Submitted to: Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/27/2009
Publication Date: 11/20/2009
Citation: Pinto-Tomas, A.A., Anderson, M.A., Suen, G., Stevenson, D.M., Chu, F.S., Cleland, W.W., Weimer, P.J., Currie, C.R. 2009. Symbiotic Nitrogen Fixation in the Fungus Gardens of Leaf-Cutter Ants. Science. 326:1120-1123. Interpretive Summary: Leaf-cutting ants, through the maintenance of gardens of fungi, manage one of nature’s most active degradations of plant material. Understanding this ecosystem will provide important clues on how to improve cellulosic biomass conversion in the bioenergy industry. We have demonstrated degradation of plant material in these gardens occurs with minimal inputs of fixed nitrogen, due to the harnessing of nitrogen-fixing bacteria that convert nitrogen from the air into a form of nitrogen that the members of this ecosystem can use. This strategy of nitrogen utilization, if transferable to industrial cellulose bioconversion systems, can reduce the use of expensive nitrogen sources in biorefinery reactors.
Technical Abstract: Bacteria-mediated acquisition of atmospheric dinitrogen by plants serves as a critical nitrogen source in terrestrial ecosystems, and through its key role in agriculture, this phenomenon has shaped the development of human civilizations. Here we show that, paralleling human agriculture, cultivation of fungal crops by leaf-cutter ants involves fertilization from N2-fixing bacteria. Employing acetylene reduction and stable isotope experiments, we demonstrate that N2 fixation occurs in fungus gardens of eight leaf-cutter species and establish that the ants directly benefit from the presence of N2-fixing symbionts. Furthermore, specialized N2-fixers were consistently isolated from fungus gardens of 80 leaf-cutter ant colonies. The discovery of N2 fixation within the leaf-cutter ant-microbe symbiosis reveals an unrecognized nitrogen source in neotropical ecosystems and highlights the importance of symbiotic N2-fixers for sustainable agriculture.