Are ant feces nutrients for plants? A metabolomics approach to elucidate the nutritional effects on plants hosting weaver ants

Weaver ants (genus Oecophylla) are tropical carnivorous ant species living in high numbers in the canopies of trees. The ants excrete copious amounts of fecal matter on leaf surfaces, and these feces may provide nutrients to host trees. This hypothesis is supported by studies of ant-plant interactions involving other ant species that have demonstrated the transfer of nutrients from ants to plants. In this 7-months study, a GC–MS-based metabolomics approach along with an analysis of total nitrogen and carbon levels was used to study metabolic changes in ant-hosting Coffea arabica plants compared with control plants. The results showed elevated levels of total nitrogen, amino acids, fatty acids, caffeine, and secondary metabolites of the phenylpropanoid pathway in leaves from ant-hosting plants. Minor effects were observed for sugars, whereas little or no effect was observed for organic acids, despite the fact that lower levels of total carbon were found in ant-hosting plants. The increased levels of total nitrogen, amino acids, fatty acids and caffeine and the decreased total carbon were consistent with changes observed in plants grown with an increased supply of nitrogen-containing nutrients. The up-regulation of the phenylpropanoid pathway could indicate biotic stress and/or nutrient deficiency. However, because nutrient deficiency was contradicted by the remaining results, this pathway up-regulation was ascribed to biotic stress caused by the physical presence of the weaver ants.