The abundance of nitrogen in fungus gardens grown by tropical ants is due to nitrogen-fixing bacteria living symbiotically with the insects. The discovery of the essential element's source solves a longstanding conundrum in insect chemical ecology.
The smallest farmers around, leaf-cutter ants play an essential role in nutrient cycling of tropical ecosystems by harvesting hundreds of kilograms of leaves off the forest floor per year and using them as fodder to grow fungus, which the ants use as food. Researchers trying to balance nutrient cycles have long been stumped by the high levels of nitrogen in the fungus gardens, compared with the low amount of nitrogen available from the leaves.
Now, researchers led by Cameron R. Currie, a bacteriologist at the University of Wisconsin, Madison, report that the extra nitrogen comes from the atmosphere, injected by nitrogen-fixing bacteria living in the fungus gardens (Science 2009, 326, 1120). Although nitrogen fixation has long been known to occur in the nodules of legumes, nitrogen-fixing symbiotic relationships between bacteria and insects are rare. Termites are one of the few well-studied insects also known to live symbiotically with nitrogen-fixing microbes.
This "really nice" work effectively rules out direct inorganic sources of nitrogen in fungus gardens, which play an essential role in tropical ecosystems, comments Dieter Spiteller, who studies leaf-cutter ants at the Max Planck Institute for Chemical Ecology, in Jena, Germany.