Bioaerosols in the Athens Metro: Metagenetic insights into the PM10 microbiome in a naturally ventilated subway station

Although numerous studies have investigated the physicochemical characteristics of airborne particulate matter in Metro transportation systems worldwide, the aerosol microbial content has been poorly explored. The aim of this study was to characterise the aerosol microbial abundance, diversity and composition in the Athens underground railway system. PM10 filter samples were collected from the naturally ventilated Athens Metro Line 3 station "Nomismatokopio". High throughput amplicon sequencing of the 16S rRNA gene and the internal transcribed spacer (ITS) region was performed on DNA extracted from PM10 samples. Results showed that the Athens Metro air microbiome was mostly dominated by bacterial and fungal taxa of environmental origin (e.g. Paracoccus, Sphingomonas, Cladosporium, Mycosphaerella, Antrodia) with a lower contribution of human commensal bacteria (e.g. Corynebacterium, Staphylococcus), underscoring the importance of both outdoor air and commuters as sources in shaping aerosol microbial communities. To our knowledge, this is the first study to characterise the mycobiome diversity in the air of a Metro environment based on amplicon sequencing of the ITS region. In conclusion, this study presents the first microbial characterisation of PM10 in the Athens Metro, contributing to the growing body of microbiome exploration within urban transit networks. Moreover, this work provides the foundation for further bioaerosol exposure monitoring and the data generated could facilitate the investigation of the vulnerability of public transport to airborne disease transmission.