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A tiny drop in the ocean with a ship's name on it: A DNA marker may make it possible to trace oil slicks back to the offending carrier, says Keith Hindell

TWO British scientists have made it possible to trace an oil slick to a particular ship long after it has disappeared over the horizon. Using the science of DNA, they have invented a biological marker for oil, patented as Microtrace.

At present, ships discharging waste fuel oil, or leaking crude oil, usually escape detection, and even when caught often escape conviction. If Microtrace works as reliably as its inventors expect, it will be possible to mark each cargo in a way that would remove all doubt as to the source of pollution.

The inventors claim that 1mg of Microtrace would be sufficient to mark 250,000 tons of crude oil, the typical load of a very large crude carrier. Microtrace is a synthetic compound that adheres to the molecular structure of the oil and can be detected at very small concentrations, even after months in the sea.

The DNA molecule consists of a double helix with chemicals, known as bases, forming the crosslinks rather like the steps on a spiral staircase. Any one of four 'bases' (adenine, cytosine, guanine and thymine) can be attached within the spiral in any order. As a typical bacterium DNA has a million bases on its spine, the permutations are almost infinite. Although manipulating the bases on a DNA molecule is a complex process, it can now be done by a machine that is able to create a unique molecule in a few minutes.

According to Howard Slater, Professor of Microbiology at the University of Wales, it is now possible to create an artificial DNA with a specific sequence of bases and then manufacture it as a unique compound. In powder form, this would then be bonded on to the surface of a tiny plastic bead which would be coated with compounds to protect it from seawater and make it adhere to the oil molecules. The finished marker would be hydrophobic and lipophilic, water-hating and oil-loving.

In theory about ten thousand million million molecules of DNA (10 to the power 15) would be needed for each batch of Microtrace. For practical use, 1mg of Microtrace would be mixed with inert chemicals to bulk it up to about 1kg of powder or 5 gallons of liquid, an amount large enough to be mixed evenly into a shipment.

John Minton, managing director of chemical analysts Minton, Treharne & Davies, in Cardiff, estimates the cost of manufacturing such a batch at around pounds 1,000, and the entire cost of shipping, storing, mixing and recording at about pounds 3,000 per shipment. Considering that the current value of a supertanker's cargo is about pounds 18m, the extra cost of the marker is modest.

If a pollution incident were to occur, the analysis of a sample to identify the marker would involve considerable further expense. However, that cost could be recovered from the polluter. In order to extract enough marker for identification, the minute amount of DNA in a sample would have to be multiplied by a method known as polymerase chain reaction. DNA's unique ability to self-replicate makes this possible.

Oil actually contains natural markers, imprinted on the carbon backbone of the molecules, which differ from oilfield to oilfield according to the geological history. However the best results obtained using this 'fingerprinting' method reveal only the field from which a particular oil comes. Such analysis can be used as evidence in pollution cases, but is not conclusive against a specific ship.

In the early Eighties experiments using tiny fragments of heavy metals as 'active' oil markers were carried out in the Baltic. Like Microtrace, the metals could be added to a cargo in a unique combination, but at that time the problems of systematic insertion could not be overcome.

Practical difficulties may be more formidable than scientific ones. The marking and recording of cargoes obviously has to be consistent, otherwise the credibility of the whole system would fail. Ian White, director of the international Tanker Owners Pollution Federation, is sceptical about the idea because he feels the staff work involved 'would be phenomenal'. 'In any case,' he says, 'it's no use unless universally applied.'

The inventors accept that irresponsible owners and masters will not mark their cargoes unless compelled to do so by law, but they argue that better companies, and particularly their insurers, will welcome a method of warding off false accusations of pollution. The penalties for pollution in the US are now so high that major companies are withdrawing from that trade because the risks cannot be covered fully by insurance.

British and European law is, as yet, less punitive, but governments might well insist that all cargoes in their waters be marked. At present, Britain's Marine Protection Control Unit operates expensive air surveillance. This may be effective in deterring ships' masters from cleaning their tanks in daylight, but its evidence has secured remarkably few convictions.

The inventors of Microtrace point out that it could have other applications. It could, for example, be used onshore, to trace leaks in a tank farm back to a particular pipe or tank, and Shell has expressed interest in using the marker to prove the presence of its additives in a particular fuel.

However, the main rewards would be in the marking of cargo and fuel oil. The pollution risks from the world's 6,768 tankers and 72,000 oil-powered ships, which all foul the sea to some degree, are enormous. The need for further deterrents to illegal discharge and spillage is overwhelming. A reliable oil marker may provide part of the answer.

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