxication processes of metal(loid)s, although this information is often lacking. Here, the toxicokinetics of two non-essential metal(loid)s (Cd and As) and one essential metal (Zn) in both the whole body and subcellular fractions of earthworm (Eisenia fetida) were assessed. Earthworms were exposed to natural soils originating from a gradient of metal(loid) pollution for 14 days followed by a 14-day elimination phase in clean soil. Clearly distinct toxicokinetic patterns were found in the earthworms according to the metal(loid) considered. An obvious concentration-dependent increase was observed in earthworms or subcellular compartments where no equilibrium was reached (with slow or no elimination) for Cd and As throughout the experiment. As for Zn, the earthworms were able to retain a steady-state concentration of Zn in its body or each fraction without a clear intake behavior via the dynamic trade-off between uptake and elimination at different pollution levels. These differences in toxicokinetics at the subcellular level supported the observed differences in bioaccumulation patterns and were indicative of the strategy by which non-essential and essential elements are handled by earthworms. Notably, the concentration of Cd and As in subcellular compartments showed the same pattern as for Zn in the order of cellular cytosol > cellular debris > metal-rich granules, which might be associated with the binding of non-essential/essential elements with metallothionein enriched in the cytosol. Our findings enhance the understanding of the underlying mechanisms for metal(loid) accumulation kinetics in earthworms from the perspective of subcellular partitioning, and will be beneficial for accurate risk assessment of Cd, As, and Zn.

Chemical availability versus bioavailability of potentially toxic elements in mining and quarry soils

Abandoned mining and quarry areas are sources of potentially toxic elements (PTEs), through lixiviates or transfer processes of bioavailable fractions from mining wastes and tailings. In this study, earthworms (Eisenia fetida Savigny, 1826) were exposed for 28 days to two mining soils from a lead/zinc mine and two quarry soils from an old serpentine quarry. Despite their pseudo total metal contents, a previous characterization of these soils pointed out for a low chemical availability of PTEs. Therefore, a multibiomarker approach was used and the response of E. fetida to soils was assessed through the analysis of neurotoxic, oxidative stress, energy metabolism and DNA damage biomarkers (acetylcholinesterase, catalase, glutathione-s-transferase, lactate dehydrogenase, lipid peroxidation and DNA strand breaks). Metal bioaccumulation was also assessed to evaluate bioavailability and organism's exposure. Results showed that high contents of PTEs were recorded in the whole body of earthworms exposed to lead/zinc mine. However, the bioaccumulation factors for worms exposed to soils from both sampling sites were <1 due to the high PTEs contents in soils. Earthworms exposed to both types of soils displayed neurotoxic and energy metabolism effects. However, significant levels of oxidative stress and DNA damage were recorded only for earthworms exposed to lead/zinc mine soils. This study demonstrated that despite the low availability of PTEs showed by previous sequential chemical extractions, the results obtained from the direct toxicity assessment performed in this study, highlight the importance of a multibiomarker approach using soil organisms to provide a better evaluation of soils pollution.

Unravelling the ZnO-NPs mechanistic pathway: Cellular changes and altered morphology in the gastrointestinal tract of the earthworm Eisenia andrei

A major uptake route of nanoparticles (NPs) occurs via the gastrointestinal (GI) tract. When GI tract cells are exposed, NPs cytotoxic effects are observed that subsequently adversely affect the GI tract morphology and have consequences for the whole organism. The aim of this study was to understand the mechanism of effects caused by ZnO-NPs compared to Zn ions on the earthworm Eisenia andrei. The following aspects of individually exposed earthworms were investigated: 1) qualitative structural alterations in the gut epithelium and chloragogen cells of the GI tract, 2) quantitative changes within chloragogen tissues after 48 h of exposure (using morphometric analysis), and 3) the ADP/ATP ratio in homogenized tissue of the whole organism after 21 days of exposure to contaminated soil (contamination phase) followed by 14 days of elimination in clean soil (decontamination phase) to identify possible recovery. Both ZnO-NPs and Zn ions adversely affect the gut epithelium and chloragogen tissue of earthworms after 48 h of exposure to contaminated soil. Morphometric measurements revealed that the proportions of debris vesicles in the chloragocytes were significantly lower in worms exposed to ZnO-NPs than in worms exposed to Zn ions. Moreover, numerous spherite granules were observed in the chloragocytes of ionic Zn-treated worms, but not the ZnO-NPs-treated worms, suggesting differential regulation of these Zn forms. The Zn cytotoxic effect was not reflected in ADP/ATP ratio measurements. Our study provides new insights into nano-specific effects that are distinctive from ion regulation inside the GI tract and furthers our understanding of the relationship between effects at the cellular and whole-body levels.

