Peat bog pools around Tamsweg (Lungau, Austria) are typical habitats of the unicellular green alga Micrasterias denticulata . By measurement of water temperature and irradiation throughout a 1-year period (2018/2019), it was intended to assess the natural environmental strain in winter. Freezing resistance of Micrasterias cells and their ability to frost harden and become tolerant to ice encasement were determined after natura...