When the Deepwater Horizon accident spewed millions of barrels of oil into the Gulf of Mexico last year, surface bacteria launched into a feeding frenzy, a new study finds. But microbes that gobbled up the surface oil did so without increasing their numbers or gaining weight.
Waters in much of the Gulf are fairly mineral poor, at least in terms of what microbes need to flourish, says chemical oceanographer Benjamin Van Mooy of the Woods Hole Oceanographic Institution in Massachusetts. He and his team expected that microbes encountering the surface oil slick would turn up their figurative noses at the petroleum smorgasbord. To the scientists’ surprise, local bacteria pigged out, more than quintupling their normal daily intake with no increase in their mass. The researchers describe their findings online August 3 in Environmental Research Letters.
“You can imagine these bugs are like Richard Simmons,” whose Sweatin’ to the Oldies aerobics videos advocate exercise to rev up the body’s calorie expenditures, Van Mooy says. In the Gulf, “microbes can gorge on the petroleum buffet but not gain weight because their metabolism is high.”
To test the microbes’ dining habits, the Woods Hole researchers sampled water during last year’s Gulf gusher from five sites inside the surface slick and seven more upwind They dumped water from six of the sites into gas-tight jars that contained a new kind of sensor. Shining light on this sensor induced a fluorescent readout of the water’s oxygen content. Because bacteria use fairly predictable amounts of oxygen when they break down oil, oxygen depletion offered an indirect measure the bugs’ dining rate.
Enzyme measurements confirmed that Gulf bacteria — and especially the rapid diners inside the slick — suffered from a shortfall of phosphorus, a nutrient essential for growth. Offering affected bacteria extra phosphorus greatly boosted their feeding rate and their proliferation. But even without the dietary aid, the team found, it appeared Gulf microbes were breaking down oil at an unprecedented rate. “It’s a real mystery as to what’s going on,” Van Mooy says.
The new paper’s findings “are very interesting but not totally surprising,” says Terry Hazen of Lawrence Berkeley National Laboratory in California. Although microbial oil degradation in the surface slick proceeded faster than had been expected, he suggests this may reflect the bugs’ adaptation over millions of years to the large number of natural oil seeps in the Gulf of Mexico.
David Valentine of the University of California, Santa Barbara, also argues that the fact that bacterial cells didn’t blimp out after gorging on oil may not be all that puzzling. Gulf bacteria have evolved several mechanisms to store oil constituents that they don’t initially use as fuel, he explains. “Some bacteria transform the oil to produce dense particles within their bodies, like giant kidney stones, which effectively store their food energy for later use.”
Whatever the explanation, Valentine says, the new paper gives the first measurement of biodegradation of oil in surface slicks and “an important ecological context” for understanding its fate.