Dispersants have been used in oil spills before and had often been considered as the main line of defense against such catastrophes. But when Joye started looking into the evidence behind this position, she became concerned. Some papers claimed that they help bacteria to break down oil, others said that they slow the process, and yet others found no effect. The problem, Joye realized, was that they were all using different measurements, most of which were indirect.

To get better answers, her team, including ex-student Sara Kleindienst, collected water from a site in the Gulf of Mexico where hydrocarbons naturally seep into the oceans. Back in the lab, they flooded the water with either oil, dispersants, or both, at concentrations and temperatures that mimicked the Deepwater spill and its aftermath. “The goal was to recreate the Deepwater plume as best we could,” says Joye.

The team analyzed these microcosms with a battery of techniques. They counted microbial cells, and sequenced them to identify the species that were present. They quantified the enzymes made by those microbes to gauge how active they were. They checked the production of microbial molecules used to break down the oil. And they used slightly radioactive oil compounds to directly measure the rate at which those substances were being degraded. “No one has ever done all of that in a single experiment,” says Joye.

These techniques showed that dispersants fueled the growth of Colwellia bacteria, which went from 1 percent of the total microbes to up to 43 percent. Colwellia has sometimes been billed as an oil-degrader, but its presence was minimal in the oil-only water. If anything, it was more of a Corexit-degrader.

Marinobacter, however, is a bona fide oil-busting specialist. In oil-only water, its went from 2 percent of the total microbes to 42 percent. But Corexit curtailed this ascendance, either by affecting Marinobacter directly, or by boosting competitors like Colwellia. Either way, the dispersants suppressed all strains of Marinobacter. And, according to the team’s varied techniques, the dispersants reduced the breakdown of hydrocarbons from the oil.

“So what happened to the hydrocarbons?” asks Joye. In another recent paper, written on the fifth anniversary of the disaster, she estimated that microbes broke down 43 to 61 percent of the oil, while 2 to 15 percent sank to the sea floor. “We we really can't account for 24 to 55 percent of the oil. Where is it? I don't know and it really bothers me. I suspect that some is in marshes, some is on beaches, and probably more than we think is on the seafloor. But we did a very poor job—and I include myself here—of measuring hydrocarbon degradation.”

Should authorities avoid dispersants in the future? “That's an extraordinarily complicated question,” says Joye. Corexit has its problems, but it does seem to keep oil away from coasts. “Nobody wants to see oiled birds, turtles, and dolphins, but the bottom line is that if you disperse that oil, it's still in the water. You feel better, but is it improving the situation? My gut instinct is that I would put my faith in the microbial communities to do their job.”