Seasonal variations in airborne fungal communities and their ice-nucleation activities at forest environment

Ice clouds in the atmosphere are formed by ice nuclei. Bioaerosols, such as fungi and bacteria, originating from forests are expected to have a primary influence on ice formation in clouds to act as ice nuclei. However, microbial community structures and their ice-nucleation activities in the forest atmosphere remain poorly understood. This study performed the three-years survey in the forest environment to investigate the seasonal changes in fungal communities, which are related to the ice nucleation. The concentration of fungal cells increased slightly from spring to summer, decreasing in winter, due to the seasonal variations of temperature and humidity. The mushroom-type fungi of the class Agaricomycetes were dominated constantly, while the mold-type fungi of the classes Dothideomycetes and Sordariomycetes increased from spring to fall. At high altitudes of 700 m, the class Agaricomycetes sequences disappeared, while mold-type fungi dominated. The mold-type fungi of the genera <i>Apiospora</i> and <i>Sarocladium</i> can be isolated from both aerosols at forest floor and high altitude, showing high ice-nucleation activities (&lt;–9 °C). The ice-nucleating fungi is thought to be suspended from the forest floor to the above canopy. The ice-nucleation activities confirmed by isolates decreased gradually during spring, when the biological metabolism is activated in forest. In summary, some fungal populations including the class Sordariomycetes changed seasonally by interrelated factors, such as vegetation, humidity and temperature, and relating to the ice-cloud formation as ice nuclei, of which activities is enhanced during vegetation growing.