MADISON Leaf-cutter ants, which cultivate fungus for food, have many remarkable qualities.
Here's a new one to add to the list: the ant farmers, like their human counterparts, depend on nitrogen-fixing bacteria to make their gardens grow. The finding, reported this week (Nov. 20) in the journal Science, documents a previously unknown symbiosis between ants and bacteria and provides insight into how leaf-cutter ants have come to dominate the American tropics and subtropics.
What's more, the work, conducted by a team led by University of Wisconsin-Madison bacteriologist Cameron Currie, identifies what is likely the primary source of terrestrial nitrogen in the tropics, a setting where nutrients are otherwise scarce.
"Nitrogen is a limiting resource," says Garret Suen, a UW-Madison postdoctoral fellow and a co-author of the new study. "If you don't have it, you can't survive."
Indeed, the partnership between ant and microbe permits leaf-cutters to be amazingly successful. Their underground nests, some the size of small houses, can harbor millions of inhabitants. In the Amazon forest they comprise four times more biomass than do all land animals combined.
"This is the first indication of bacterial garden symbionts in the fungus-growing ant system," says Currie, a UW-Madison professor of bacteriology.
A critical finding in the new study, according to the Wisconsin scientist, is that the nitrogen, which is extracted from the air by the bacteria, ends up in the ants themselves and, ultimately, benefits the nitrogen-poor ecosystems where the ants thrive.
The fungus-growing ants, Currie notes, are technically herbivores. They make their living by carving up foliage and carrying it back to their nests in endless columns to provide the raw material for the fungus they grow as food. "But plant-feeding insects are known to be nitrogen limited," explains Currie, "and the plant biomass nitrogen is lower
|Contact: Cameron Currie|
University of Wisconsin-Madison