Do flower visitors introduce unique nectar microbial communities to strawberry flowers?
Plant-pollinator interactions have historically been viewed as bipartite interactions, but are now recognized to have another set of key interactors: floral microbes. Nectar-inhabiting microbes can modulate floral attractiveness and thus shape the success of pollination services. Floral resources are utilized by a wide variety of animals, which vary from efficient pollinators to opportunistic flower visitors. All flower visitors, not just the primary pollinators, could be important vectors of nectar microbes. This study sought to determine whether the identity of flower visitors has consequences for nectar microbial community structure.
We assessed the possibility that nectar microbe communities of strawberry flowers, Fragaria x ananassa (Rosaceae), might vary by flower visitor type. Virgin (un-visited) flowers were exposed to the natural flower visitor community and monitored until the first flower-visiting insect arrived. The flower visitor was collected and the flower was re-bagged. After 24 hours, nectar was collected to assess nectar microbe abundance and community composition via culturing on non-selective media. A subset of samples were examined with Illumina high-throughput sequencing for culture-independent analysis of community composition. Ultra-performance liquid chromatography (UPLC) was also used to determine if microbe communities differentially affected nectar sugar concentrations.
Nectar was collected and analyzed for 70 visited flowers and 53 un-visited (control) flowers. Honeybees and solitary bees were the most frequent flower visitors, but various flies, hemipterans, ants, and beetles were also observed. Culturing the nectar samples showed that all taxa, not just pollinating bees, are associated with culturable fungi and bacteria. However, there was no significant difference in the proportion of flowers with nectar microbes between different visitor types (including no difference between control and visited plates). Further, microbial abundance, as estimated by the percent cover of plate growth, did not differ in control versus visited flowers for bacteria or fungi. UPLC revealed that strawberry nectar is composed of equal concentrations of fructose and glucose.
Thus, in commercial strawberry flowers, the identity of flower visitors may not be the most important factor in nectar microbe community assembly. Dispersal of spores and bacterial propagules by wind may be important in flowers like strawberries, which have exposed nectar. Also, thrips (which could not be excluded from control flowers) might play an important role in nectar microbe dispersal, especially in agricultural systems where they are present in high densities.