<Emphasis Type="Italic">Polytrichum commune</Emphasis> spores nucleate ice and associated microorganisms increase the temperature of ice nucleation activity onset

Moss spores disperse via wind and have been found previously in precipitation and air samples. Their presence in the atmosphere led to this study’s examining the potential of moss spores to contribute to ice nucleation, a process necessary for ice formation in clouds prior to precipitation. Ice nucleation assays were conducted using Polytrichum commune spores that were either associated with natural assemblages of microbes or extracted aseptically from capsules and subsequently confirmed to be free of culturable microbes. Liquid suspensions of capsule spores and non-sterile spores nucleated ice at temperatures as high as ?12 and ?7 °C, respectively. When capsule and non-sterile spores were heated at 95 °C for 10 min, which killed all culturable microbes on non-sterile spores, both nucleated ice from ?10 to ?13 °C. An additional non-sterile spore sample collected from partially opened capsules in a forested ecosystem (ID, USA) nucleated ice at temperatures as high as ?7 °C, similar to non-sterile P. commune spores. This is the first set of results to indicate that P. commune spores themselves are capable of nucleating ice at temperatures higher than many abiological particles such as mineral dust (≤?15 °C) and that natural assemblages of microbes can increase their ice nucleation efficiency. Future studies aimed at determining the abundance of moss spores in the atmosphere and the identity of ice-nucleating microbes associated with them will provide further insights into the ability of moss spores to impact precipitation patterns.