Leaf-cutter ant refuse dumps are nutrient reservoirs harboring diverse microbial assemblages

Colonies of the leaf-cutter ant Atta colombica deposit hundreds of kilograms of waste material per year in concentrated locations on the forest floor. These refuse dumps contain significant-ly greater amounts of many important nutrients compared to surrounding soil and are primari-ly composed of partially degraded plant material and spent biomass from fungus gardens that the ants cultivate for food. Over the life of a colony, refuse material accumulates and dumps develop distinct vertical stratification. To better understand community dynamics within these nutrient-rich reservoirs, we generated almost 16,000 16S rRNA sequences and 508,000 16S rRNA pyrotag reads from five distinct strata within dumps of three A. colombica colonies. We also generated over 28,000 18S rRNA sequences from fungus gardens and refuse dumps. Proteobacteria, Actinobacteria, and Bacteroidetes dominated bacterial diversity. Estimates of beta diversity indicated that community structure was generally more similar among samples collected from equal depths, rather than from the same dump, suggesting a potential succession in community structure. Network analysis of co-occurring operational taxonomic units revealed numerous inter-lineage associations ranging from solitary pairs to larger assemblages, some of which were specific to particular strata. Our findings indicate that refuse dumps host a diverse and specialized community, likely evolved to exploit this unusual niche. This work represents a significant step in understanding the role microbial communities play in degrading recalcitrant plant biomass within leaf-cutter ant refuse dumps and highlights the potential of refuse dumps as a model for exploring patterns in microbial ecology.