Floral nectar is an important reward for pollinators, but is often colonized by pollinator-vectored yeast and bacteria that can attain high densities in nectar. These nectar inhabitants have been hypothesized to degrade floral resources, or conversely, to improve pollinator effectiveness. Changes in nectar chemistry have been implicated in both effects. Multiple species of yeast and bacteria can inhabit the nectaries of a hummingbird-pollinated shrub native to California, Mimulus aurantiacus. We conducted field and laboratory experiments to examine if nectar microbes affect plant-pollinator interactions via chemical changes in nectar. In a common garden, we inoculated newly opened flowers with yeast and bacteria that had been isolated from nectar in our field system: the yeast Metschnikowia reukaufii, an acetic acid bacterium (most likely Gluconobacter sp.), or a sucrose solution. We then exposed flowers to pollinators and measured stigma closure (a measure of pollinator visitation) two, four, and six days after inoculation. Additionally, to examine microbial effects on nectar chemistry, we inoculated PCR tubes containing 9 uL of M. aurantiacus nectar with the yeast M. reukaufii, Starmerella bombicola or the acetic acid bacterium. Six days after inoculation, we quantified changes in nectar chemistry, including changes in pH, hydrogen peroxide, sugars and amino acids.
We found that inoculation with the bacterium, but not Metschnikowia reukaufii, decreased hummingbird visitation of Mimulus aurantiacus, measured by stigma closure, compared to the sucrose control. In addition, microbial taxa differentially affected the chemistry of M. aurantiacus nectar. Specifically, M. reukaufii dramatically decreased nectar pH compared to the sucrose control and other microbe-inoculated nectar. In contrast, inoculation with S. bombicola only slightly decreased nectar pH, but quadrupled peroxide concentrations after six days of growth. The growth of microbes did not substantially affect the concentration of nectar sugars glucose, sucrose or fructose relative to the sterile control, but shifted the sugar composition to increase the ratio of monosaccharides:disaccharides. Our results indicate that 1) microbial taxa vary in their effects on plant-pollinator interactions and 2) microbial taxa also vary in their effects on nectar chemistry. Changes in nectar chemistry may play a key role in mediating the effects of nectar-dwelling yeast and bacteria on plant-pollinator interactions. Ongoing experiments seek to explicitly link taxon-specific changes in nectar chemistry to nectar removal and pollination rates in Mimulus.