A newly discovered organelle may hold the key to how much methane cattle burp out.
The organelle doesn’t belong to cows. It’s part of fuzzy single-celled protozoa called ciliates. The microbes live in cattle’s rumens, the first stomach of cud-chewing animals where grass and other plants are fermented and broken down. The new organelle is called a hydrogenobody. It makes hydrogen, which then stimulates other microbes in the rumen to produce the greenhouse gas methane, researchers report April 30 in Science.
The discovery could point to new ways to control methane emissions from cud-chewing animals such as cattle, sheep, goats and deer. Those ruminants account for about 30 percent of methane produced by agriculture.
Ciliates make up about a quarter of the microbes that live in the rumen but have not been studied much, says Ivan Čepička, a protistologist at Charles University in Prague who was not involved in the study.
Researchers in China have now filled some of that knowledge gap by cataloging DNA from ciliates living in the rumens of cattle and other ruminants. They found 65 species of ciliates, 45 of which had never had their DNA examined. Those species fell into major groups: Vestibuliferida, Entodiniomorphida and another unclassified family. Vestibuliferida species resemble Koosh balls because they are covered in cilia, while Entodiniomorphida tend to have a shock of cilia concentrated in one part of the cell.
This catalog of ciliates is a treasure trove of information for scientists who study ruminant microbiology, says Rainer Roehe of Scotland’s Rural College in Midlothian. He investigates how cattle’s genetics influence the microbes living in their rumens. Such libraries have been difficult to assemble because ciliates have lots of repetitive DNA and frequently exchange DNA with other microbes. That makes it hard to read parts of the DNA and to disentangle what DNA actually belongs to ciliates versus contamination from other organisms. To get around the contamination problem, the Chinese researchers had to isolate single ciliate cells to conduct their studies.
The team studied 100 dairy cows and found that the more ciliates cattle had, the more methane-producing microbes they had, and the more methane the animals produced.
Previous research has shown that hydrogen made by some organisms can stimulate microbes called archaea to make methane. Usually, hydrogen-producing organisms have organelles called hydrogenosomes, which are related to energy-producing mitochondria.
But studies have failed to definitively show where rumen ciliates produce hydrogen, Čepička says. In the new study, they finally show where in the cell is hydrogen produced. It’s in this newly detected compartment,” the hydrogenobody.
Hydrogenosomes have a double membrane like mitochondria do. But ciliates’ hydrogen factories have only a single membrane encasing them. Hydrogenobodies are located at the base of hairlike projections called cilia that give ciliates their fuzzy appearance.
Ciliates in the family Vestibuliferida are especially furry, have more hydrogenobodies, and stimulate more methane production than Entodiniomorphida, the researchers found. Strategies to remove Vestibuliferida ciliates from the rumen or inhibit their growth may reduce the amount of methane in ruminant burps, the researchers suggest.
Others have tried to wipe out ciliate protozoa from the rumen and have seen a drop in methane production, says Todd Callaway, a microbiologist and ruminant nutritionist at the University of Georgia in Athens. But that came at the cost of reduced milk and meat production.
Keeping protozoa out of the rumen is also a challenge, Callaway says. Cattle must be isolated in sealed barns, given sterilized feed and kept at least 200 meters from other cattle to avoid airborne transmission. Any intervention to reduce ciliate numbers would probably have to be ongoing, he says. Knowing the physiological differences between protozoa species may help in devising treatments to deplete specific ones to reduce methane emissions without compromising milk and meat production.
Any such treatments are still in the future, Callaway says. “This is step one of probably 25, but it’s a good step.”
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