Comparative analysis of bacterial diversity in clouds and aerosols

While the presence of condensed water in clouds could have influence on the diversity of bacteria circulating in the high atmosphere (multiplication or disruption of specific taxa), aerosols and clouds have been poorly studied comparatively in terms of microbiological content. Here we investigated the airborne bacterial diversity during cloudy and clear situations collected throughout the year at a high mountain site. Samples were collected using high flow rate impingers to examine total cell and DNA concentrations, as well as bacterial diversity by high -throughput sequencing. Overall very similar bacteria diversity was observed in clouds and clear air, with as high as 73.8% of the richness shared at the amplicon sequence variant (ASV) level. Bacterial diversity was more variable between sampling dates in the absence of cloud due to a wider range of meteorological conditions, but within single air masses, cloudy situations were more heterogeneous due to higher proportions of low-abundance taxa. Some taxa were found more abundant in clouds compared with clear situations, such as Staphylococcus, Acinetobacter, Kocuria and Enhydrobacter, or conversely, like Bacillus. Except for Kocuria, these could all be explained by concomitants factors other than the presence of cloud, i.e., season and altitude of the atmospheric boundary layer. This work emphasizes that airborne bacterial assemblages are driven by numerous inter-connected environmental factors hardly distinguishable from each other. Replicated sampling help deciphering their respective roles, and such practice must be applied more widely in the field of aerobiology.