Survival, Pb-uptake and behaviour of three species of earthworm in Pb treated soils determined using an OECD-style toxicity test and a soil avoidance test

Eisenia fetida avoidance behavior as a tool for assessing the efficiency of remediation of Pb, Zn and Cd polluted soil

Remediation by means of soil leaching with ethylenediaminetetraacetic acid (EDTA) is capable of extracting the most labile soil fractions, leaving the residual metals in biologically non-available forms. We evaluated the feasibility of the standardized earthworm (Eisenia fetida) avoidance test for assessing the efficiency of soil remediation of Pb, Zn and Cd polluted soil. Chemical extraction tests (six-step sequential extraction, toxicity characteristic leaching procedure, physiologically based extraction test, diethylenediaminepentaacetic acid extraction) indicated that the mobility, oral bioaccessibility and phytoavailability of Pb, Zn and Cd were consistently reduced. However, the avoidance test showed no significant avoidance of polluted soil in favor of that which had been remediated. Pb, Zn and Cd accumulation in E. fetida mirrored the decreasing pattern of metal potential bioavailability gained by leaching the soil with increasing EDTA concentrations. The calculated bioaccumulation factors indicated the possibility of underestimating the metal bioavailability in soil using chemical extraction tests.

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.

Molecular genetic differentiation in earthworms inhabiting a heterogeneous Pb-polluted landscape

A Pb-mine site situated on acidic soil, but comprising of Ca-enriched islands around derelict buildings was used to study the spatial pattern of genetic diversity in Lumbricus rubellus. Two distinct genetic lineages ('A' and 'B'), differentiated at both the mitochondrial (mtDNA COII) and nuclear level (AFLPs) were revealed with a mean inter-lineage mtDNA sequence divergence of approximately 13%, indicative of a cryptic species complex. AFLP analysis indicates that lineage A individuals within one central 'ecological island' site are uniquely clustered, with little genetic overlap with lineage A individuals at the two peripheral sites. FTIR microspectroscopy of Pb-sequestering chloragocytes revealed different phosphate profiles in residents of adjacent acidic and calcareous islands. Bioinformatics found over-representation of Ca pathway genes in EST(Pb) libraries. Subsequent sequencing of a Ca-transport gene, SERCA, revealed mutations in the protein's cytosolic domain. We recommend the mandatory genotyping of all individuals prior to field-based ecotoxicological assays, particularly those using discriminating genomic technologies.

Uptake kinetics of metals by the earthworm Eisenia fetida exposed to field-contaminated soils

It is well known that earthworms can accumulate metals. However, most accumulation studies focus on Cd-, Cu-, Pb- or Zn-amended soils, additionally few studies consider accumulation kinetics. Here we model the accumulation kinetics of 18 elements by Eisenia fetida, exposed to 8 metal-contaminated and 2 uncontaminated soils. Tissue metal concentration was determined after 3, 7, 14, 21, 28 and 42 days. Metal elimination rate was important in determining time to reach steady-state tissue metal concentration. Uptake flux to elimination rate ratios showed less variation and lower values for essential than for non-essential metals. In theory kinetic rate constants are dependent only on species and metal. Therefore it should be possible to predict steady-state tissue metal concentrations on the basis of very few measurements using the rate constants. However, our experiments show that it is difficult to extrapolate the accumulation kinetic constants derived using one soil to another.

Ecological risks of an old wood impregnation mill: application of the Triad approach

Although many studies deal with the distribution and mobility of chromated copper arsenate (CCA) metals in soil, the ecotoxicity of CCA-contaminated soils is rarely studied. The Triad approach was applied to determine the ecological risks posed by a CCA mixture at a decommissioned wood impregnation mill in southern Finland. A combination of (1) chemical analyses; (2) toxicity tests with plants (aquatic: Lemna minor; terrestrial: Lactuca sativa), earthworms (Lumbricus rubellus), and enchytraeids (Enchytraeus albidus) conducted on contaminated soils, their aqueous extracts, and well water collected from the site; and (3) determination of the abundance of enchytraeids and nematodes and the bioaccumulation of metals in plants (horsetail) collected from the field were used to assess the actual risk. Although metal concentrations were low, L. minor growth appeared to be reduced by As contamination of the well water. In soil, metals were heterogeneously distributed with total concentrations of 14.8 to 4360 mg As/kg, 15.2 to 1740 mg Cr/kg, and 4.83 to 790 mg Cu/kg. In several samples, concentrations were above Finnish regulatory guideline values and exceeded the half maximal effective concentration (EC50) or 50% lethal concentration (LC50) values for the toxicity of the individual metals to earthworms and enchytraeids, indicating hazards to the ecosystem. (Bio)availability of metals was high, as indicated by weak electrolyte extractions and body residues in L. rubellus and E. albidus exposed in bioassays. Earthworm survival correlated significantly with body metal concentrations, but not with soil total metal concentrations. Enchytraeid responses in the soil bioassays were less sensitive to CCA metal exposure. Plant growth was affected by CCA pollution, with L. sativa root elongation correlating significantly with total and available As concentrations and L. minor development being significantly reduced in H2O extracts of the most contaminated soil sample. Abundance of enchytraeids and nematodes in the field was much lower than in nonpolluted Finnish soils but did not significantly correlate with CCA contamination. Arsenic accumulation in horsetail did not correlate with As concentrations in soil. Overall, the results of the 3 lines of evidence of the Triad approach indicate possible increased risks to the ecosystem at the most contaminated sites of the CCA treatment area.

