Leaf-Cut­ters Get Their Fix (ni­tro­gen fix, that is)

by Elio

Ants span a great dis­tance within the hu­man psy­che, from be­ing in­signif­i­cant lit­tle nui­sances to oc­cu­py­ing a promi­nent place in the bi­o­log­i­cal scheme of things. They surely de­serve the lat­ter. Not only do they make up a large pro­por­tion of the an­i­mal bio­mass (up to 25% of the to­tal in the trop­ics), but their so­cial qual­i­ties make for sub­stantial mind food. It was not al­ways so. Their very name is ten­den­tious: it is de­rived from the West Ger­manic amaitjo, mean­ing "the biter," from Proto-Ger­manic ai-, "off, away" + mait- "cut." (If you see lit­tle con­nec­tion be­tween these roots and the word ant, keep in mind the Rules of Et­y­mol­ogy, as ex­pressed by a lin­guist friend: The vow­els don't count and the con­so­nants have ar­bi­trary val­ues.)

A sin­gle leaf-cut­ter ant fun­gus gar­den cham­ber with the queen (black ar­row). Scale bar: 1 cm. Source

Few things have the psy­che-tick­ling value of the leaf-cut­ter ants, those mas­ter agron­o­mists that some 50 mil­lion years ago learned to cul­ti­vate their fun­gal food­stuff. The leaves they so assidu­ous­ly bring into their nest be­come the sub­strate on which the fungi grow. To de­ter un­wanted fun­gal species that would oth­er­wise move into their "fun­gus gar­dens," the ants har­bor bac­te­ria (Pseudono­car­dia) that make se­lec­tive an­ti­fun­gal prod­ucts. But their agri­cul­tural en­deav­ors still leave us with a ques­tion: Where do the ants get their ni­tro­gen? The plant tis­sues they use are poor in ni­troge­nous com­pounds and, of course, nei­ther they nor their fungi can fix ni­tro­gen. We'll tell you in a minute, but first a star­tling di­gres­sion.

A partly ex­ca­vated nest of a ma­ture colo­ny. Source

The nests these leaf­cut­ter ants make are as­tound­ing in size, con­tain­ing as many as eight mil­lion worker ants and oc­cu­py­ing an un­der­ground vol­ume of more than 20 m³! That's about the size of a 100 square foot room, about the size of my small den. Re­searchers ex­ca­vated such a nest, with a stun­ning vi­sual ef­fect (al­though we don't ex­pect the ants ap­proved of it).

Now, how about the source of ni­tro­gen? The re­cent pa­per by the pi­o­neer myco-myrme­col­o­gist (that's a term we just made up for fun­gus-ant ex­pert) Cameron Cur­rie and col­leagues comes up with the an­swer: the fun­gus gar­dens con­tain ni­tro­gen-fix­ing bac­te­ria that pro­vide a sub­stantial pro­por­tion of the needed ni­tro­gen. The ev­i­dence? The ni­tro­gen con­tent is low in the plant food, greater in the fun­gus gar­dens, and greater still in the spent plant ma­te­r­ial that the ants carry to their mid­den-like refuse dumps. In ad­di­tion, the au­thors mea­sured acety­lene re­duc­tion, a tried and true method for mea­sur­ing the ac­tiv­ity of ni­tro­ge­nase, the en­zyme that car­ries out ni­tro­gen fix­a­tion. Ni­tro­ge­nase ac­tiv­ity was high­est in the fun­gus gar­dens, al­though the level var­ied with the species of ants. Lit­tle en­zyme ac­tiv­ity was as­so­ci­ated with the ants or their Pseudono­car­dia sym­bionts, but ac­tiv­ity was found in bac­te­ria iso­lated from the fun­gus gar­dens us­ing ni­tro­gen-free me­dia. Preva­lent were ni­tro­gen-fix­ing species of Kleb­siella and Pan­toea.

What does it take for these sym­bi­otic con­sor­tia to be suc­cess­ful? As the au­thors point out:"The eco­log­i­cal suc­cess of leaf-cut­ter ants is de­rived, in large part, from the com­bined abil­ity of the ants to break down plant an­ti­fun­gal bar­ri­ers and of the fun­gus gar­den to neu­tral­ize plant an­ti­in­sect tox­ins." As a re­sult, leaf-cut­ters can sub­sist on a large va­ri­ety of plants, many more than other her­bi­vores. But they must rely on ni­tro­gen-fix­ing sym­bionts for their ni­tro­gen. In this, they resem­ble ter­mites, which are known to have such bac­te­r­ial sym­bionts. Not only does ni­tro­gen fix­a­tion by the sym­bionts help the ants, it may also add fixed ni­tro­gen to the en­vi­ron­ment, via the de­tri­tus from the spent fun­gus gar­dens.

Leaf cut­ting ants left their hunt­ing-gath­er­ing mode of life and be­came farm­ers long be­fore hu­mans did. In the process, they have ac­quired some pretty amaz­ing uses for sym­bi­otic relation­ships. They sure know how to farm sus­tain­ably!

Ref­er­ence

Pinto-Tomas, A., An­der­son, M., Suen, G., Steven­son, D., Chu, F., Cle­land, W., Weimer, P., & Cur­rie, C. (2009). Sym­bi­otic Ni­tro­gen Fix­a­tion in the Fun­gus Gar­dens of Leaf-Cut­ter Ants Sci­ence, 326 (5956), 1120–1123 DOI 10.1126/science.1173036