Kangaroo gut microbes make eco-friendly farts

When kangaroos let one rip, the gas may be offensive to the nose but easy on the planet.

Marsupial toots and burps contain little or no methane, a potent greenhouse gas. A new study suggests that the scanty emissions are thanks to the distinct mix of microbes in the kangaroos’ gut. The study appears March 13 in the ISME Journal, a microbial ecology journal. By sniffing out the microbes responsible for the “green” gas, Australian researchers hope to glean ways of curtailing methane blasts from other grazing animals.

Fumes from farm ruminants, such as cows and goats, account for up to 25 percent of methane emissions per year from human-related activities. Since methane can cause about 20 times as much atmospheric warming as carbon dioxide, curbing methane would help slow global warming.

To get to the bottom of kangaroos’ low-methane farts and belches, microbiologist Scott Godwin of the Queensland Department of Agriculture, Fisheries and Forestry in Brisbane, Australia, and colleagues collected the foregut microbes of three wild eastern gray kangaroos, after they had been hunted for food. The researchers let the harvested microbes ferment in bottles alongside bottles of cow gut microbes, and then added a dash of molecules that produce carbon dioxide containing carbon-13, a heavy form of the element.

“The idea is to trace where the carbon goes,” says Godwin. In the foregut, a kangaroo’s last meal of greenery is broken down by fermentation, with carbon dioxide and hydrogen left over. In cows and other ruminants, microbes called methanogens gobble up those leftovers and transform them into methane. But in the kangaroos’ guts, the team found something different.

When the researchers sucked out gas from the kangaroo bottles, which smelled like manure with a hint of vinegar and parmesan cheese, they found little methane, as expected. In the milieu, they discovered a spike in acetate, a volatile fatty acid made by bacteria called acetogens. These microbes compete with methanogens to use the carbon dioxide and hydrogen.

To pinpoint the acetogens in the kangaroo mix, Godwin harvested all of the microbial RNA from one batch of kangaroo gut microbes. Any organisms using the carbon-13 labeled molecules would incorporate some of it into their RNA, he reasoned.

After sequencing the RNA labeled with carbon-13, the researchers identified known acetogens, methanogens and some unclassified microbes. Godwin says he’s not sure what all of the microbes do, but he was heartened to find the acetogen Blautia coccoides. The bacteria live in cows as well as in kangaroos, he says, so if researchers could find a way to enliven them in cattle, it could be a route to environmentally friendlier flatulence and belches.

In cows, methanogens usually outcompete acetogens because producing methane from fermentation’s leftovers is more energy efficient than making acetate, explains rumen microbiologist Peter Janssen of the New Zealand Agricultural Greenhouse Gas Research Centre in Palmerston North. “If you put them head to head, the methanogens will win, unless they’re suppressed in some way,” Janssen says.

Questions remain about how the kangaroo’s methanogens are subdued, Janssen says. The marsupial’s gut may be inhospitable to methanogens, or the animal’s microbial mix could fight off the methane-makers through their own defenses. “It’s an important first study,” he says, but it’s not the full story. “It’s the clue where to look.”