Nycterohemeral airborne fungal and bacterial communities and health risks of potential pathogens in Shanghai†

Urbanization-influenced airborne microorganisms and megacity bioaerosols have garnered particular attention due to their importance in the atmospheric environment. Herein, the diurnal variation of airborne microbes in PM_2.5 between day and night was unveiled by a culture-independent approach and morphological analyses. The results demonstrated that airborne fungi, with values of 8636 copies per m³ (daytime) and 9443 copies per m³ (nighttime), and bacteria, with values of 38 725 copies per m³ (daytime) and 38 613 copies per m³ (nighttime), were observed under improved air quality. The recurrent diel cycle's airborne fungal and bacterial concentrations had no discernible difference. The airborne microbial community structure displayed robustness, in which Formitopsidaceae, Meruliaceae, Aspergillaceae, Rhizobiaceae, Caulobacteraceae, and Moraxellaceae dominated. The LefSE analysis revealed Sistotrema, Moesziomyces and Acinetobacter as diurnal biomarkers and Cercospora and Blastomonas as nocturnal biomarkers. In assessing the health risks of potential pathogens, the nocturnal samples contained a higher fungal relative abundance (38.52%), and diurnal samples had a higher proportion of potentially pathogenic bacteria (21.92%). SO₂ inhibited the fungal concentrations significantly. The observation of fungi by Scanning Electron Microscope (SEM) revealed the intact cell structure presumably annotated to Basidiomycota and Ascomycota and acted as the independent atmospheric particle matters of which surface harboured chemical composition. This study offers important insights into airborne fungi and bacteria in metropolitan cities, coupled with high throughput sequencing technologies and morphological observation under low PM_2.5 concentration.