Ant fungus gardens fix atmospheric nitrogen

Sericomyrmex ants in a laboratory fungus garden

The textbook version of the leafcutter ant and its fungus is a simple story: attine ants cultivate an edible fungus in their nests.  They are obligate farmers, eating only the fungus, and the fungus is a specialized cultivar found only in ant nests.

It's a nice tale, but as researchers probe deeper they continually uncover just how complex the ant-fungus interaction is.  For example, about a decade ago Cameron Currie discovered that ants employ bacterial antibiotics to keep the garden clean of diseases.  Microbes, too, are integral to the system.

In this week's Science, a team led by Adrián Pinto-Thomás reveals another player in the partnership: bacteria in the genus Klebsiella. These organisms capture nitrogen from the atmosphere and make it available to the fungus and the ants.  They aren't just bit actors, either.  The bacteria may provide the primary fertilizer for the gardens.  Here's the abstract:

Bacteria-mediated acquisition of atmospheric N₂ 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 N₂ fixation occurred in the fungus gardens of eight leaf-cutter ant species and, further, that this fixed nitrogen was incorporated into ant biomass. Symbiotic N₂-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 N₂ fixation within the leaf-cutter antâmicrobe symbiosis reveals a previously unrecognized nitrogen source in neotropical ecosystems.

At this point I'd not be surprised if further research reveals ants to have a stash of John Deere tractors and little radios for tuning in to the morning report on mushroom futures.

source: Pinto-Tomás, Adrián A. et al. Symbiotic nitrogen fixation in the fungus gardens of leaf-cutter ants. Science 326, 1120-1123.

Doi: 10.1126/science.1173036

Additional coverage at Not Exactly Rocket Science and Science News.