Biogeographic patterns and processes that impact the assembly and structure of microbial communities have recently been illuminated. However, the importance of interactions between microbial communities (i.e., microbiomes) and higher trophic levels in structuring continental scale patterns of microbial biodiversity is not yet understood. Here, we examined the impact of environment-host-microbial interactions on the biogeography of the microbiome inhabiting the northern pitcher plant (Sarracenia purpurea), which supports a multi-trophic level food web that is a model system in community ecology. S. purpurea pitchers were sampled at 36 sites across the north-south distribution of its North American range, from Florida to Québec. For each plant sampled, pitcher physical characteristics (e.g., pitcher length and volume) were recorded in the field, the abundance of higher-order organisms (e.g., flesh flies, midges) was quantified through direct counts, and high-throughput sequencing of ribosomal rRNA genes was utilized to characterize the pitcher microbiome. From this, distance-based linear models were used to explore relationships between microbial diversity, pitcher characteristics, food web structure, and geographic distance.
Results/Conclusions
Bacterial communities inhabiting S. purpurea pitchers exhibited biogeographic structure across the plant’s North American range and were impacted by the location, environment and morphology of their plant host as well as species encompassing higher trophic levels in the pitcher ecosystem. A strong latitudinal effect was observed, as the pitcher microbiome decreased in richness with increasing latitude (-30%), and further, geographic distance accounted for the greatest proportion of variation in microbiome composition across the range (11%). In addition to latitudinal effects, the abundance of organisms encompassing higher trophic levels in the S. purpurea food web accounted for a significant proportion of the variation in microbiome composition (14%). Further, members of higher trophic levels exerted contrasting forces on bacterial richness, with the mosquito and midge being associated with greater bacterial richness (+14 and +12%, respectively), whereas bacterial richness decreased when flesh flies were present (-9%). Pitcher morphology also impacted the pitcher microbiome, with pitcher volume, rosette diameter, and chlorophyll content accounting for ~15% of the variation in microbiome composition across the range. Overall, the results suggest that the microbiome inhabiting S. purpurea pitchers exhibits biogeographic characteristics across its North American range and is constrained by a combination of plant-associated, environmental, and geographic factors.