Nitrogen fixation by diverse diazotrophic communities can support population growth of arboreal ants

Abstract

Background Symbiotic ant-plant associations, in which ants live on plants, feed on plant-provided food, and protect host trees against threats, are ubiquitous across the tropics, with the Azteca-Cecropia associations being amongst the most widespread interactions in the Neotropics. Upon colonization of Cecropia’s hollow internodes, Azteca queens form small patches with plant parenchyma, which are then used as waste piles when the colony grows. Patches—found in many ant-plant mutualisms—are present throughout the colony life cycle and may supplement larval food. Despite their initial nitrogen (N)-poor substrate, patches in Cecropia accommodate fungi, nematodes, and bacteria. In this study, we investigated the atmospheric N2 fixation as an N source in patches of early and established ant colonies. Results Via 15N2 tracer assays, N2 fixation was frequently detected in all investigated patch types formed by three Azteca ant species. Quantified fixation rates were similar in early and established ant colonies and higher than in various tropical habitats. Based on amplicon sequencing, the identified microbial functional guild—the diazotrophs—harboring and transcribing the dinitrogenase reductase (nifH) gene was highly diverse and heterogeneous across Azteca colonies. The community composition differed between early and established ant colonies and partly between the ant species. Conclusions Our data show that N2 fixation can result in reasonable amounts of N in ant colonies, which might not only enable bacterial, fungal, and nematode growth in the patch ecosystems but according to our calculations can even support the growth of ant populations. The diverse and heterogeneous diazotrophic community implies a functional redundancy, which could provide the ant-plant-patch system with a higher resilience towards changing environmental conditions. Hence, we propose that N2 fixation represents a previously unknown potential to overcome N limitations in arboreal ant colonies.

Dynamics and drivers of fungal communities in a multipartite ant-plant association

“…Second, Azteca workers deposit plant tissue, ant feces, and ant corpses onto patches and constantly shape and manipulate them [24,32,47,48]. Third, patch structures were found in almost every internode of the 93 colonies investigated, even in those with brood [48]. Last, none of the Azteca colonies inhabiting Cecropia stems from this study were found without patches in their nest.…”

“…First, it was observed in 180 Cecropia saplings that the Azteca queens form the first patch before they start to lay their eggs [47]. Second, Azteca workers deposit plant tissue, ant feces, and ant corpses onto patches and constantly shape and manipulate them [24,32,47,48]. Third, patch structures were found in almost every internode of the 93 colonies investigated, even in those with brood [48].…”

“…In return, Cecropia trees provide ants with a nesting space inside the hollow stem (domatium) and plant-derived food bodies known as Müllerian bodies [44][45][46]. In this association, fungi, as well as bacteria and nematodes, are transgenerationally transmitted by the foundress queen who transfers these organisms to a self-made pile of parenchyma known as "patch" [25,35,[47][48][49][50]. Several observations provide evidence of the importance of these patches for the Azteca-Cecropia association (Fig.…”

“…Although the patches and the fungi they contain are recognized as permanent components in the Azteca-Cecropia mutualism [25,31,47,48], nothing is currently known about the establishment of the fungal communities during the life cycle of ant colonies nor of the influence of the inhabiting ant species and the host-plant species. By analyzing amplicon sequence data of the ITS2 region, we investigated the fungal communities inhabiting patches of 93 colonies from three different Azteca species inhabiting Cecropia spp.…”

“…By analyzing amplicon sequence data of the ITS2 region, we investigated the fungal communities inhabiting patches of 93 colonies from three different Azteca species inhabiting Cecropia spp. Based on previous research [42,47,48,51], we hypothesize that fungal diversity increases during ant colony development due to the increasing foraging and patrolling activity while the colony grows. This leads to the incorporation of spores or hyphal fragments from the environment into the patches.…”