Effect of lead on survival, locomotion and sperm morphology of Asian earthworm, Pheretima guillelmi

To provide basic toxicity data for formulating risk characterization benchmarks, the effects of lead on survival, locomotion, and sperm morphology were investigated in the Asian earthworm Pheretima guillelmi. The LC50 of P. guillelmi for 7 and 14 d were 4285 +/- 339 mg/kg and 3207 +/- 248 mg/kg, which shows P. guillelmi can tolerate a higher concentration of lead nitrate. The average weight of the surviving earthworms decreased at concentration of 2800 mg Pb/kg soil, and the locomotor ability of earthworms exposed to a range of soil Pb concentrations showed a general decrease with increasing Pb concentrations. We also presented data depicting the sperm morphology of earthworms, which shows potential as a sensitive biomarker for measuring the effects of heavy metal on reproduction.

Mercury, cadmium and lead concentrations in different ecophysiological groups of earthworms in forest soils

Bioaccumulation of Hg, Cd and Pb by eight ecophysiologically distinct earthworm species was studied in 27 polluted and uncontaminated forest soils. Lowest tissue concentrations of Hg and Cd occurred in epigeic Lumbricus rubellus and highest in endogeic Octolasion cyaneum. Soils dominated by Dendrodrilus rubidus possess a high potential of risk of Pb biomagnification for secondary predators. Bioconcentration factors (soil-earthworm) followed the sequence ranked Cd>Hg>Pb. Ordination plots of redundancy analysis were used to compare HM concentrations in earthworm tissues with soil, leaf litter and root concentrations and with soil pH and CEC. Different ecological categories of earthworms are exposed to Hg, Cd and Pb in the topsoil by atmospheric deposition and accumulate them in their bodies. Species differences in HM concentrations largely reflect differences in food selectivity and niche separation.

Validation of avoidance assays for the screening assessment of soils under different anthropogenic disturbances

A chemical characterization of the soil compartment per se does not supply any information about the synergistic/antagonistic effects of mixtures of chemicals, resulting in an under- or overestimation of the risks. Therefore the existence of rapid and ecologically relevant toxicity assays becomes of paramount importance, allowing the evaluation of invertebrate's behavioural parameters with equal consequences in terms of functionality of the edaphic community. The aim of this study was to evaluate the sensitivity of avoidance assays, with the earthworm Eisenia andrei, in discriminating natural soils under different anthropogenic disturbances. Although earthworms were able to discriminate soils with similar contamination, it is nonetheless questionable whether the preference for some soils is determined by the reduced bioavailability of contaminants, the great affinity of the species for organic matter-rich soils, or the inability of chemoreceptors to detect some contaminants from complex environmental mixtures, usually present in natural contaminated soils.

A Cu tolerant population of the earthworm Dendrodrilus rubidus (Savigny, 1862) at Coniston Copper Mines, Cumbria, UK

Dendrodrilus rubidus were sampled from a mine spoil soil at Coniston Copper Mine, an abandoned Cu mine in Cumbria, UK and a Cu-free control site. Earthworms were maintained for 14d in both Kettering loam and a Moorland soil amended with Cu nitrate. Mortality, condition index, weight change and tissue concentration were determined. In both soils D. rubidus native to the mine site were able to tolerate significantly higher soil Cu concentrations (MWRT, p<or=0.001), and exhibited significantly less change in weight (t-test, p<or=0.001) and a lower loss in condition (t-test, p<or=0.001) than control earthworms. For a given soil Cu concentration tissue Cu concentrations were greater in the mine site earthworms. Low cocoon production and viability from the mine site population prevented the determination of toxicity parameters on the F1 generation and may be an indicator of the cost of tolerance to the population.

