Effects of freezing and thawing on a species of Zygnema (Chlorophyta) from the Antarctic

Using a variety of techniques, the susceptibility of a species of Zygnerna (Chlorophyta) from the Antarctic to freezing was investigated with freshly collected and cultured material. There was no evidence of resting spore formation. Intracellular ice nucleation only occurred at high cooling rates and was lethal. The effect of exposure to extracellular ice at slow cooling rates depended on the duration and temperature of exposure. During repeated overnight exposures down to −4°C, photosynthetic capacity was maintained and there was little loss of intracellular solutes. An increase in the duration of exposure or a decrease in temperature resulted in loss of photosynthetic capacity, leakage of solutes and cell death. The alga is well suited to growth during the Antarctic summer when diurnal freeze-thaw cycles rarely fall below −3°C, but survival during winter may depend on a few highly resistant cells.