Micropredators and Oily Prey

by John Fischman

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.

It’s just a hypothesis, Juhl cautions. But it rests on some observations of  oil-soaked sediments. The oil gets broken down faster when these micropredators are active than when they are not. Think about it, Juhl says. “If bacteria chow down on the oil, they get fat and lock up all the nutrients their bodies. But if you have an active predatory community eating up these big fatsos, they release the nutrients and make it available for other bacteria to use.” Metabolism goes up, but the population stays low.

And that could affect the ability of natural oil-degraders to eat, or not eat, leftovers from spills like Deepwater Horizon. “The point is that you can’t just look at one component of the food chain,” Juhl says. “You have to look at it holistically, and understand what is happening up and down the food web.”

And that is something, scientists on the Endeavor all say, that hasn’t been done, because they don’t have a lot of data from the gulf on these organisms. Juhl would like to begin to rectify that with simple counts of bacteria and their predators at different depths. After taking samples of water, he’s planning to add oil to some samples,  and add oil and nutrients to others. In still other samples, he plans to reduce the “encounter frequency” between micropredators and bacteria by adding filtered, predator-free seawater. Essentially he’ll be giving all the organisms more space to swim around, reducing their chances of engulfing and devouring.

Some of the experiments can be done on the ship, using an expensive piece of equipment that counts cells (something that’s more typically found in land laboratories and not lashed down to a bobbing water-borne lab table) and others will take place back at Lamont-Doherty, where a sophisticated microscope can determine the identity of those cells: Predator or prey.

“It’s just a little piece of the puzzle, but we’ll put it all together at some point,” Juhl says. He expects to have initial results by January, when there is a large oil spill research meeting in New Orleans.

Juhl talked about bacteria and doing experiments on a ship (“You have to be flexible. If something breaks, it’s not like you can run out to the hardware store for a part.”) late into the night. On Monday morning it was clear that Debby was going east, opening the door for the Endeavor to head back west. But first we headed south, making 10 knots in surprisingly calm seas, heading for Deepwater Horizon itself, roughly 100 miles from Pascagoula. The rig is gone, and the ruins of the wellhead only show up on nautical charts as the symbol for an underwater obstruction. The Endeavor arrived at 1 in the morning.

And the first thing we saw was another oil rig, perhaps a mile away. It was a stark reminder that drilling in the gulf goes on and is, as one of the scientists noted, a push to understand the effects of spills before another one happens.