While a spectrum of things, from cars to crops, have undergone adjustments to reduce their carbon footprints, a more unusual and difficult accomplishment is being explored in New Zealand: a low-emission sheep.

Ruminant animals like sheep and cows rely on a series of convoluted passageways and chambers called a rumen to ferment and digest the vegetable material they consume. They also employ vast quantities of microbes to break down otherwise indigestible plant components, such as grass, within this fermenting stomach.

Unfortunately, one player in this process -- an aptly named group of bacteria called methanogens -- produces methane, a gas 10 to 20 times more harmful for the atmosphere than carbon dioxide.

Sheep

This makes sheep, cows and other ruminants responsible for 28 percent of human-related methane releases, so researchers eager to create more planet-friendly pasture animals have been trying to understand both the rumen and its inhabitants, but so far without great success.

The difficulty primarily revolves around the fact that the atmosphere inside the sheep is an environment totally different from the outside air, one entirely devoid of oxygen.

"Just getting the samples out of the animals and into test tubes is difficult without exposing it to air," said Graeme Attwood, who calls himself a microbial geneticist, a profession he has evolved into over the past two decades as the field has changed.

"That's nothing on trying to replicate the complex nutrients they require to survive. We try, but it's hard."

To avoid this problem, researchers have abandoned cultivation altogether by sequencing the entire contents of a rumen sample, then slowly piecing back the results like a puzzle.


Last week, a team in New Zealand, led by Attwood of AgResearch Ltd., published some of the first major data future researchers may be able to use to control emission levels in ruminants, relying on this genetic approach to decode the stomach contents of low- and high-emitting sheep from a group his team has selectively bred for the past four years.

With help from the Department of Energy's Joint Genome Institute in California, the Lawrence Berkeley National Laboratory and the University of California, Merced, samples from Attwood's subjects revealed three new types of methanogens, ones previously elusive to classification.

Where do the electrons go in sheep?

Even more importantly, they found that these new bacteria's digestion pathways were responsible for more than 90 percent of methane production in their subjects.

Such specific findings will give future researchers more of a targeted view of what to try and control when trying to reduce emissions in ruminants.

Rod Mackie, a researcher at the University of Illinois who has worked with ruminant microbes for 45 years, said Attwood's study is a perfect example of the kind of genomic technology that has drastically changed his field, but added that this hasn't really made their work easier.

"Skipping cultivation has revolutionized the field, but you pay for it," he said. "In the huge soup of genetic material, you're left to reconstruct into some sort of sensible sketch of what's going on in an environment I equate to a zoo."

Mackie said the rumen supports levels of life not really seen anywhere else on Earth, and the diversity of the inhabitants is just as elaborate. "Really, Attwood's success is very good for the field," he said. "And honestly, it's very new."

To answer the question of how to reduce methane production, it is necessary to first understand an inevitable process of life, explained Mackie: "All organisms face the challenge of getting rid of electrons from what we eat."

It doesn't matter if you're a sheep, a human or a centipede; there is some part of every organism that is devoted to this reduction process, and in almost every case, there are microbes in tow.

Microbes: the ultimate garbage collectors

Microbes are like the ultimate garbage collectors in this sense, for unlike their human counterparts, they don't require customers to sort their trash, or even take it to the curb.

Microbes are completely devoted to their jobs, never leaving their place of work, picking though waste in perfect order and disposing of each element until there is nothing left.

But even in this disposal-centric community, life is still limited by the physical and chemical properties of their environment. Although there are different routes that this system of microbe and non-microbe life in the digestive tract can take to dispose of electrons, in the rumen, choices are limited.

"In some cases, animals produce ammonia or sulfates, but in the rumen, there is not enough of either of these chemicals to really make anything viable other than methane production," Mackie said.

This is why both the microbes and pathways favoring methane production dominate the world inside the rumen.

"The most common pathway is one in which, electron by electron, over a series of steps, CO2 is reduced to CH4 using hydrogen," Attwood said, "predominantly done by methanogens."

But knowing the culprits behind methane production by no means gives researchers any way to actually control them. For this, the genes of the microbes involved have to be traced and targeted, an expensive process, Attwood said. So the first step was the time-honored method, selective breeding.

Attwood first looked for sheep with enough variation to possibly note differences in their vast microbial communities, then, from their breed lines, the team selected 22 rams, feeding them alfalfa pellets typical of North American and New Zealand diets, and then used open-circuit respiration chambers to gather data on their emission rates.

Within the first generations, they found sheep producing roughly 40 percent less methane than others.

From this group, the team chose an even smaller set of animals for genomic sequencing. Selecting four of the highest and lowest emitters, plus two intermediates, they collected rumen samples and sent them off to California, waiting for the soup of data that would tell them what was going on in the stomachs of their drastically different emitting sheep.

Getting closer to the 'super' sheep?

Although the researchers expected many differences, what the genomic data revealed was a starling similarity between the samples. The only real difference between low and high emitters data was the activities of the different bacteria groups.

Pinpointing just who is producing the most methane in the rumen and how they do it could give hope to scientists' efforts to kill the high producers with vaccines. Mackie said a vaccine would likely continue to fail because the rumen itself remains the same; its conditions will continue to favor the same forms of life. Sheep will adapt back to their old, smelly ways, and using vaccines would require repeat dosing.

And there are other variables to consider. Most sheep in the world eat grasses and other natural materials that produce more hydrogen and less methane than the emissions of the laboratory animals. On top of this, suppressing methane production in the rumen could have damaging side effects.

"It's not like you can just change one part of the puzzle and hope the whole picture changes," Mackie said.

Mackie said he was struck by how large a difference existed among Attwood's sheep. On a given day, the low-emitting sheep produced 8 liters less methane than the more gassy sheep. It's a stark difference that points to the tantalizing possibility of a super low-emitting sheep that could help the world mitigate climate change.

Attwood said that right now, the team is happy with the progress it has made, a lot of which could be practically used for selecting future subjects.

"We can use this information, at the very least, to continue to select for lower emitters until we reach the theoretical lowest emitter, because, you know, this is still the natural world; there are always limits," he said.