Unveiling climate- and resource-driven Variation in Bumble Bee Gut Microbiomes: Perspectives from Elevational Gradients, Colony Translocation, and Climate Chamber Experiments

Rising temperatures negatively affect bumble bee fitness directly through effects on physiology, but they may also disrupt mutualistic interactions between bees and other organisms, which ultimately determine species-specific response to climate change. Gut symbionts, key regulators of host nutrition and health, may be the Achilles' heel of thermal responses in insects. They not only modulate biotic interactions with plants and pathogens but also exhibit varying thermal sensitivity themselves. Understanding microbiome community disruption due to environmental change is a crucial first step to determine potential consequences for host population responses. Here, we analysed the gut bacteria communities of six bumble bee species inhabiting different climatic niches along an elevational gradient in the German Alpes, using 16S ribosomal DNA amplicon sequencing. We first investigated whether inter- and intraspecific differences in gut bacterial communities can be linked to species' elevational niches, which differ e.g., in temperature, flower resource composition and, likely, pathogen pressure. With a reciprocal translocation experiment, attempted between two distinct climatic regions within Germany, we then tested in B. terrestris and B. lucorum, how gut bacterial communities of bumble bee workers change in short-term when exposing colonies to new environments. Finally, we exposed these species to heat and cold wave scenarios within climate chambers to disentangle pure temperature-driven effects on the microbiome from other environmental effects. Species-specific microbiome characteristics indicated strong evolutionary influences on microbial associations. E.g., the loss of gut microbiome stability, characterized species inhabiting predominantly high and cooler elevations. Short-term adjustment of the gut microbiome to a warmer region was more subtle and characterized by an uptake of Lactobacillaceae. However, the gut microbiomes of these bumble bees did not change under laboratory temperature scenarios. Cumulatively, our data suggest that the gut microbiome is rather altered by climate-related environmental conditions rather than by temperature effects alone, with bumble bees response to climate change being determined by prevailing species-specific differences that co-evolve over time.