Eating oil

Fertilisers accelerate growth not only in plants but also in certain microbes that feast on oil

Published: Tuesday 31 May 1994

-- SCIENTISTS have established beyond doubt the efficacy of inorganic fertilisers in helping mop up oil spills, an idea suggested over 2 decades ago.

Although nitrogen and phosphorus fertilisers had been shown to encourage, in the laboratory, the action of microorganisms capable of breaking down oil, the technique when applied to real-life spills had failed almost as often as it succeeded. Another problem in establishing the role of fertilisers in hastening the breakdown of oil was identifying components of oil that were non-biodegradable and could be used as reference to observe how well the technique worked. But now, a fresh analysis by US scientists -- including Ronald Atlas, one of those who had originally suggested the technique -- of the Exxon Valdez oil spill clean-up unit has put the idea on a sound footing.

In March 1989, oil tanker Exxon Valdez spilled 41 million litres of crude oil cargo in the Prince William Sound, a strait in Alaska. Clean-up experts rushed in and, instead of using detergents, they treated the shorelines with nitrogen and phosphorous fertilisers.

Scientist James R Bragg of the Exxon Production Research Co, and colleagues at Exxon and in US universities, monitored the massive clean up operation. They found that the application of fertilisers significantly increased rates of oil biodegradation in nature as in the lab (Nature, Vol 368, No 6470).

Although, in field tests in 1989 on the Exxon Valdez oil residues, the US Environmental Protection Agency (EPA) showed striking visual evidence that bioremediation works -- the fertiliser treated areas were cleaner than those which were untreated -- EPA scientists were unable to conclusively prove that fertiliser application had enhanced rates of biodegradation. This was because the microorganisms found in the region were extremely efficient at degrading the oil -- including certain fractions which are usually more resistant to biodegradation and are, therefore, used as "markers".

Starting 1990, Bragg and his colleagues undertook a new field monitoring programme which involved extensive chemical analyses of the oil. They identified a stable, high molecular weight compound in the oil called hopane, which is non-biodegradable. Using hopane as a reference, the scientists were able to prove conclusively that by applying fertilisers, biodegradation rates increased 5 times.

Bragg and his colleagues, however, caution that the success of the technique depends on the nature of the sediments contaminated. If the sediments are fine-grained, more oil droplets adhere to them and at the same time less oxygen is available to the microbes, retarding their efficacy.

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