The indoor mycobiome of daycare centers is affected by occupancy and climate

Many children spend considerable time in daycare centers and may here be influenced by indoor microorganisms, including fungi. In this study, we investigate the indoor mycobiome of 125 daycare centers distributed along strong environmental gradients throughout Norway. Dust samples were collected from doorframes outside and inside buildings using a citizen science sampling approach. Fungal communities in the dust samples were analyzed using DNA metabarcoding of the ITS2 region. We observed a marked difference between the outdoor and indoor mycobiomes. The indoor mycobiome included considerably more yeasts and molds compared to the outdoor samples, with Saccharomyces, Mucor, Malassezia and Penicillium among the most dominant fungal genera. Changes in the indoor fungal richness and composition correlated to numerous variables related to both outdoor and indoor conditions; there was a clear geographic structure in the indoor mycobiome composition that mirrored the outdoor climate, ranging from humid areas in western Norway to drier and colder areas in eastern Norway. Moreover, the number of children in the daycare centers, as well as various building features, influenced the indoor mycobiome composition. We conclude that the indoor mycobiome in Norwegian daycare centers is structured by multiple factors and is dominated by yeasts and molds. This study exemplifies how citizen science sampling enables DNA-based analyses of a high number of samples covering wide geographic areas.