Earthworms tackle environmental problems

Earthworm populations are helping scientists to understand more about soil that is contaminated with metal, how metals can drive evolution and what affect they themselves have on potentially toxic elements in the earth. Thanks to the latest X-ray technology, earthworm tissue, excreted soil, and earthworm burrow walls can be studied as never before.

Lumbricus rubellus earthworms from uncontaminted & contaminated sites. Copyright Jane Andre, University of Reading

The questions that we are currently addressing are:

1. Why is it that some populations of earthworms can inhabit contaminated soil and other populations, of the same earthworm species, can’t? It appears that metal tolerant populations of earthworms are evolving and we are studying this by seeing if the toxic elements are stored in the earthworm population using different mechanisms. Super metal munching earthworms might have a role to play in soil clean up operations, helping the soil to live again, and our understanding of their evolution is important if we are to successfully tap into this potential.
2. Do earthworms actually affect metal mobility in soils? We are interested to see what impact earthworms have on potentially toxic elements in soils - if an earthworm ingests and excretes soil that contains potentially toxic elements will the potentially toxic elements in the excreted soil be more readily taken up by plants, leached into water courses etc. If plants are able to take up metals from soils after earthworms have excreted the soil we could use earthworms to help plants extract metals from contaminated sites for metal clean up operations or even plant assisted mining!
3. Do earthworm burrow walls play a role in metal transport and availability? The walls of earthworm burrows have a lining of mucus and a rich bacterial population. There is some evidence that metals are concentrated in burrow walls. We are interested to see if metal speciation is different between bulk soil and earthworm burrows.

A combination of laboratory, field and synchrotron X-ray experiments have led to the finding that metal tolerant populations of super earthworms are evolving. Watching evolution in action is exciting in itself, but if we are to tap into the potential of these earthworms as Ecosystem Engineers we need to deepen our understanding of what is happening to the metals while they are inside the earthworms and how the excreted metals are speciated.

Modern X-ray absorption spectroscopy techniques available at Diamond, such as EXAFS (Extended X-ray Absorption Fine Structure) and XANES (X-ray Absorption Near Edge Structure), are allowing us to examine the earthworms, the soil and the burrows as never before. As a surgeon can examine your vital organs to gain an understanding of how your body is functioning, we can now look inside an earthworm and see what is happening to the metals that have been ingested along with the soil. The size of the metal samples we are tracking is around one thousand times smaller than a grain of salt (think of a book on a football pitch), so invisible to the human eye and impossible to detect in this level of detail with our standard laboratory microscopes.

Some earthworms have been able to adapt to the metal polluted environments that they inhabit and their relationship with this toxic soil could help humans to correct the soil damage that has occurred due to our industrial past.

Dr Mark Hodson, University of Reading