Honey bees discriminate between nectars based on microbial, but not pollen, content

Background/Question/Methods: Microbes that inhabit floral nectar play an important and understudied role in the plant/pollinator mutualism. Fungi and bacteria may enhance or reduce pollinator affinity in a species-specific manner, ultimately impacting plant fitness. Microbial growth in nectar is limited by high osmotic pressure, antimicrobial plant secondary metabolites, and low nitrogen content, yet microorganisms are frequently detected at high cell densities. Pollen is commonly found in nectar samples and has been proposed as a nitrogen source that supports microbial growth. Further research has demonstrated that pollen increases yeast population growth within nectar. However, the specific chemical modifications of pollen on nectar chemistry, and pollinator response to the interacting effects of microbial inoculation and pollen content, have not been evaluated.

We investigated the impact of sunflower (Helianthus annus) pollen on nectar chemistry, population growth of a nectar specialist yeast, and the impact of these factors on pollinator preference in a full factorial design. We tested the impact of Metschinikowia reukaufii inoculation and pollen addition (0, 10, or 100 pollen grains/µL) in artificial nectar and measured yeast density, microbial- and pollen-derived volatile emission, nectar sugar and amino acid content, and honey bee (Apis mellifera) preference.

Results/Conclusions: Pollen significantly increased microbial population growth in nectar in a dose-dependent manner. Both pollen content and yeast inoculation impacted the volatile profile of nectar. Pollen contributed a variety of secondary metabolites, including some which exhibit antimicrobial activity, and increased the amino acid content of nectar. M. reukaufii reduced the abundance of pollen-derived volatiles and contributed microbial volatiles to nectar aroma. The addition of pollen dramatically impacted the proportion of sugars present in nectar, decreasing sucrose and increasing glucose and fructose, while yeast inoculation had no detectable effect on sugar abundance or composition. Honey bees preferentially foraged on uninoculated nectar over nectar containing yeast, and did not differentiate between nectars based on pollen content.

This work is the first to simultaneously examine the composite impact of pollen and yeast to nectar chemistry and pollinator affinity. Both yeast and pollen altered nectar chemistry, but pollinators responded only to the presence of yeast and did not differentiate between nectars based on pollen content. This work underscores the importance of nectar microbes and their metabolites in mediating pollinator preference and provides direct evidence for the importance of pollen in microbial growth dynamics within floral nectar.