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Snails snack on poison metals

December 18, 2002

SOILS tainted with heavy metals from industrial pollution and sewage sludge may poison organisms that live in the soil far more readily than thought. The finding raises fears that unexpectedly high levels of toxins are getting into the food chain.

Contaminated soils are given hazard ratings that are based on the key assumption that organisms can only take up heavy metals and other pollutants if they are dissolved or suspended in water percolating through the soil. A large proportion of pollutants are assumed to bind tightly to soil particles, making them inaccessible to organisms feeding on the soil.

But Renaud Scheifler and his team of soil biologists at the University of Franche-Comté in Besan'§on, France, have shown that this assumption is wrong. They allowed snails to rummage around a container filled with cadmium-contaminated soil taken from near a disused lead and zinc smelter. After two weeks, they analysed tissue from the snails and found that around 16 per cent of the cadmium they had absorbed was from the supposedly inaccessible cadmium bound tightly to the soil. "Our work is, to our knowledge, the first evidence of this," says Scheifler.

The finding suggests other heavy metals could also be more "bioavailable" than assumed, and could be entering the food chain.

"While a lot of research has been done on availability to plants, this paper addresses the much neglected soil fauna," says British-based researcher Ian Pulford, who studies the fate of heavy metals in soil at the University of Glasgow. "It means that more cadmium than predicted is getting into the food chain," he says.

Cadmium can cause kidney damage, anaemia and a painful bone disorder known as "itai itai", Japanese for "ouch ouch". Hundreds of people living in Yokkaichi, 300 kilometres south-west of Tokyo, developed the disease in the 1950s after exposure to cadmium from industrial pollution.

It is too early to know whether the finding will lead to improved risk assessment of tainted soils, says Scheifler, whose study is due to be reported in the journal Environmental Science and Technology (DOI: 10.1021/ es025677w). But he says it is now necessary to see whether other heavy metals such as zinc, copper, lead and mercury are also getting into the food chain more easily than anyone imagined.

His team also wants to check if other organisms that feed on soil can absorb heavy metals as readily as the snails. "We have to work with other invertebrates to find whether the snail is a particular case," he says. Pulford says we need to understand much more about the role of soil animals in bringing heavy metals into the food chain.

Author: Andy Coghlan

New Scientist