What will gobble the spilled oil?

Whether or not ecological disaster follows the BP spill may hinge on what eats the oil first.

"Right now it's a race between the microbes and the fish," said marine biologist Larry McKinney of Texas A&M University, a specialist in the Gulf of Mexico.

Ideally, microbes will win, transforming the oil into less toxic substances. If, on the other hand, fish or crustaceans or other organisms absorb it, the oil could spread through the food chain.

With constant reports about the latest environmental catastrophe or hope - oddities in blue crab larvae one week, marsh grass growing back the next - scientists say it is important to remember that the true impact of the spill may not be known for a year or more.

Nevertheless, they are encouraged by government estimates that about half the oil has already either evaporated or been recovered.

There are several reasons the situation could have been worse, said David Velinsky, a biogeochemist at Philadelphia's Academy of Natural Sciences. For one thing, the type of oil that poured out after the Deepwater Horizon explosion in April is made of lighter, more easily evaporated hydrocarbons than the heavier crude that spilled when the Exxon Valdez tanker struck a reef off Alaska in 1989.

And the gulf's warm temperatures increase the rate of evaporation. Wind can speed that up as long as the water stays smooth, Velinsky said. If wind - or, worse, a hurricane - churns up the water, it can mix the oil into an "emulsion," slowing down its evaporation.

The warm water temperatures also encourage the growth of oil-eating microbes, he said. These bacteria digest the oil in somewhat the same way that higher animals do sugar or fat, breaking it down into carbon dioxide and other less toxic molecules.

Fish and other creatures, however, may absorb oil but are incapable of breaking it down.

There's still a huge amount of oil under water, said McKinney - about 10 times the Exxon Valdez spill. If the goverment estimates are correct, about a quarter of it was broken up by chemical dispersants into tiny droplets that are easier for the microbes to digest. Another quarter is unaccounted for, either on shore or in the sea.

Where this oil ends up remains unknown, and definitive answers may take many months.

Never before has anyone sprayed chemical dispersants underwater. While breaking up the oil, they also kept it from rising to the surface.

The toxic effects of the underwater oil is just one worry. Another is the possibility that the oil and the methane mixed with it will feed a microbial bloom that will suck oxygen out of the water and suffocate other marine organisms, McKinney said.

The gulf already suffers from a huge hypoxic "dead zone" caused by agricultural runoff.

Another potential problem: The oil could stick to particles carried into the gulf by the Mississippi River and then sink to the bottom, creating what McKinney termed a "carpet of oiled sediment." That could damage unique communities of deep sea corals and other organisms that have colonized the sea floor.

Government estimates of the remaining oil contain huge uncertainties, McKinney said. "There's a lot of wiggle room."

Reports of damage also are difficult to pin down. Several months ago, for example, biologists noticed strange orange particles lodged under the shells of millimeter-long crab larvae. "We suspect it has something to do with the oil spill," said Caz Taylor, a biologist on a Tulane University team that is investigating the phenomenon.

The group is still running tests to see if the droplets carry any signature of the spilled oil or the chemicals added as dispersants.

"We haven't gotten anything conclusive yet," Taylor said.

In another ongoing investigation, researchers are comparing oyster shells collected after the spill with samples from previous eras, including some 100-year-old specimens from the Academy of Natural Sciences. How the post-spill shells incorporate the hydrocarbons and the chemical dispersants will help reveal whether these substances are moving through the food chain.

Even if the oil isn't concentrated enough to kill marine organisms, it could cause chronic toxicity, said the academy's Velinsky.

Tumors that have appeared on catfish in other polluted waters, including the Schuylkill, may have been caused by low levels of hydrocarbons, he said.

A major unknown is exactly where in the gulf the submerged plumes of chemical-laced oil are lurking. The scientists can go out in boats and take a few samples, Velinsky said, but that's a slow way to get at the location and extent of the plumes. "They're not going to be easy to find."

Over the coming months, teams of scientists from around the country will sample and test and examine the region in boats and submersibles.

"This is one giant experiment," said McKinney.