Abstract

Green algae of the genus Zygnema form extensive mats and produce large amounts of biomass in shallow freshwater habitats. Environmental stresses including freezing may perturb these mats, which usually have only annual character. To estimate the limits of survival at subzero temperatures, freezing resistance of young Zygnema sp. (strain MP2011Skan) cells and pre-akinetes was investigated. Young, 2-week-old cultures were exposed to temperatures of 0 to – 14 °C at 2-K steps, whereas 8-month-old cultures were frozen from − 10 to – 70 °C at 10-K intervals. Cell viability after freezing was determined by 0.1% auramine O vital fluorescence staining and measurements of the effective quantum yield of photosystem II (ФPSII). At – 8 °C, the young vegetative cells were unable to recover from severe frost damage. But temperatures even slightly below zero (− 2 °C) negatively affected the cells’ physiology. Single pre-akinetes could survive even at – 70 °C, but their LT50 value was − 26.2 °C. Severe freezing cytorrhysis was observed via cryo-microscopy at – 10 °C, a temperature found to be lethal for young cells. The ultrastructure of young cells appeared unchanged at – 2 °C, but severe damage to biomembranes and formation of small foamy vacuoles was observed at – 10 °C. Pre-akinetes did not show ultrastructural changes at – 20 °C; however, vacuolization increased, and gas bubbles appeared at – 70 °C. Our results demonstrate that the formation of pre-akinetes increases freezing resistance. This adaptation is crucial for surviving the harsh temperature conditions prevailing in the High Arctic in winter and a key feature in seasonal dynamics of Zygnema sp.

Highlights

  • Polar hydro-terrestrial ecosystems are characterized by changing and harsh environmental conditions

  • Young vegetative cells develop during the summer season into hardened mature cells termed preakinetes; these are characterized by smaller vacuoles, thicker cell walls, increased lipids, and their typical stellate chloroplasts are reduced (McLean and Pessoney 1970; Pichrtová et al 2014a, b; Pichrtová et al 2016a, b)

  • The present study focused on freezing resistance by an Arctic Zygnema sp

Read more

Summary

Introduction

Polar hydro-terrestrial ecosystems are characterized by changing and harsh environmental conditions. Terrestrial algae inhabiting this harsh environment had to adapt to these stresses. As other filamentous representatives of the order Zygnematales (Zygnematophyceae), it forms extensive mats during the spring and summer seasons. Such mats were reported and studied mainly in localities fed by slow meltwater streams in the High Arctic (Holzinger et al 2009, 2011; Pichrtová et al 2014a, 2016b, 2018) and Antarctic regions (Hawes 1989, 1990). The transition between young vegetative cells and pre-akinetes and vice versa is a continuous process (Herburger et al 2015)

Objectives
Results
Discussion
Conclusion
Full Text
Published version (Free)

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call