The plant microbiome can influence plant phenotype in disparate ways, but predicting when microbial communities influence plant-insect interactions remains challenging. A first step is to examine patterns of natural variation in microbial abundance across plant species and corresponding effects on plant phenotype. In some flowering plant species, bacteria and yeasts can influence nectar chemistry and pollinator behavior. We conducted a survey and field experiment to compare microbial abundance among plant species and link this naturally-occurring variation to floral traits and patterns of pollinator visitation. Standing crop nectar was collected from flowering plant species throughout the peak flowering season at Stebbins Cold Canyon Reserve in California, from March-June 2016. The presence, abundance, and species composition of bacteria and fungi in floral nectar was assessed using culture-based and culture-independent methods. Flower morphological, chemical, and pollinator traits were gathered for plant species included in the survey.
Plant species varied in the incidence and abundance of bacteria and fungi in floral nectar, and microbial abundance varied depending throughout the flowering period, with average population densities generally decreasing through the flowering season. Plant traits, including floral morphology and corolla depth, flower color, and pollination syndrome were linked to microbial abundance in nectar. Microbial abundance was related to the degree of microbial modification of floral nectar chemistry. This study generates hypotheses about when microbial communities may influence plant-pollinator interactions, and which plant traits may predict this effect. Our results suggest that pollinator identity and plant traits jointly influence microbial abundance and may feedback to determine when microbes attain high abundance and influence floral characteristics. More broadly, this comparative survey suggests that plant traits can influence microbial abundance and may be useful in predicting when the microbiome can influence plant-pollinator interactions.