Influence of metal-contamination on distribution in subcellular fractions of the earthworm (Metaphire californica) from Hunan Province, China

Earthworms have the ability to accumulate of heavy metals, however, there was few studies that addressed the metals in earthworm at subcellular levels in fields. The distributions of metals (Cd, Cu, Zn, and Pb) in subcellular fractions (cytosol, debris, and granules) of earthworm Metaphire californica were investigated. The relationship between soil metals and earthworms were analyzed to explain its high plasticity to inhabit in situ contaminated soil of Hunan Province, south China. The concentration of Cd in subcellular compartments showed the same pattern as Cu in the order of cytosol > debris > granules. The distribution of Zn and Pb in earthworms indicated a similar propensity for different subcellular fractions that ranked as granules > debris > cytosol for Zn, and granules > cytosol > debris for Pb. The internal metal concentrations in earthworms increased with the soil metals (p<0.05). Significant positive correlations were found between soil Cd and Cd concentrations in cytosol and debris (p<0.01). Moreover, the soil Pb concentration significantly influenced the Pb concentrations in cytosol and debris (p<0.01), similar to that of Cd. The soil Cu concentrations was only associated with the Cu in granules (p<0.05). Soil Zn concentrations correlated with the Zn concentrations in each subcellular fraction (p<0.05). Our results provide insights into the variations of metals partitioning in earthworms at subcellular levels and the relationships of soil metals, which could be one of the detoxification strategies to adapt the long-term contaminated environment.

Structural equation model of the relationship between metals in contaminated soil and in earthworm (Metaphire californica) in Hunan Province, subtropical China

Earthworms have the ability to take up heavy metals in soil and partition them in different subcellular compartments. In this study, we used a structural equation model (SEM) to investigate the two-step causal relationship between environmental availability (EA) and environmental bioavailability (EB) of heavy metals (Cd, Cu, Zn, and Pb), as reflected by their levels in soil fractions and in earthworms from field-contaminated areas in Southern China. In the SEM, the correlation between EA and EB reflected the bioavailability of Cd, Zn, and Pb. For Cd, the causal relationship between the latent variables EA and EB was reflected by DTPA fractions in soil as well as by earthworm internal and subcellular cytosol fractions. The extractable and oxidizable fractions of Zn in soil influenced Zn concentrations in the cytosol and debris. The DTPA and reducible Pb fractions were bioavailable to earthworm internal Pb concentrations and those in cytosol fractions. These results implied that the DTPA, extractable, oxidizable, or reducible fractions of different metals could be the bioavailable sources to earthworm internal metals and partitioned in their subcellular compartments.

Impact of heavy metal bioaccumulation on antioxidant activities and DNA profile in two earthworm species and freshwater prawn from Ogun River

The use of freshwater invertebrates as bioindicator of heavy metal pollution is an important tool for environmental biomonitoring. This study investigated antioxidant activities and DNA profile in two limicolous earthworms (Alma millsoni and Libyodrilus violaceus) and freshwater prawns (Macrobrachium vollenhovenii) at selected points of Ogun River, Abeokuta. Heavy metal concentrations and DNA profile in the earthworms and prawn tissues were measured using standard procedures. Zn concentration was higher than other heavy metals in A. millsoni (685.83±114.42mg/kg), L. violaceus (1913.3±1098.7mg/kg) and M. vollenhovenii (134.7±13.61mg/kg). Superoxide dismutase activity ranged from 62.44±7.16-79.82±11.18U/g tissues, 60.26±11.18-71.07±7.54U/g tissues and 74.07±16.69-87.79±8.50U/g tissues in A. millsoni, L. violaceus and M. vollenhovenii respectively. RAPD-PCR revealed varying DNA profile among the earthworms samples; the UPGMA dendrogram formed two distinct clusters at genetic similarity coefficient of 0.15-0.2 with one cluster consisting of Alma millsoni and Libyodrilus violaceus from Sokori, Enugada and Iberekodo sampling points and the second cluster forming two distinct sub-clusters comprising Arakanga and Ago-ika's L. violaceus in one and A. millsoni in the other. High genetic variability was recorded among the earthworm species while the freshwater prawn showed no variability. Antioxidant activities and genetic variability in earthworms could serve as biomarkers of heavy metal pollution in freshwater environment.

