Tolerance of pennate diatoms (Bacillariophyceae) to experimental freezing: comparison of polar and temperate strains

ABSTRACT

Although polar regions are characterised by extreme environment conditions with overall low temperatures and often pronounced diurnal to seasonal temperature fluctuations, diatoms (Bacillariophyceae) thrive in a wide range of nonmarine polar habitats, suggesting that they are well adapted to these harsh conditions. In this study, tolerance to freezing of vegetative and resting cells of 17 polar and nine temperate diatom strains, belonging to different morphospecies and originating from both freshwater and terrestrial habitats, was experimentally tested. Most strains were newly isolated from sites in Maritime Antarctica (James Ross Island and Vega Island), the High Arctic (Spitsbergen) and Europe. Cultures were exposed to five freezing treatments differing in temperature (−4, −20, −40 and −180 °C), freezing rate (gradual, abrupt), time (1 h, 12 h) and thawing rate (slow, fast). Results indicated that diatoms were sensitive to experimental freezing. Freezing temperatures had a significant effect on strain survival: all strains survived −4 °C; most survived −20 °C; five survived −40 °C, and four of these (all belonging to the Pinnularia borealis complex) survived freezing in liquid nitrogen (−180 °C). The cooling and thawing rate had a significant impact on survival: abrupt cooling and slow thawing resulted in much lower survival rates than gradual cooling and fast thawing. Resting cells showed better growth than vegetative cells after freezing treatment but only in the −4 °C treatment. Surprisingly, no striking differences in growth recovery were observed between polar and temperate strains.

KEYWORDS:

• Antarctica
• Arctic
• Cryopreservation
• Freezing
• Stress tolerance

ACKNOWLEDGEMENTS

We acknowledge Kateřina Kopalová and Petra Vinšová for collecting natural samples. We are grateful to Prof. Bart Van de Vijver of the Botanic Garden Meise, Belgium, for identification of the diatom strains. For technical support, we thank Jana Šnokhousová and for statistical help we are grateful to Jana Duchoslavová and Koen Van den Berge.

SUPPLEMENTAL DATA

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Additional information

Funding

The research was supported by the Grant Agency of Charles University (Project No. 20217), a Charles University Mobility grant, the Hlávka Foundation, Institutional Research Concept RVO67985939 (funding to EH, LN, JE), and the Fund for Scientific Research – Flanders (FWO–Flanders, Belgium, funding to EP). We thank the Argentine Antarctic Institute for the logistical support during the Antarctic expedition in 2014 (Picto Project No. 2010–0096) and the Ministry of Education, Youth and Sports of the Czech Republic for use of their facilities (Project LM2015078 CzechPolar 2).