Science: Poisons that point to good health: A test for toxins may offer early warning about pollution. Martyn Kelly explains

There was a time when it was easy to assess how toxic a substance was. You stuffed a batch of test animals full of the chemical and recorded the concentration at which half of them died.

Unfortunately, it is now clear that pollution can devastate an ecosystem at much lower concentrations than these tests implied. An adult animal may not be killed outright, but its ability to breed or compete with other species for food can be impaired.

But many organisms can fight back when exposed to pollution, 'switching on' biochemical processes to neutralise a toxin or expel it. According to Stephen George, of the Institute of Aquatic Biochemistry at the University of Stirling, these reactions represent a potential early-warning system for pollutants.

To an animal, some pollutants are little different to the breakdown products of natural body substances such as blood and hormones. Others may resemble the compounds that some plants and animals produce as chemical defences against predators.

Animals can therefore apply the biochemical 'waste treatment' system they have evolved to deal with pollutants. A vital part of the armoury for overcoming the chemical defences of plants is a class of compounds known as mixed-function oxidases. These enzymes break down the proteins in the plant toxins, rather as the enzymes in modern washing powders break down protein stains.

Crude oil contains polyaromatic hydrocarbons (PAHs), which trigger expression of the oxidases due to their resemblance to natural compounds. 'It is almost certain that the natural substances that switch on the pathway which handles PAH are the flavenoids, produced by plants,' Dr George says. Animals have evolved mechanisms to deal with low concentrations of these compounds, which they encounter in their diets.

An increase in enzyme activity above the level associated with its normal 'housekeeping' function within the cell can be used as an indication that PAH is in the organism. By exposing cells from fish livers to PAHs and polychlorinated biphenyls (PCBs), Dr George and his team located the gene responsible for making this enzyme. They were then able to measure the activity of the gene under different conditions.

A chance to test these ideas came when the tanker Braer grounded in Shetland in January 1993. Crude oil is a complex mixture of molecules, including small quantities of PAHs. Dr George's team was asked by Greenpeace to investigate the effects on fish around the islands. Four months after the incident they found high levels of enzyme activity in the livers of fish caught close to the site of the wreck.

Although levels subsequently returned to normal, these analyses suggest that biologists should keep a watchful eye on the breeding success of fish around the wreck site.

People were also concerned that oil in sediments might be stirred up by winter storms, releasing more PAHs into the water. However, Dr George examined another batch of samples this winter and found only a slight rise in enzyme activity.

The alternative is to monitor fully developed cancers in fish. By this stage, however, it may be too late for remedial action. Dr George is also doing a study in the Barents Sea, which receives industrial pollution from rivers draining northern Russia. 'It is an extremely fragile ecosystem as well as an important fishery.' he says, 'All the biological activity takes place in a three-month period when the ice melts.'

Statoil, which is about to start oil exploration in the area, is anxious to know whether there is any evidence of PAHs in Arctic cod. This will serve as a baseline against which changes can be assessed once the company starts drilling, in effect a primary healthcare programme.