Plastic Bag Derived-Microplastics as a Vector for Metal Exposure in Terrestrial Invertebrates

Microplastics are widespread contaminants in terrestrial environments but comparatively little is known about interactions between microplastics and common terrestrial contaminants such as zinc (Zn). In adsorption experiments fragmented HDPE bags c. one mm² in size showed similar sorption characteristics to soil. However, when present in combination with soil, concentrations of adsorbed Zn on a per mass basis were over an order of magnitude lower on microplastics. Desorption of the Zn was minimal from both microplastics and soil in synthetic soil solution (0.01 M CaCl₂), but in synthetic earthworm guts desorption was higher from microplastics (40-60%) than soil (2-15%), suggesting microplastics could increase Zn bioavailability. Individual Lumbricus terrestris earthworms exposed for 28 days in mesocosms of 260 g moist soil containing 0.35 wt % of Zn-bearing microplastic (236-4505 mg kg^-1) ingested the microplastics, but there was no evidence of Zn accumulation, mortality, or weight change. Digestion of the earthworms showed that they did not retain microplastics in their gut. These findings indicate that microplastics could act as vectors to increase metal exposure in earthworms, but that the associated risk is unlikely to be significant for essential metals such as Zn that are well regulated by metabolic processes.

Impacts of BDE209 addition on Pb uptake, subcellular partitioning and gene toxicity in earthworm (Eisenia fetida)

Lead (Pb) and decabromodiphenyl ether (BDE209) are the mainly co-existed contaminants at e-waste recycling sites. The potential toxicity of Pb (250 μg g(-1)) to earthworm Eisenia fetida in the presence of BDE209 (1, 10 and 100 μg g(-1)) was determined during 14-d incubation period. Compared to Pb treatment alone, the co-exposure with 1 μg g(-1) BDE209 barely affected Pb uptake, subcellular partitioning and gene expression; however, histopathological changes in earthworms' body wall (epidermal, circular and longitudinal muscles) demonstrated that 10 and 100 μg g(-1) BDE209 additions enhanced Pb uptake and altered its subcellular partitioning, indicating that Pb redistributed from fractions E (cell debris) and D (metal-rich granules) to fraction C (cytosols); Additionally, BDE209 supply significantly inhibited (p<0.05) the induction of SOD (superoxide dismutase) and CAT (catalase) gene expressions (maximum down-regulation 59% for SOD gene at Pb+100 μg g(-1) BDE209 and 89% for CAT gene at Pb+10 μg g(-1) BDE209), while facilitated (p<0.05) Hsp90 (heat shock protein 90) gene expression with maximum induction rate of 120% after exposure to Pb+10 μg g(-1) BDE209. These findings illustrate the importance of considering environmental BDE209 co-exposure when assessing Pb bioaccumulation and toxicity in multi-contaminated soil ecosystems.

Metallothionein induction in the coelomic fluid of the earthworm Lumbricus terrestris following heavy metal exposure: a short report

Earthworms are useful bioindicator organisms for soil biomonitoring. Recently the use of pollution biomarkers in earthworms has been increasingly investigated for soil monitoring and assessment. Earthworm coelomic fluid is particularly interesting from a toxicological perspective, because it is responsible for pollutant disposition and tissue distribution to the whole organism. The aim of the present work was to study the effect of heavy metal exposure on metallothionein (Mt) induction in the coelomic fluid of Lumbricus terrestris in view of future use as sensitive biomarker suitable for application to metal polluted soil monitoring and assessment. L. terrestris coelomic fluid showed a detectable Mt concentration of about 4.0 ± 0.6 μg/mL (mean ± SEM, n = 10) in basal physiological condition. When the animals were exposed to CuSO₄ or CdCl₂ or to a mixture of the two metals in OECD soils for 72 h, the Mt specific concentration significantly (P < 0.001) increased. The Mt response in the coelomic fluid perfectly reflected the commonly used Mt response in the whole organism when the two responses were compared on the same specimens. These findings indicate the suitability of Mt determination in L. terrestris coelomic fluid as a sensitive biomarker for application to metal polluted soil monitoring and assessment.

