By Susan Milius
No pigs or chickens yet. But the vast farms where leaf-cutter ants raise their fungal crops may harbor a crew of previously overlooked farmhands — nitrogen-fixing bacteria.
At least eight species of leaf-cutter ants typically live with bacteria that capture nitrogen from the air and turn it into a form that living organisms can use, says microbial ecologist Adrián Pinto-Tomás of the University of Costa Rica in San José. He and his colleagues propose that these bacterial helpers might explain how the ants feed up to 8 million workers in a single colony just by harvesting bits of nitrogen-poor leaves and letting a fungus grow on them.
Neither the fungus nor the ants, nor any other multicellular organisms, can use the atmosphere’s abundant nitrogen directly. Pinto-Tomás and his colleagues tracked the path of nitrogen through ant nests and tested inhabitants for genes active in capturing the nutrient from the air. Live-in bacteria, particularly in the genus Klebsiella, could provide an estimated 45 to 60 percent of the nitrogen in the ants’ food, the researchers report in the Nov. 20 Science.
“I find it very exciting,” says entomologist Ted Schultz of the Smithsonian’s National Museum of Natural History in Washington, D.C. Fungus-growing ants “have become a model system for studying symbioses and coevolution,” Schultz says. “This discovery makes the system that much more interesting and that much more complicated.”