Symbiotic Nitrogen Fixation in the Fungus Gardens of Leaf-Cutter Ants

Bacteria-mediated acquisition of atmospheric N-2 serves as a critical source of nitrogen in terrestrial ecosystems. Here we reveal that symbiotic nitrogen fixation facilitates the cultivation of specialized fungal crops by leaf-cutter ants. By using acetylene reduction and stable isotope experiments, we demonstrated that N-2 fixation occurred in the fungus gardens of eight leaf-cutter ant species and, further, that this fixed nitrogen was incorporated into ant biomass. Symbiotic N-2-fixing bacteria were consistently isolated from the fungus gardens of 80 leaf-cutter ant colonies collected in Argentina, Costa Rica, and Panama. The discovery of N-2 fixation within the leaf-cutter ant-microbe symbiosis reveals a previously unrecognized nitrogen source in neotropical ecosystems.