Figure 46.8 Where Does the Extra Nitrogen Come From?

Although nitrogen gas (N₂) is a major component of Earth’s atmosphere (Figure 46.7), most organisms are unable to use nitrogen in this form. Thus, nitrogen is often a limiting nutrient in ecosystems, with most organisms dependent on microbes that are able to fix nitrogen, breaking N₂ into usable ammonium (NH₄⁺). Pinto-Tomás and colleagues recently investigated how leaf-cutter ants overcome their inherent nitrogen limitation. Leaf-cutter ants have a mutualistic relationship with fungus, which they cultivate in fungus gardens. Furthermore, they also have a symbiotic relationship with Actinobacteria that produce antibiotics, protecting fungus gardens from parasites. Do leaf-cutter ants have a third mutualist: a nitrogen-fixing bacterium? To test this, Pinto-Tomás et al. (2009) maintained ant colonies in the laboratory, preventing possible nitrogen inputs from the soil or via consumption of protein-rich insects by the ants in addition to their normal fungus diet. The researchers measured nitrogen contents at different levels of the colonies and observed that they differed significantly in nitrogen content, with ant workers containing the most nitrogen. Thus, nitrogen is clearly added as leaves are consumed by the fungus and the fungus is then eaten by the ants. To provide further evidence that symbiotic nitrogen-fixing bacteria are present, Pinto-Tomás et al. measured nitrogenase activity using acetylene reduction assays and found that such activity is highest in the fungus garden. The researchers also used a ¹⁵N₂-enrichment experiment to demonstrate that the nitrogen that is fixed in the fungus gardens is actually taken-up by the ants. After establishing the presence and localizing the nitrogen-fixing mutualist to the fungus gardens, the researchers went on to determine that the microbial mutualists are bacterial species in the genus Klebsiella. Interestingly Klebsiella species are also known to have symbiotic relationships with termites and fruit flies. In these taxa, the mutualism takes place in the host’s gut; the fungus garden in analogous to the host’s gut, given that the garden is the location of the digestion of the leaves collected by the ants.

Original Paper

Pinto-Tomás, A., M. A. Anderson, G. Suen, D. M. Stevenson, F. S. T. Chu, W. W. Cleland, P. J. Weimer, C. R. Currie. 2009. Symbiotic Nitrogen Fixation in the Fungus Gardens of Leaf-Cutter Ants. Science 326: 1120–1123.
http://dx.doi.org/10.1126/science.1173036

Links

Kimball’s Biology Pages: The Nitrogen Cycle
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/N/NitrogenCycle.html

Bernhard, A. 2010. The Nitrogen Cycle: Processes, Players, and Human Impact. Nature Education Knowledge 1(10): 12.
http://www.nature.com/scitable/knowledge/library/the-nitrogen-cycle-processes-players-and-human-15644632

National Science Foundation: Science Nation: Leaf-cutter Ants
http://www.nsf.gov/news/special_reports/science_nation/leafcutterants.jsp

ScienceDaily: Science News: Ants Use Bacteria to Make Their Gardens Grow
http://www.sciencedaily.com/releases/2009/11/091119141032.htm

University of Wisconsin: Department of Bacteriology: Currie Lab
http://currielab.wisc.edu/