Analyzing indoor mycobiomes through a large-scale citizen science study of houses from Norway

Background: In the built environment, fungi can cause important deterioration of building materials and have adverse health effects on occupants. We here investigate what are the most important environmental variables (climate, building features or occupant characteristics) driving the indoor mycobiome structure and diversity in houses. We used Norway as study area since it spans a wide range of environmental gradients such as temperature and precipitation. We organized a large citizen science sampling campaign including 271 houses and 807 dust samples collected from three different house compartments: outside of the building, living room and bathroom. Fungal community composition and diversity were determined by DNA metabarcoding, and correlated to the environmental variables by multivariate statistical analyses.Results: The fungal community composition was clearly different between indoor and outdoor samples, but there were no significant differences between the two indoor compartments (i.e living room vs. bathroom). The 32 selected variables, related to the outdoor environment, building features and occupant characteristics, accounted for 15% of the overall variation in community composition, with the house compartment as the most important factor (7.6%). Next, the regional-scale climate was the main driver of the dust mycobiomes (4.2%), while building and occupant variables had significant but minor influences (1.4% and 1.1%, respectively). The house dust mycobiomes were mainly dominated by ascomycetes (⁓70%) with Capnodiales and Eurotiales as the most abundant orders. Compared to the outdoor, the indoor mycobiomes showed higher species richness, which is probably due to the mixture of fungi from outdoor and indoor sources. The main indoor indicator fungi belonged to two ecological groups with allergenic potential: xerophilic molds (mainly Penicillium and Aspergillus) and skin-associated yeasts (Malassezia, Debaryomyces, Candida and Rhodotorula).Conclusions: We show that indoor mycobiomes represent a mixture of fungi from indoor and outdoor sources, and that a multitude of indoor and outdoor variables structure indoor mycobiomes. We also demonstrate that citizen science is a successful and effective strategy to obtain samples for characterizing the indoor microbiomes at large geographical scales.