SPATIO-TEMPORAL VARIABILITY AND COMMUNITY-ASSEMBLY PROCESSES OF AIRBORNE BACTERIA ACROSS TWO URBAN AREAS

The study of spatio-temporal variability of airborne bacterial communities and the identification of their assembly processes have recently gained importance, due to the evidence that airborne bacteria are involved in atmospheric processes and can affect human health. In our work, we described the structure of airborne microbial communities in two urban areas (Milan and Venice, Northern Italy) through the sequencing, with the Illumina platform, of libraries containing the V5-V6 hypervariable regions of the 16S rRNA gene, and estimated the abundance of airborne bacteria with qPCR. We assessed the impact of environmental and meteorological conditions and of chemical composition of atmospheric particulate matter on microbial community structure. Our results indicated that airborne microbial communities in both cities were dominated by few orders, particularly Burkholderiales and Actinomycetales, more abundant in colder seasons, and Chloroplasts, more abundant in warmer seasons. Statistical analyses revealed that environmental factors, particularly relative humidity, explained a large part (approximately 50%) of the variation in bacterial community structure, and that these factors also exhibit a strong seasonality. Both local sources of bacteria, as suggested by the large abundance of Rhodobacterales in spring samples from Venice, and long-range dispersal, as indicated by the significant impact of wind speed, were involved in the assembling of airborne microbial communities. Moreover, the existence of possible selection processes on airborne bacteria was suggested by the significant effect of Ba, SO42- and Mg2+ on microbial community structure.