The combined effect of dissolved organic carbon and salinity on the bioaccumulation of copper in marine mussel larvae

Larvae of Mytilus spp. are among the most Cu sensitive marine species. In this study we assessed the combined effect of salinity and dissolved organic carbon (DOC) on Cu accumulation on mussel larvae. Larvae were exposed for 48 h to three Cu concentrations in each of nine salinity/DOC treatments. Synchrotron radiation X-ray fluorescence was used to determine the Cu concentration in 36 individual larvae with a spatial resolution of 10 × 10 μm. Cu body burden concentrations varied between 1.1 and 27.6 μg/g DW larvae across all treatments and Cu was homogeneously distributed at this spatial resolution level. Our results indicate decreasing Cu accumulation with increasing DOC concentrations which can be explained by an increase in Cu complexation. In contrast, salinity had a nonlinear effect on Cu. This cannot be explained by copper speciation or competition processes and suggests a salinity-induced alteration in physiology.

Adaptation of Saccharomyces cerevisiae to toxic manganese concentration triggers changes in inorganic polyphosphates

The ability of Saccharomyces cerevisiae to adapt to toxic Mn(2+) concentration (4 mM) after an unusually long lag phase has been demonstrated for the first time. The mutants lacking exopolyphosphatase PPX1 did not change the adaptation time, whereas the mutants lacking exopolyphosphatase PPN1 reduced the lag period compared with the wild-type strains. The cell populations of WT and ΔPPN1 in the stationary phase at cultivation with Mn(2+) contained a substantial number of enlarged cells with a giant vacuole. The adaptation correlated with the triggering of polyphosphate metabolism: the drastic increase in the rate and chain length of acid-soluble polyphosphate. The share of this fraction, which is believed to be localized in the cytoplasm, increased to 76%. Its average chain length increased to 200 phosphate residues compared with 15 at the cultivation in the absence of manganese. DAPI-stained inclusions in the cytoplasm were accumulated in the lag phase during the cultivation with Mn(2+).

Earthworms and soil pollutants

Although the toxicity of metal contaminated soils has been assessed with various bioassays, more information is needed about the biochemical responses, which may help to elucidate the mechanisms involved in metal toxicity. We previously reported that the earthworm, Eisenia fetida, accumulates cadmium in its seminal vesicles. The bio-accumulative ability of earthworms is well known, and thus the earthworm could be a useful living organism for the bio-monitoring of soil pollution. In this short review, we describe recent studies concerning the relationship between earthworms and soil pollutants, and discuss the possibility of using the earthworm as a bio-monitoring organism for soil pollution.

Comparative chronic toxicity of nanoparticulate and ionic zinc to the earthworm Eisenia veneta in a soil matrix

Manufactured nanoparticles (NPs) are increasingly being used in a range of consumer products and are already entering the environment. NP ZnO is one of the most widely used and potentially toxic NPs in aquatic exposures. It is likely that ZnO nanoparticles will also be bioavailable to soil organisms, studies on ZnO NP toxicity in a soil matrix are lacking. We exposed the earthworm Eisenia veneta to uncoated NP ZnO (<100 nm) dosed to soil and food at 250 and 750 mg Zn kg(-1) for 21 d. Concurrent exposures of equivalent ionic Zn were conducted with ZnCl(2) and for both forms effects on life history traits, immune activity and Zn body concentrations were compared. Despite slightly higher measured body concentrations, NP ZnO generally had less impact than ZnCl(2) on measured traits. At 750 mg Zn kg(-1), reproduction declined by 50% when exposed to NP ZnO but was almost completely inhibited by ZnCl(2). Similarly, immune activity was unaffected by NP ZnO but was suppressed by 20% when exposed to ZnCl(2). Scanning electron microscopy analysis of worm tissues following 24h aqueous exposure showed the presence of ZnO particles suggesting that NPs can be taken up in particulate form. This may explain the reduced effects at similar body concentrations seen in the soil study. Our findings suggest that risk assessments do not need to go beyond considering the metal component of NP ZnO in soils at least for the larger size uncoated particles considered here.

