Addition of pollen increases growth of nectar-living yeasts

Nectar is frequently inhabited by a limited number of microorganisms. Nonetheless, these species can quickly attain relatively high cell densities. This is quite surprising because of the limited availability of nutrients and unbalanced Carbon/Nitrogen ratios. Because nectar yeasts commonly aggregate around pollen and pollen grains are particularly rich in proteins, it has been suggested that the presence of pollen in nectar contributes to enhanced growth of yeasts in nectar, but compelling experimental evidence is still lacking. In this study, we conducted in vitro growth experiments to investigate whether the addition of pollen to sugar water increased growth of yeasts that naturally occur in nectar and honey provisions: Metschnikowia reukaufii, Starmerella orientalis and Torulaspora delbueckii. Our results indicate that yeasts benefit from the addition of pollen to a sugar-dominated medium, but the effects depended on type of pollen used. Overall, these results demonstrate that pollen plays an important role in the population dynamics of nectar-inhabiting yeasts and supports the idea that the chemical composition and the concentration of dehisced pollen may be more important factors determining the population growth of nectar yeasts than the chemistry of the nectar itself.