Kai Ryssdal: It's been two years since the Deepwater Horizon oil rig exploded in the Gulf of Mexico -- 11 people were killed, the rig was destroyed, and by the time the wellhead was capped 87 days later, nearly 5 million barrels of crude had poured into the Gulf. Studies put the costs in the multiple billions of dollars for the fishing industry, the energy industry, tourism and environmental damage. The clean up continues to this day. One of the big questions people are trying to answer is where all the oil went.

Alex Chadwick, the host of the public radio series Burn: An Energy Journal, went to find out.

Alex Chadwick: Port Fourchon, La., with the last fragment of Mississippi Delta, a barrier island 9-miles long, 200-yards wide, the Gulf on the south, a marsh on the north. It stormed so hard the last two days, local schools closed. I see more squalls coming in the Gulf. John Pardue glances at the weather and then turns back to study the dark, damp gray sand, puddled from storm surge last night or from the rain.

John Pardue: The water and the waves, the erosions that have happened over the last 24 hours have exposed these areas.

He's an environmental engineer from Louisiana state, in Baton Rouge, looking for oil residue. This island is part of a reserve, he's trying to both help clean it and studying the impact of the spill.

Pardue: What you begin to realize when you're out here is that what you don't see it, it tends to be buried by the sand. So..

Chadwick: Just because you don't see it doesn't mean it is not here?

Pardue: It doesn't mean it's not here.

Deepwater Horizon was only about 40 miles away. This shore was hit pretty hard. Now Dr. Pardue stops and he bends down to a small clump.

Pardue: This is an oil mat that was exposed by this recent storm overnight. This is oil that has been deposited on the beach and then been uncovered as the sand has washed away.

Chadwick: It looks like a kind of a small, sandy brownie.

Pardue: Exactly. It certainly, unless you knew it was oil, if you walked upon on the beach and started touching it, you wouldn't know that that's what it was.

The clumps are slightly darker than the sand, slightly chocolate, and now I see many of them pocking the surface. Lightening flashes three miles offshore -- there are oil tankers out there and maybe a dozen drilling platforms.

Chadwick: You see all those oil platforms off there, those are all operating platforms?

Pardue: Some of them are random. Many of them are. This is definitely a working coast. You come down here and you're on the beach, but you're seeing a lot of oil activity immediately offshore.

There are accidental spills hundreds a year -- almost all of them small. But even without spills, the Gulf of Mexico has so much oil that maybe one million barrels seep naturally out of the seafloor every year. Dr. Pardue has been studying this ecosystem for decades, especially the role of microbes, bacteria. That natural oil seep coming out of the seafloor, they eat it.

Pardue: So it's using it as a food source, as a carbon source, just like you and I consume food.

They're too small to see, one-cell creatures -- a million of them would fit in the space of a pencil eraser.

Chadwick: The whole idea the microbes are eating oil -- I've read this before, but still it's hard to kind of accept that because you don't think of oil as being anything that you could eat.

Pardue: Oil is a product of organic matter that was deposited many, many years ago. It's gone through many, many changes deep in earth under high pressure, high temperature -- but fundamentally it's an organic material.

This is hard to grasp. Oil is food, an organic buffet for microbes. So, two years ago, when the BP oil plumed in the water, very soon the bacteria plumed, too. Different kinds in different parts of the ecosystem -- deep water, shallow, shoreline. But they all eat oil, and a lot of it, and quickly.

Pardue: So as long as the oil is available, as long as the bacteria can get access to it and all the conditions right for their growth, then you start seeing very dramatic kinds of changes that can happen over a short period of time.

It's not just the microbes; some of the oil burned off, some evaporated, some on the surface oil was swept up by skimmers, and some of it dispersed. But Dr. Pardue estimates the microbes are degrading about 20 percent, and maybe as much as 40 percent of all that came out of Deepwater Horizon.

The goop that washed up ashore in the days and weeks after the spill smothered whatever it encountered. You recall the pictures. Thousands and thousands of birds dead or injured, along with sea turtles, dolphins and whales. The exact numbers are still uncertain.

But the oil that reached the shoreline was not the same as what erupted out of the seafloor. The lightest elements, the most toxic, had evaporated. What washed up was not so dangerous and lasting. Gunk smeared on wildlife and marshes is bad. But there should not be what scientists call legacy effects, lingering deaths from toxins.

What should we do about all that oil? Political leaders were pushing scientists hard for an answer -- some techno-wizardry to get rid of it all. The answer was... nothing. We should do nothing.

The marsh habitat is delicate -- tromping all over it will make things worse. Leave it to the microbes. And after a flurry of beach clean up and bird rescue, and as hard as it is to do nothing in a crisis, that's what they did. The sweepers are gone now, the fires are out, the microbes are continuing to enjoy an extended free lunch.

Pardue: As the oil disappears, they die off -- but there's a core group of bacteria there that are able to respond when the spill in fact happens.

Chadwick: When I hear you say this I think to myself, the world has got some kind of immune system.

Pardue: Certainly these bacteria break things down in a predictable way, a consistent way, and they tend to be around to respond to lots of different kinds of things.

Dr. John Pardue, an environmental engineer from Louisiana State University in Baton Rouge. He's studying the lessons from the big oil spill two years ago. And the way oil development goes in the Gulf, we'll likely need those lessons again, and the tools we used last time -- and the microbes.

In Port Fourchon, La., I'm Alex Chadwick for Marketplace.

Ryssdal: Alex Chadwick is host of the public radio series Burn: An Energy Journal. It's from SoundVision Productions and produced in association with APM. Special thanks to the National Science Foundation.