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Microbial farm. A leaf-cutter queen ant tends her colony's fungus garden filled with a hodgepodge of microbes.

Michael Poulsen © Science/AAAS

Ants Eat Well, Thanks to Bacteria

Tropical leaf-cutter ants can't eat without a little help from their microbial friends. The insects drag inedible leaves into their massive subterranean lairs, where "gardens" of fungi break them down into a palatable, spongy white material. Meanwhile, bacteria keep the fungi healthy by secreting antibiotics. Now, it turns out that another microbe is needed to ensure that the ant has a balanced diet.

Leaves are rich in sugars, but they're low in a crucial nutrient: nitrogen. Some experts wondered whether the ants met their nitrogen needs by simply harvesting tons of leaves and then throwing away the excess. But evolutionary biologist Cameron Currie of the University of Wisconsin, Madison, couldn't believe that insects so efficient in their division of labor would be so inefficient in their eating habits. So he and colleagues decided to look for another source of nitrogen.

To rule out that nitrogen wasn't coming from the ground, the researchers collected ant colonies with their gardens and put them in plastic containers. Here, the only nitrogen source was the air, yet the leaves' nitrogen content still increased by 77% after passing through the garden.

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Farmer ant. A small worker leaf-cutter ant grooms its colony's fungus garden, picking out deleterious fungi pathogens.
Credit: Cameron R. Currie and Alison E. Stuart

Further investigation revealed two genera of nitrogen-fixing bacteria living in the fungal gardens. These bacteria convert nitrogen from the air into ammonia, which organisms can use to synthesize proteins. "We expected the fixers to be inside the ants' guts," as with the microbes termites use to get nitrogen, says Currie. "But ... the garden really is an external digestive system, so it makes sense for the fixers to be in the garden."

To see how much nitrogen the ants get from these microbes, Currie and colleagues measured the ants' levels of an isotope, nitrogen-15, which accumulates as organisms move up the food chain. The insects receive between 45% and 61% of their nitrogen from their bacterial friends, the team reports tomorrow's issue of Science. Over a year, a mature leaf-cutter colony may pull down about 1.8 kilograms of nitrogen from the air--equivalent to the nitrogen in 10,000 square meters of surrounding tropical soil. That may be why trees in the tropics clump around leaf-cutter nests, snaking their roots into the ants' dumps, Currie says.

The discovery of another symbiotic microbe in leaf-cutter ant fungal gardens is "very exciting," says etymologist Ted Schultz of the Smithsonian Institution in Washington, D.C. "When I first got into this stuff, we thought it was a ... two-partner symbiosis. It's turning out to be very complicated."