Microbial community structure and function in floral nectar are shaped by variation in dispersal

Background/Question/Methods

Symbiotic microbes can substantially influence how their hosts interact with the environment. Recent work suggests that plant-associated microbes can affect plant interactions with their biotic environment, pollinators, by changing nectar chemistry. Because microbes can vary greatly in their role as symbionts, determining the processes that shape species composition of symbiotic microbes is important to understanding their influence on host-environment interactions. Here, we examine how one process, dispersal, shapes microbial communities in floral nectar and their effects on nectar chemistry. Multiple modes of dispersal potentially exist, including via different pollinators and other flower-visiting animals and via wind and water. These characteristics of nectar microbial communities make them uniquely conducive to addressing the effects of dispersal mode.

At the Jasper Ridge Biological Preserve in California, we manipulated microbial access to flowers of Mimulus aurantiacus to examine how different dispersal modes influenced microbial community assembly within floral nectar. Mimulus aurantiacus is primarily pollinated by hummingbirds, but insects that may also vector nectar microbes visit flowers. Flowers were bagged (no visitors), caged (excluding large-bodied visitors), or exposed (no visitors excluded). Yeast and bacterial communities within floral nectar were described using plating, Illumina sequencing and molecular fingerprinting, and nectar chemistry was examined by liquid chromatography.

Results/Conclusions

Bagged flowers hosted the lowest abundance of nectar-dwelling bacteria and yeasts, whereas flowers exposed to large-bodied pollinators hosted the highest microbial abundance. Pollinator exclusion treatment shaped the community composition of bacteria in nectar, where distinct communities formed in the nectar of bagged, caged, and exposed flowers. Bacterial communities in caged flowers (dispersed by small insects), but not bagged or exposed flowers, showed significant spatial signal at the scale examined (~0.5-1 km). One dominant bacterial genus, Acinetobacter, was found primarily in flowers exposed to large bodied pollinators. Nectar chemistry also varied with pollinator exclusion treatments.

Taken together, these results indicate that different dispersal modes shape the community composition of symbiotic nectar microbes differently. Furthermore, the results show variation among microbial taxa in reliance on pollinators for dispersal, where yeasts and some bacterial taxa were found only in flowers where large-bodied pollinators were allowed access. The strong influence of variation in dispersal mode suggests that the assembly process can have substantial influence over the structure and function of symbiotic microbial communities that are environmentally acquired.