Culturable bacteria are more common than fungi in floral nectar and are more easily dispersed by thrips, a ubiquitous flower visitor.

Variation in dispersal ability among taxa affects community assembly and biodiversity maintenance within metacommunities. Although fungi and bacteria frequently coexist, their relative dispersal abilities are poorly understood. Nectar-inhabiting microbial communities affect plant reproduction and pollinator behavior, and are excellent models for studying dispersal of bacteria and fungi in a metacommunity framework. Here, we assay dispersal ability of common nectar bacteria and fungi in an insect-based dispersal experiment. We then compare these results with the incidence and abundance of culturable flower-inhabiting bacteria and fungi within naturally occurring flowers across two coflowering communities in California across two flowering seasons. Our microbial dispersal experiment demonstrates that bacteria disperse via thrips among artificial habitat patches more readily than fungi. In the field, incidence and abundance of culturable bacteria and fungi were positively correlated, but bacteria were much more widespread. These patterns suggest shared dispersal routes or habitat requirements among culturable bacteria and fungi, but differences in dispersal or colonization frequency by thrips, common flower visitors. The finding that culturable bacteria are more common among nectar sampled here, in part due to superior thrips-mediated dispersal, may have relevance for microbial life history, community assembly of microbes, and plant-pollinator interactions.

Nectar bacteria stimulate pollen germination and bursting to enhance microbial fitness.

Many organisms consume pollen, yet mechanisms of its digestion remain a fundamental enigma in pollination biology,1-3 as pollen is protected by a recalcitrant outer shell.4-8 Pollen is commonly found in floral nectar,9,10 as are nectar microbes, which are nearly ubiquitous among flowers.11-13 Nectar specialist bacteria, like Acinetobacter, can reach high densities (up to 109 cells/mL), despite the fact that floral nectar is nitrogen poor.14-17 Here, we show evidence that the genus Acinetobacter, prevalent nectar- and bee-associated bacteria,12,18-20 can induce pollen germination and bursting, gain access to protoplasm nutrients, and thereby grow to higher densities. Although induced germination had been suggested as a potential method in macroscopic pollen consumers,2,21-23 and fungal inhibition of pollen germination has been shown,24-27 direct biological induction of germination has not been empirically documented outside of plants.28-32Acinetobacter pollinis SCC47719 induced over 5× greater pollen germination and 20× greater pollen bursting than that of uninoculated pollen by 45 min. When provided with germinable pollen, A. pollinis stimulates protein release and grows to nearly twice the density compared to growth with ungerminable pollen, indicating that stimulation of germination benefits bacterial fitness. In contrast, a common nectar-inhabiting yeast (Metschnikowia)33 neither induced nor benefited from pollen germination. We conclude that Acinetobacter both specifically causes and benefits from inducing pollen germination and bursting. Further study of microbe-pollen interactions may inform many aspects of pollination ecology, including floral microbial ecology,34,35 pollinator nutrient acquisition from pollen,2,3,21,36 and cues of pollen germination for plant reproduction.37-39.