Important Issues in Ecotoxicological Investigations Using Earthworms

Eisenia fetida impact on cadmium availability and distribution in specific components of the earthworm drilosphere

Although the potential of vermiremediation for restoring metal-contaminated soils is promising, the effects of earthworms on the availability of soil metals are still debatable. Most previous studies considered the soil as a "whole black box." Mobilization or immobilization of metals are affected by earthworm activities within drilosphere hotspots under different soil conditions, which has not been specifically studied. Therefore, an improved 2D terrarium was designed to study the impact of earthworm activities on cadmium (Cd) fate in the drilosphere hotspots (burrow wall soils, burrow casts, and surface casts) of different artificially spiked Cd treatments (CK: 0 mg kg-1; LM: 1 mg kg-1; and HM: 5 mg kg-1) with different organic amendments (2% and 10%). The results revealed that Cd increased earthworm activities with the highest cast production in HM and the highest burrow length in LM. Earthworms exhibited a stronger tendency to reduce total Cd concentration by 4.48-13.58% in casts of LM soils, while 3.37-5.22% in burrow walls under HM treatments. Overall, earthworms could increase the availability of Cd in casts under all conditions (55.46-121.01%). The organic amendments decreased the total Cd concentration and increased the availability of Cd in the disturbed soil. A higher amount of organic amendment significantly decreased total Cd concentration of the drilosphere by 1.16-5.83% in LM and HM treatments, while increasing DTPA-Cd concentrations in all components by 23.13-55.20 %, 14.63-35.11%, and 3.30-11.41% in CK, LM, and HM treatments, respectively, except for earthworm non-disturbed soil and no-earthworm soil in HM treatments. Redundancy analysis (RDA) revealed that the moisture, pH, and total carbon contents in soil are the main factors affecting Cd bioavailability. In this study, we decoded the "black box" of soil by making it relatively simple to better understand the effects and mechanisms of earthworm activities on soil metal availability and consequently provided comprehensive insights for using earthworms in soil vermiremediation.

Environmental Epigenetics in Soil Ecosystems: Earthworms as Model Organisms

One of the major emerging concerns within ecotoxicology is the effect of environmental pollutants on epigenetic changes, including DNA methylation, histone modifications, and non-coding RNAs. Epigenetic mechanisms regulate gene expression, meaning that the alterations of epigenetic marks can induce long-term physiological effects that can even be inherited across generations. Many invertebrate species have been used as models in environmental epigenetics, with a special focus on DNA methylation changes caused by environmental perturbations (e.g., pollution). Among soil organisms, earthworms are considered the most relevant sentinel organisms for anthropogenic stress assessment and are widely used as standard models in ecotoxicological testing of soil toxicity. In the last decade, several research groups have focused on assessing the impact of environmental stress on earthworm epigenetic mechanisms and tried to link these mechanisms to the physiological effects. The aim of this review is to give an overview and to critically examine the available literature covering this topic. The high level of earthworm genome methylation for an invertebrate species, responsiveness of epigenome to environmental stimuli, availability of molecular resources, and the possibility to study epigenetic inheritance make earthworms adequate models in environmental epigenomics. However, there are still many knowledge gaps that need to be filled in, before we can fully explore earthworms as models in this field. These include detailed characterization of the methylome using next-generation sequencing tools, exploration of multigenerational and transgenerational effects of pollutants, and information about other epigenetic mechanisms apart from DNA methylation. Moreover, the connection between epigenetic effects and phenotype has to be further explored.

Ecotoxicological Effects of Aflatoxins on Earthworms under Different Temperature and Moisture Conditions

Aflatoxin contamination remains one of the most important threats to food safety and human health. Aflatoxins are mainly found in soil, decaying plant material and food storage systems and are particularly abundant during drought stress. Regulations suggest the disposal of aflatoxin-contaminated crops by incorporation into the soil for natural degradation. However, the fate and consequences of aflatoxin in soil and on soil organisms providing essential ecological services remain unclear and could potentially pose a risk to soil health and productivity. The protection of soil biodiversity and ecosystem services are essential for the success of the declared United Nations Decade on Ecosystem Restoration. The focus of this study was to investigate the toxicological consequences of aflatoxins to earthworms’ survival, growth, reproduction and genotoxicity under different temperature and moisture conditions. Results indicated an insignificant effect of aflatoxin concentrations between 10 and 100 µg/kg on the survival, growth and reproduction but indicated a concentration-dependent increase in DNA damage at standard testing conditions. However, the interaction of the toxin with different environmental conditions, particularly low moisture, resulted in significantly reduced reproduction rates and increased DNA damage in earthworms.