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Earthworms Indicate Soil Toxicity

Earthworms

Scientists can examine the tissue of earthworms and observe the effects of pollution.

By Jenny Bennett

Earthworms ingest soil contaminants and absorb them through the skin, making them an ideal indicator of soil toxicity.

The contamination of soil with polycyclic aromatic hydrocarbons (PAHs) is of much concern due to the possible harmful effects these substances can have on human health. PAHs are lipophilic materials; this means they mix more easily with oil than water. Because of these properties, PAHs in the environment are found predominantly in soil, sediment and oily substances rather than in water or in air. In addition to their presence in fossil fuels they are also formed by incomplete combustion of carbon-containing fuels such as wood, coal, diesel, fat, tobacco and incense.

Earthworms are excellent model organisms in ecotoxicological studies of soil toxicity due to their exposure to soil contaminants via both ingestion and passive absorption through their skin. The recommended species of earthworm in such toxicity studies is Eisenia fetida, which is known under various common names such as redworm, brandling worm, tiger worm and red wiggler worm. These worms thrive in rotting vegetation, compost and manure. Scientists can examine the tissue of these worms and observe the effects of pollution, enabling them to identify the most efficient and effective action to prevent any detrimental outcome.

Many studies have examined the exposure of earthworms to lethal contaminants such as PAHs. The problem with these studies is that, because they kill the worm, they often do not provide sufficient information concerning the toxic mode of action (MOA) of these contaminants.

To rectify this problem, these creatures need to be exposed to sub-lethal levels of contaminant. By monitoring the fluctuations in metabolite levels in response to these concentrations of toxins, scientists can potentially elucidate the contaminant’s MOA. However, studies that explore changes in cellular metabolism, such as the breakdown of simple metabolites like amino acids and sugars due to exposure to very low or sub-lethal concentrations of contaminants, are not often carried out.

Phenanthrene is one of 16 PAHs that have been classified as priority pollutants by the US Environmental Protection Agency because they possess toxic, mutagenic and carcinogenic properties. The toxic mode of action of phenanthrene in earthworms has not yet been fully examined but Myrna Simpson and co-workers from the University of Toronto in Canada have adopted a novel approach using a technique called 1H nuclear magnetic resonance (NMR) spectroscopy to measure the response of earthworms to sub-lethal exposure of phenanthrene.

Their approach uses a technique called metabolomics. Metabolomics is the study of chemical processes involving metabolites, the unique chemical fingerprints that specific cellular processes leave behind.

In this study, E. fetida was exposed to six sub-lethal concentrations of phenanthrene for 48 hours before the tissue extracts were examined by 1H NMR spectroscopy. The results revealed that a two-phase MOA was taking place.

At ultra-low sub-lethal exposures of phenanthrene, the MOA was characterised by a linear correlation between the metabolites produced and the exposure concentration. At higher exposures, the metabolite response to phenanthrene appeared to level out, indicating a distinct change in the MOA.

Further data analysis suggested that the metabolites alanine, lysine, arginine, isoleucine, maltose, ATP and betaine may be potential indicators for sub-lethal phenanthrene exposure.

These novel results, reported in Environmental Chemistry, suggest that the technique of NMR-based earthworm metabolomics has great potential for development as a routine tool in the ecotoxicological assessment of low and sub-lethal levels of contaminants, such as polycyclic aromatic hydrocarbons, in the environment.

Jenny Bennett is Publisher – Chemical Sciences at CSIRO Publishing.