Characteristics of immune-competent amoebocytes non-invasively retrieved from populations of the sentinel earthworm Lumbricus rubellus (Annelida; Oligochaeta; Lumbricidae) inhabiting metal polluted field soils

Lumbricus rubellus is a cosmopolitan earthworm devoid of riboflavin-storing eleocytes; its immune competent coelomocytes are predominantly amoebocytes. Our aim was to determine whether amoebocyte cytometrics in L. rubellus are robust biomarkers for innate immunological responses to environmental pollutants. Investigations were conducted on populations inhabiting three unpolluted and five metalliferous (mainly Pb+Zn+Cd) habitats in the UK and Poland. Inter-population differences in worm mass and amoebocyte numbers did not consistently reflect soil or tissue metal concentrations. Flow cytometry indicated that autofluorescence of the amoebocytes differs between cells from the unpolluted and metal-polluted worms, and pinocytosis of neutral red by amoebocytes was lower (especially at 15 versus 60 min incubation) in worms from the polluted Poland site compared with the reference population. To conclude, amoebocyte cytometrics and functionality are potentially useful for environmental diagnostics; deployment is contingent on better understanding potential confounders.

Synthesis of isotopically modified ZnO nanoparticles and their potential as nanotoxicity tracers

Understanding the behavior of engineered nanoparticles in the environment and within organisms is perhaps the biggest obstacle to the safe development of nanotechnologies. Reliable tracing is a particular issue for nanoparticles such as ZnO, because Zn is an essential element and a common pollutant thus present at elevated background concentrations. We synthesized isotopically enriched (89.6%) with a rare isotope of Zn (67Zn) ZnO nanoparticles and measured the uptake of 67Zn by L. stagnalis exposed to diatoms amended with the particles. Stable isotope technique is sufficiently sensitive to determine the uptake of Zn at an exposure equivalent to lower concentration range (<15 μg g(-1)). Without a tracer, detection of newly accumulated Zn is significant at Zn exposure concentration only above 5000 μg g(-1) which represents some of the most contaminated Zn conditions. Only by using a tracer we can study Zn uptake at a range of environmentally realistic exposure conditions.

Body metal concentrations and glycogen reserves in earthworms (Dendrobaena octaedra) from contaminated and uncontaminated forest soil

Stress originating from toxicants such as heavy metals can induce compensatory changes in the energy metabolism of organisms due to increased energy expenses associated with detoxification and excretion processes. These energy expenses may be reflected in the available energy reserves such as glycogen. In a field study the earthworm, Dendrobaena octaedra, was collected from polluted areas, and from unpolluted reference areas. If present in the environment, cadmium, lead and copper accumulated to high concentrations in D. octaedra. In contrast, other toxic metals such as aluminium, nickel and zinc appeared to be regulated and kept at low internal concentrations compared to soil concentrations. Lead, cadmium and copper accumulation did not correlate with glycogen reserves of individual worms. In contrast, aluminium, nickel and zinc were negatively correlated with glycogen reserves. These results suggest that coping with different metals in earthworms is associated with differential energy demands depending on the associated detoxification strategy.

Riboflavin mobilization from eleocyte stores in the earthworm Dendrodrilus rubidus inhabiting aerially-contaminated Ni smelter soil

A 6-week reciprocal transfer laboratory exposure experiment was conducted with two populations of the epigeic earthworm Dendrodrilus rubidus; one population inhabited a site approx. 200 m downwind of an active Ni smelter co-contaminated with Ni and Cu (3648 and 977 microg g(-1)d.w., respectively), the other inhabited uncontaminated soil. Worms transferred from unpolluted to Ni/Cu-polluted soil lost body mass (62%); they also had reduced (70%) total coelomocyte number, including autofluorescent eleocytes, and had significantly decreased (92%) riboflavin-derived fluorescence emission measured at 525 nm. Coelomocyte counts were low, and 525 nm emission was negligible in worms maintained on their native Ni/Cu soil. Earthworms and their coelomocytes were unaffected when transferred from Ni/Cu-polluted soil to unpolluted soil. In conclusion, exposing worms to stress-inducing factors, including metal pollution, alters the riboflavin status within the immune-competent cells of D. rubidus, but it requires further in vivo studies to establish whether the reduction in the fluorescence signal is predominantly due to depletion of riboflavin-containing eleocytes, or to riboflavin quenching, or to enzymatic conversion (and thus depletion) of stored riboflavin into its functional immune-potentiating flavin derivatives, FMN and FAD. The flavin budget of D. rubidus coelomocytes recovered by a reproducible extrusion procedure is a potentially useful biomarker for assessing sublethal stress in this early colonizer of disturbed soils.

