2008-10-08

Newly evolved "superworms" that feast on toxic waste could help cleanse polluted industrial land, a new study says.

These hardcore heavy metal fans, unearthed at disused mining sites in Britain and Wales, devour lead, zinc, arsenic, and copper.

The earthworms excrete a slightly different version of the metals, making them easier for plants to suck up. Harvesting the plants would leave cleaner soil behind.

"These worms seem to be able to tolerate incredibly high concentrations of heavy metals, and the metals seem to be driving their evolution," said lead researcher Mark Hodson of the University of Reading in England.

"If you took an earthworm from the back of your garden and put it in these soils, it would die," Hodson said.

DNA analysis of lead-tolerant worms living at Cwmystwyth, Wales, show they belong to a newly evolved species that has yet to be named, he said.

Two other superworms, including an arsenic-munching population from southwest England, are also likely new to science, Hodson said.

"It's a good bet they are also different species, but we haven't categorically proved that," he said.

The findings were announced in September at the British Association Festival of Science in Liverpool.

Micro Processors

Hodson's team's investigation used x-rays to zap worms with intense light, allowing them to track metal particles a thousand times smaller than a grain of salt.

The findings suggest the arsenic-tolerant population produces a special protein that "wraps up the metal and keeps it inert and safe so it doesn't interact with the earthworms," Hodson said.

The lead-eating Welsh worms likewise use a protein to render the metal harmless inside their bodies, he added.

The toxicity of the metal particles once they have passed through the worms isn't yet known, since the protective protein wrappings will degrade over time, the study authors noted.

But experiments suggest the superworms make the metals easier for plants to extract from the soil, Hodson said.

"The earthworms don't necessarily render the metals less toxic, but they do seem to make them available for plant uptake," he said. This raises this possibility of using the earthworms as part of efforts to clean up land contaminated by mining and heavy industry.

(Related: "Microorganism Cleans Up Toxic Groundwater" [April 7, 2004].)

Plant Mining

The long-term aim is to breed and then release the worms at polluted sites to speed up the process of soil development and help kick-start the ecosystem's rehabilitation, Hodson said.

Plants could be used to extract toxic metals once the superworms have got to work, he added.

This in turn could boost the development of methods for using plants to mine metals.

"The goal at the end of the rainbow is that the plants become so efficient at it that you can use them as a source of metal in industrial processes," Hodson said. "So you just crop off the plants and take them to a processing plant."

Peter Kille of the School of Biosciences at Cardiff University in Wales has also been tracking the metal-eating worms.

He said previous studies show it takes earthworms many years to improve polluted soils. While the new superworms should prove a useful tool, even they can't compete with industrial cleanup processes that take one to two years.

The worms, however, are an excellent way to diagnose metal concentrations in contaminated land, Kille said.

"Basically you can see the earthworms as biological dipsticks of the soil toxicity and the metal levels," he said.

And the superworms are perfect subjects for studying evolution in action, Kille added.

"What's really interesting is that each patch of high metal creates a unique evolutionary event," he said. The worms either develop new ways of dealing with the metals or find solutions similar to other populations.

"Each time it happens it's a localized event, and it allows us to study the processes of evolution that create the adaptation," he said.

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