DIFFERENTIAL SCANNING CALORIMETRY STUDIES ON AN ANTARCTIC NEMATODE (PANAGROLAIMUS-DAVIDI) WHICH SURVIVES INTRACELLULAR FREEZING

Differential scanning calorimetry was used to characterize thermal eve nts associated with freezing and melting of suspensions and extracts o f Panagrolaimus davidi, an Antarctic nematode which can survive intrac ellular freezing. Nematode suspensions produced a single freezing exot herm with a shoulder on the peak representing the freezing of the nema todes. A shoulder on the peak of melting endotherms indicates the melt ing of the nematodes and of the water surrounding them. Exotherms were also detected from individual nematodes mounted in liquid paraffin. T he freezing of nematodes was very rapid and in marked contrast to that of freezing-tolerant insects and vertebrates, which take hours or day s to freeze. Eighty-two percent of the nematodes' body wafer froze. Hi gh levels of survival were obtained in nematodes exposed to temperatur es down to -40 degrees C. No additional thermal events were observed a fter the freezing event and before the melting of samples cooled to -4 0 degrees C, indicating no changes in the proportion of body water fro zen. Ice nucleating activity is present in nematode suspensions but no t in supernatants from nematode extracts. No thermal hysteresis activi ty was detected in nematode extracts. (C) 1997 Academic Press.