Systems toxicology approaches for understanding the joint effects of environmental chemical mixtures

Environmental mixtures of chemicals constitute a prevalent issue in ecotoxicology and the development of new methods to reduce the uncertainties associated with their ecological risk assessment is a critical research need. Historically, a number of models have been explored to predict the potential combined effects of chemicals on species. These models, especially concentration addition and the independent action, have been applied to a number of mixtures. While often providing a good prediction of joint effect, there are cases where these models can have limitations: notably in cases where there are interactions for which they fail to adequately predict joint effects. To support the better mechanistic understanding of interactions in mixture toxicology a framework to support experimental studies to investigate the basis of observed interactions is proposed. The conceptual framework is derived from the extension of a three stage scheme which has previously been applied to understand chemical bioavailability. The framework considers that interactions in mixtures result from processes related to 1) the speciation, binding and transport of chemicals in the exposure medium (external exposure); 2) the adsorption, distribution, metabolism and excretion of chemicals within the organisms (toxicokinetics); 3) associations governing the binding and toxicity of the chemical(s) at the target site (toxicodynamics). The current state of the art in (eco)toxicology in relation to investigation of the mechanisms of interactions between chemicals is discussed with particular emphasis towards the multi-disciplinary tools and techniques within environmental chemistry; toxicology; biochemistry and systems biology that can be used to address such effects.

Spatial imaging and speciation of lead in the accumulator plant Sedum alfredii by microscopically focused synchrotron X-ray investigation

Sedum alfredii (Crassulaceae), a species native to China, has been characterized as a Zn/Cd cohyperaccumulator and Pb accumulator though the mechanisms of metal tolerance and accumulation are largely unknown. Here, the spatial distribution and speciation of Pb in tissues of the accumulator plant was investigated using synchrotron-based X-ray microfluorescence and powder Extended X-ray absorption fine structure (EXAFS) spectroscopy. Lead was predominantly restricted to the vascular bundles of both leaf and stem of the accumulator. Micro-XRF analysis revealed that Pb distributed predominantly within the areas of vascular bundles, and a positive correlation between the distribution patterns of S and Pb was observed. The dominant chemical form of Pb (>60%) in tissues of both accumulating (AE) and nonaccumulating ecotype (NAE) S. alfredii was similar to prepared Pb-cell wall compounds. However, the percentage of the Pb-cell wall complex is lower in the stem and leaf of AE, and a small amount of Pb appeared to be associated with SH-compounds. These results suggested a very low mobility of Pb out of vascular bundles, and that the metal is largely retained in the cell walls during transportation in plants of S. alfredii.

The two Caenorhabditis elegans metallothioneins (CeMT-1 and CeMT-2) discriminate between essential zinc and toxic cadmium

The nematode Caenorhabditis elegans expresses two metallothioneins (MTs), CeMT-1 and CeMT-2, that are believed to be key players in the protection against metal toxicity. In this study, both isoforms were expressed in vitro in the presence of either Zn(II) or Cd(II). Metal binding stoichiometries and affinities were determined by ESI-MS and NMR, respectively. Both isoforms had equal zinc binding ability, but differed in their cadmium binding behaviour, with higher affinity found for CeMT-2. In addition, wild-type C. elegans, single MT knockouts and a double MT knockout allele were exposed to zinc (340 microm) or cadmium (25 microm) to investigate effects in vivo. Zinc levels were significantly increased in all knockout strains, but were most pronounced in the CeMT-1 knockout, mtl-1 (tm1770), while cadmium accumulation was highest in the CeMT-2 knockout, mtl-2 (gk125) and the double knockout mtl-1;mtl-2 (zs1). In addition, metal speciation was assessed by X-ray absorption fine-structure spectroscopy. This showed that O-donating, probably phosphate-rich, ligands play a dominant role in maintaining the physiological concentration of zinc, independently of metallothionein status. In contrast, cadmium was shown to coordinate with thiol groups, and the cadmium speciation of the wild-type and the CeMT-2 knockout strain was distinctly different to the CeMT-1 and double knockouts. Taken together, and supported by a simple model calculation, these findings show for the first time that the two MT isoforms have differential affinities towards Cd(II) and Zn(II) at a cellular level, and this is reflected at the protein level. This suggests that the two MT isoforms have distinct in vivo roles.