Micropredators and Oily Prey

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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 byNASA’s Earth Observatory, NASA Goddard Space Flight Center. This image is in the public domain.

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