Pollen and yeast change nectar aroma and nutritional content alone and together, but honey bee foraging reflects only the avoidance of yeast

Abstract

Summary Floral nectar often contains pollen and microorganisms, which may change nectar's chemical composition, and in turn impact pollinator affinity. However, their individual and combined effects remain understudied. Here, we examined the impacts of the nectar specialist yeast, Metschnikowia reukaufii, and the addition of sunflower (Hellianthus annus) pollen. Pollen grains remained intact, yet still increased yeast growth and amino acid concentrations in nectar, whereas yeast depleted amino acids. Pollen, but not yeast, changed nectar sugar concentrations by converting sucrose to its monomers. Both pollen and yeast contributed emissions from nectar, though yeast volatiles were more abundant than pollen volatiles. Yeast volatile emission was positively correlated with pollen concentration and cell density, and yeast depleted a subset of pollen‐derived volatiles. Honey bees avoided foraging on yeast‐inoculated nectar and foraged equally among uninoculated nectars regardless of pollen content, underscoring the importance of microbial metabolites in mediating pollinator foraging.

Sweet solutions: nectar chemistry and quality

“…Contamination with pollen falling into nectar could lead to high levels of proline and other amino acids [ 89 , 90 ]. While deliberate exposure to pollen did not increase amino acids in nectar of Aloe marlothii [ 86 ], sunflower pollen added to synthetic nectar leached amino acids, and pollen contamination in visited flowers of Gentiana lutea enriched the amino acid profile in nectar [ 90 , 91 ]. Recently, bacteria in nectar have been shown to induce pollen germination and bursting, thus increasing protein levels [ 92 ].…”

Do amino and fatty acid profiles of pollen provisions correlate with bacterial microbiomes in the mason beeOsmia bicornis?

“…Differences in floral microbial community composition interact with differences in floral chemistry. For example, microbial communities in nectar and on flowers vary with floral volatile compounds or plant secondary metabolites, indicating a tight link between flower chemistry and the present microbiota [44][45][46][47][48]. Moreover, pollen protein content can vary between 2.5% and 61% [49] and pollen lipid content between 1% and 20% [50,51] dry mass.…”

The effects of urban land use gradients on wild bee microbiomes

“…Areas with a low percentage of development were found to have a greater abundance of Acinetobacter, Ascosphaera, Saccharibacter, and Taphrina ( Supplementary Table S4 ). Acinetobacter is a flower-associated species of bacteria also commonly associated with yeasts in nectar which can induce germination and pollen bursting that then benefits pollinators by way of improved nutrition from nectar ( Christensen et al, 2021 ; Rering et al, 2021 ). Another flower-associated bacteria, Saccharibacter , is closely related to the bacteria Bombella apis which is known to protect developing honey bees from fungal pathogens and contains genetic loci involved with nutrition, microbial and host interactions, and immunity ( Smith and Newton, 2020 ).…”