Effects of metals on life cycle parameters of the earthworm Eisenia fetida exposed to field-contaminated, metal-polluted soils

Two control and eight field-contaminated, metal-polluted soils were inoculated with Eisenia fetida (Savigny, 1826). Three, 7, 14, 21, 28 and 42 days after inoculation, earthworm survival, body weight, cocoon production and hatching rate were measured. Seventeen metals were analysed in E. fetida tissue, bulk soil and soil solution. Soil organic carbon content, texture, pH and cation exchange capacity were also measured. Cocoon production and hatching rate were more sensitive to adverse conditions than survival or weight change. Soil properties other than metal concentration impacted toxicity. The most toxic soils were organic-poor (1-10 g C kg(-1)), sandy soils (c. 74% sand), with intermediate metal concentrations (e.g. 7150-13,100 mg Pb kg(-1), 2970-53,400 mg Zn kg(-1)). Significant relationships between soil properties and the life cycle parameters were determined. The best coefficients of correlation were generally found for texture, pH, Ag, Cd, Mg, Pb, Tl, and Zn both singularly and in multivariate regressions. Studies that use metal-amended artificial soils are not useful to predict toxicity of field multi-contaminated soils.

Effect of time and mode of depuration on tissue copper concentrations of the earthworms Eisenia andrei, Lumbricus rubellus and Lumbricus terrestris

Eisenia andrei, Lumbricus rubellus and Lumbricus terrestris were exposed to 250, 250 and 350mgkg(-1) Cu respectively in Cu(NO(3))(2(aq)) amended soil for 28 d. Earthworms were then depurated for 24 to 72h, digested and analysed for Cu and Ti or, subsequent to depuration were dissected to remove any remaining soil particles from the alimentary canal and then digested and analysed. This latter treatment proved impossible for E. andrei due to its small size. Regardless of depuration time, soil particles were retained in the alimentary canal of L. rubellus and L. terrestris. Tissue concentration determinations indicate that E. andrei should be depurated for 24h, L. rubellus for 48h and L. terrestris should be dissected. Ti was bioaccumulated and therefore could not be used as an inert tracer to determine mass of retained soil. Calculations indicate that after 28 d earthworms were still absorbing Cu from soil.

A review of studies performed to assess metal uptake by earthworms

Earthworms perform a number of essential functions in soil; the impacts of metals on earthworms are often investigated. In this review we consider the range of earthworm species, types of soil and forms of metal for which metal uptake and accumulation have been studied, the design of these experiments and the quantitative relationships that have been derived to predict earthworm metal body burden. We conclude that there is a need for more studies on earthworm species other than Eisenia fetida in order to apply the large existing database on this earthworm to other, soil dwelling species. To aid comparisons between studies agreement is needed on standard protocols that define exposure and depuration periods and the parameters, such as soil solution composition, soil chemical and physical properties to be measured. It is recommended that more field or terrestrial model ecosystem studies using real contaminated soil rather than metal-amended artificial soils are performed.

Earthworm biomarkers in ecological risk assessment

Earthworms have had a notable contribution in terrestrial ecotoxicology. They have been broadly used to assess environmental impact from metal pollution, and they are typical test organisms (e.g., Eisenia) in standardized toxicity tests. Several reviews and international workshops have stressed the need for increasing the understanding and applicability of earthworm biomarkers in the ecological risk assessment (ERA) process. This review summarizes recent available information concerning the most investigated earthworm biomarkers. In earthworms, the use of biomarkers has been focused on assessing metal pollution, and available data on biomarker responses to organic contaminants are rather limited. The potential for applying earthworm biomarkers in the standardized toxicity tests is suggested in view of their significant contribution to the risk assessment of contaminated soils (e.g., estimation of bioavailable and bioactive fraction or sublethal effects). Field studies involving earthworm biomarkers are still scarce and are summarized according to their main practical approaches in retrospective ERA: biological surveys, laboratory tests of the soil, simulated field studies, and in situ exposure bioassays. Despite the great volume of laboratory studies on earthworm biomarkers, future lines of research are suggested besides the recommendations made by others: (1) the potential and limitations of the inclusion of biomarkers in the standardized toxicity tests should be examined under a well-defined weight-of-evidence framework; (2) it is necessary to develop operating guidelines to standardize earthworm biomarker assays, an important step to apply biomarkers in a regulatory context; (3) molecular and physiological biomarkers should be directly linked to behavioral changes with significant ecological implications, an important step in considering them as ecotoxicological biomarkers; and (4) biomarkers to organic pollutants of current concern (e.g., polycyclic aromatic hydrocarbons, anti-ChE and pyrethroid insecticides, polybrominated flame retardants, etc.) need to be developed and validated in the field. Also, an increase in the knowledge of earthworm biomarkers is undoubtedly useful in assessing the effectiveness of procedures for recovering/protecting the environment (e.g., phytoremediation or agrienvironment schemes) besides its potential use in the ERA framework.