Microbial diversity in the floral nectar of Linaria vulgaris along an urbanization gradient

Microbes are common inhabitants of floral nectar and are capable of influencing plant-pollinator interactions. All studies so far investigated microbial communities in floral nectar in plant populations that were located in natural environments, but nothing is known about these communities in nectar of plants inhabiting urban environments. However, at least some microbes are vectored into floral nectar by pollinators, and because urbanization can have a profound impact on pollinator communities and plant-pollinator interactions, it can be expected that it affects nectar microbes as well. To test this hypothesis, we related microbial diversity in floral nectar to the degree of urbanization in the late-flowering plant Linaria vulgaris. Floral nectar was collected from twenty populations along an urbanization gradient and culturable bacteria and yeasts were isolated and identified by partially sequencing the genes coding for small and large ribosome subunits, respectively.

A total of seven yeast and 13 bacterial operational taxonomic units (OTUs) were found at 3 and 1% sequence dissimilarity cut-offs, respectively. In agreement with previous studies, Metschnikowia reukaufii and M. gruessi were the main yeast constituents of nectar yeast communities, whereas Acinetobacter nectaris and Rosenbergiella epipactidis were the most frequently found bacterial species. Microbial incidence was high and did not change along the investigated urbanization gradient. However, microbial communities showed a nested subset structure, indicating that species-poor communities were a subset of species-rich communities.

The level of urbanization was putatively identified as an important driver of nestedness, suggesting that environmental changes related to urbanization may impact microbial communities in floral nectar of plants growing in urban environments.