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Micropredators and Oily Prey

 Deepwater Horizon 

The ecological effects of the 2010 Deepwater Horizon oil spill are still largely unknown. Josh Fischman, senior writer, is on the research vessel Endeavor in the Gulf of Mexico, with a team of university scientists seeking answers. He is filing reports from the ship. 

—100 miles off Pascagoula, Miss. 

Debby did Gulfport this past weekend. Or threatened to, enough to toss the Endeavor’s cruise plan up in the air. Tropical Storm Debby was barreling north across the gulf with 50-knot winds and 15-foot waves, but the forecasts were vague about whether she would turn east across Florida or west, right across Gulfport, Miss., and the area we want to study. The harbor in Gulfport is fairly exposed, and the captain didn’t relish the idea of staying in port and getting banged against the pier. So on Sunday we jogged four hours east, to a Coast Guard station and shipyard protected by an island at Pascagoula. It was fly-infested—the biting buggers were still on the ship days later—but it was quiet and it was safe.

And it gave Andrew Juhl a chance to talk about why he was on the ship. He was hunting for predators. Small single-celled predators, but still bigger than the oil-eating bacteria which they engulf with tiny whiplike appendages called flagella.

Juhl is a biological oceanographer who “didn’t even see the ocean until I was a teenager, because I grew up in Madison, Wisconsin,” he says. “But I was always interested in it, probably because I watched a lot of Jacques Cousteau as a kid.” He sees a lot of it now, as a research scientist at Lamont-Doherty Earth Observatory, part of Columbia University, where he holds an adjunct appointment and teaches. A slender, quiet man, Juhl spends a lot of his time near the water in Alaska, where he studies algae that grow inside sea ice, and on the water here in the South, where he has been part of the Ecogig, a group studying gulf ecology since 2010.

Here his interest is bacteria, in particular the kind that live off hydrocarbons like oil, or pieces of hydrocarbons, and a puzzle about them spewed by Deepwater Horizon. Every milliliter of seawater has about a million bacteria. What researchers found in the aftermath of the 2010 accident was that particular bacteria had started to degrade the oil.  But although their metabolic rates went up—the bacteria were more active—the population wasn’t growing by much.

“That’s sort of a paradox,” Juhl says. “You’d think if there’s a food source they’d start dividing more, and the population would increase a lot.” (Scarcity of nutrients like nitrogen, which are not a part of the oil, can limit population size, as one of Juhl’s colleagues, Samantha Joye of the University of Georgia, has pointed out. But not in this case, Juhl says. If lack of nitrogen was holding bacteria back then the metabolism would have stayed low along with population size.) The composition of the community changed—there were more bacteria that degraded alkanes, an oil component—but the overall population size didn’t go up much.

The explanation, Juhl thinks, lies in the next step up the ocean food chain: Micropredators, single cells just a few microns across that look like spheres with hairs sticking out of them, are grazing on the bacteria, thinning their ranks.

Read the rest at Chronicle.com. 

Image: Deepwater Horizon oil spill as seen from NASA's Terra Satellites, May 24, 2010. Photo by NASA's Earth Observatory, NASA Goddard Space Flight Center. This image is in the public domain.