Season and city identity structure urban bioaerosols, with minor differences across vegetation and socioeconomic gradients

Urban vegetation is unevenly distributed across socio-economic gradients, with lower-income neighborhoods often hosting less dense and diverse green spaces. This disparity may affect respiratory health, by influencing exposure to bioaerosols, airborne biological particles such as bacteria, fungi, and pollen. Understanding the composition, diversity, and abundance of bioaerosols could be essential for anticipating risks related to allergies and other respiratory conditions. In this study, we hypothesized that urban vegetation cover and neighborhood socio-economic status shape bioaerosol communities within and across cities. We sampled air across 65 sites in three Canadian cities of varying population size and density, Montréal, Québec City, and Sherbrooke, using an active air sampler throughout the growing season. We used amplicon sequencing to characterize bacterial, fungal, and pollen bioaerosols. Our results show that sampling period is the dominant driver of bioaerosol community composition, explaining up to 40% of variation in pollen, 29% in fungi, and 11% in bacteria. Significant differences in alpha diversity across seasons were also detected for fungi and pollen, but not for bacteria. In contrast, we found a small effect of vegetation cover on bioaerosols and almost none for median household income. These findings provide a critical baseline for understanding airborne biodiversity in cities and highlight the need for further research into how urban vegetation identity and diversity, rather than cover alone, may shape bioaerosol dynamics under changing climatic and urban conditions.