Thursday, 19 August 2010

Where did the oil go? The recent oil spill in the Gulf of Mexico released, as we have all seen on tv, a lot of oil. Quite how much is a "lot" is a bit of a guess, but roughly 4.9 million barrels, or 784 million litres*. What actually happened to this oil was reviewed recently in an article in Science (Kerr 2010). Only about 0.1% was recovered from beaches and marshes (that´s still an awful lot of oil!). About 17% was siphoned away at the well head, 5% burned off at the surface, and only 3% skimmed off by booms, despite a lot of effort and money spent. And the other 75%? It's, er, disappeared.

So where did this oil go? Some evaporated, but with luck most of it was eaten.

Oil is energy, that's why we use it in our cars and power stations. And energy means food. There are actually quite a few bacteria that digest and breakdown crude oil, and these are massively important in the recovery of the ocean from disasters like this. They work as a consortium, each concentrating on a particular fraction of the oil, and as one hydrocarbon is degraded to another, other bacteria take over. The first, and so in many ways the most important, are Alcanivorax species (Vila et al 2010). These are found in tiny quantities in unpolluted waters, but their numbers rocket when in the presence of linear and branched alkanes, common in crude oil. In fact they are so specialised for this type of hydrocarbon that without long chain alkenes they grow very poorly, but by then their job is done. Now other species such as Roseovarius and Marinobacter take over.

This breakdown was helped by the massive release of chemical dispersants at the oil head, 1.1 million gallons (Kintisch 2010). These are similar to the detergent in your kitchen, breaking down lumps of oil into tiny droplets, which are "dispersed" and can be attacked much more efficiently by bacteria. This was very controversial, as dispersants are pretty toxic and an immense quantity was involved. Still, it seemed to work, and much of the oil was broken down into 1-10 micrometer droplets. In fact, it started to raise fears that it was working TOO well, a microbial explosion depriving the ocean floor of oxygen and creating a huge dead zone. But this seems not to have happened, and in fact so far the prognosis is good.

We´re not out of the woods yet, the oil could yet turn up in unwanted places, and chemical damage by detergents might yet, for instance, devastate the local tuna population. But there have been lessons learnt for next time - and there will be a next time.

Kerr RA (2010). Gulf Oil Spill. A lot of oil on the loose, not so much to be found. Science (New York, N.Y.), 329 (5993), 734-5 PMID: 20705818;329/5993/734?maxtoshow=&hits=10&RESULTFORMAT=&fulltext=oil+biodegradation&searchid=1&FIRSTINDEX=0&sortspec=date&resourcetype=HWCIT

Kintisch E (2010). Gulf Oil Spill. An audacious decision in crisis gets cautious praise. Science (New York, N.Y.), 329 (5993), 735-6 PMID: 20705819

Vila, J., Nieto, J., Mertens, J., Springael, D., & Grifoll, M. (2010). Microbial community structure of a heavy fuel oil-degrading marine consortium: linking microbial dynamics with polycyclic aromatic hydrocarbon utilization FEMS Microbiology Ecology DOI: 10.1111/j.1574-6941.2010.00902.x

* the oil "barrel" is actually based on a type of old English wine barrel or "teirce" holding 35 gallons.

1 comment:

Sarah Stephen said...

Very interesting post! Thanks, David.
Amazing that the bulk of the work was probably done by these bacteria. Man, despite all his progress, still has miles to go.


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