Abstract
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 natural hardening and exposure to a cold acclimation treatment that simulated the natural temperature decrease in autumn. Transmission electron microscopy (TEM) was performed in laboratory-cultivated cells, after artificial cold acclimation treatment and in cells collected from field. Throughout winter, the peat bog pools inhabited by Micrasterias remained unfrozen. Despite air temperature minima down to −17.3 °C, the water temperature was mostly close to +0.8 °C. The alga was unable to frost harden, and upon ice encasement, the cells showed successive frost damage. Despite an unchanged freezing stress tolerance, significant ultrastructural changes were observed in field-sampled cells and in response to the artificial cold acclimation treatment: organelles such as the endoplasmic reticulum and thylakoids of the chloroplast showed distinct membrane bloating. Still, in the field samples, the Golgi apparatus appeared in an impeccable condition, and multivesicular bodies were less frequently observed suggesting a lower overall stress strain. The observed ultrastructural changes in winter and after cold acclimation are interpreted as cytological adjustments to winter or a resting state but are not related to frost hardening as Micrasterias cells were unable to improve their freezing stress tolerance.
Highlights
For half a century, the unicellular green alga Micrasterias denticulata has been used as a common cell biological research object (Kiermayer 1981; Lütz-Meindl 2016; Meindl 1993)
We hypothesize that (A) in peat bog pools inhabited by Micrasterias water remains unfrozen allowing the cells to escape freezing throughout winter. (B) We suggest that Micrasterias cells cannot survive ice encasement and are not able to frost harden
In the course of the year, the water temperature varied only by 23.8 °C which is less than half of the 54.5 °C air temperature variation
Summary
The unicellular green alga Micrasterias denticulata has been used as a common cell biological research object (Kiermayer 1981; Lütz-Meindl 2016; Meindl 1993). By means of high-resolution 2-D and 3-D electron microscopy, it was shown that Micrasterias cells aggregate and fuse their mitochondria to local networks during chilling and freezing stress, assuming to be important for low-temperature stress management (Steiner et al 2020). Despite these extensive studies, (1) only little is known under which environmental conditions Micrasterias survives winter and (2) if it tolerates ice encasement or can frost harden. Environmental conditions under which green algae, representing poikilohydric plants, survive winter are less investigated, but aquatic algae likely escape freezing by staying in the water body (Hampton et al 2017). Until now, ecophysiological aspects of winter survival in Micrasterias have not been investigated, and environmental parameters allowing winter survival have not been measured
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