Ant farmers use bacteria to make their gardens grow

In a new research, scientists have found that ant farmers, like their human counterparts, depend on nitrogen-fixing bacteria to make their gardens grow.

The finding 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.

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," said University of Wisconsin-Madison bacteriologist Cameron Currie.

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 and the plant biomass nitrogen is lower than what the insects need for survival," said Currie.

Enter the nitrogen-fixing bacteria, two species of which were isolated in laboratory and field colonies of the ants.

But merely finding the bacteria, Suen emphasizes, wasn't enough. It was necessary to prove that the ants were actually utilizing the nutrient to confirm a true mutualism.

"This is important because it could be that the bacteria are fixing nitrogen for themselves and not actually benefiting the ants," said Garret Suen, a UW-Madison postdoctoral fellow and a co-author of the new study.

"Showing that the nitrogen fixed by the bacteria is incorporated into the ants establishes that these bacteria aren't just transient visitors," Suen added.

However, the discovery of the nitrogen-fixing mutualism in ants has significant ecological implications given the dominance of ants in virtually all of the word's terrestrial ecosystems.

The new work suggests that an important source of nitrogen in the American tropics and subtropics is derived through the partnership of ant and bacteria.

"It is possible that this fixed nitrogen can have ecosystem scale impacts," said Currie.