Nutritional sources of the fungus cultured by leaf-cutting ants

Leaf-cutting ants are an important component of Neotropical ecosystems, both as herbivores and soil modifying. Their influence on soils includes: (a) soil horizon alteration through soil removal; and (b) concentration of nutrients released from the refuse material dumped outside the fungus gardens. Accordingly, it is important to understand the nutritional characteristics of the fungus cultivated by the ants since they in turn reflect the ants' selectivity of the plant material collected as well as the chemical characteristics of the refuse material. We assessed the nutritional characteristics of the fungus from leaf-cutting ant nests of four Atta and Acromyrmex ant species from Cordoba, Argentina. Fungus samples were cultivated with different carbon and nitrogen sources. In carbon tests, the fungus grew with pectin and glucose only. In the nitrogen tests, the fungus developed with soluble sources only, both organic and inorganic (ammonium, asparagine, and urea). Although, it is widely accepted today that the fungus cultivated by leaf-cutting ants is a saprophytic basidiomycetes, our results indicate that the fungus utilizes soluble nutrients only thus lacking the ability to break down complex polymers that characterize saprophytic basidiomycetes. Accordingly, the fungus appears to behave as a typical biotrophic basidiomycete organism, functionally similar to a mycorrhiza. Ecological implications of the non-saprophytic nature of the fungus are wide-ranging, which change our perception of the ecological role of leaf-cutting ants. For example, a much larger volume of plant material needs to be harvested to sustain an ant's nest than if the fungus was saprophytic, which may explain the importance of leaf-cutting ants as agricultural pests. Moreover, a significant volume of non-used plant material has to be dumped as refuse which favors development of new food chains and changes in soil characteristics. (C) 2004 Elsevier B.V. All rights reserved.