Phyllosphere bacterial communities in urban green areas throughout Europe relate to urban intensity

The phyllosphere, the total leaf surface of plants, is regarded as one of the largest microbial habitats on earth. At this plant-atmosphere interface, a diverse and specific phyllosphere bacterial community (PBC) develops, which influences host plant health, productivity and ecosystem functioning. Insight in how PBC in urban trees are influenced by urban stressors is scarce. To our knowledge, none of the studies on urban phyllosphere bacteria evaluated the influence of urban green areas (UGAs) connectivity and size on the composition and diversity of PBCs. We studied the diversity and composition of PBCs of 233 Platanus x acerifolia and Acer pseudoplatanus trees in 77 UGAs throughout six European cities. Sampling was done twice, early and late in the growing season, and bacterial communities were studied using 16s rRNA sequencing. The most abundant families were the Sphingomonadaceae, the Hymenobacteraceae and the Acetobacteraceae. At the lower taxonomic level of amplicon sequence variants (ASVs), the community composition and alpha diversity differed between cities but not between tree species. Although no relationships with the alpha diversity were observed, the community composition of PBC related significantly with indicators of urban intensity, i.e. air pollution, surface temperature and size of the UGAs where the sample was taken. Our results showed that the PBC community composition became more distinct between geographic locations later in the growing season. We also found a significant correlation between the relative abundances for 29 out of the 50 most abundant families and the urban intensity. The relative abundances of more typical phyllosphere families, such as Acetobacteraceae, Planctomycetes and Beijerinkiaceae, decreased with urban intensity (i.e. more abundant in areas with more green, lower air pollution and lower temperature), whereas those related to human activities, such as Enterobacteriaceae and Bacillaceae, increased with urban intensity. The results of this study show that shifts in PBCs in European cities relate to urban intensity, an indicator of several combined stress factors.