Bound water freezing in AntarcticUmbilicaria aprinafrom Schirmacher Oasis

Abstract

AbstractThe effect of low temperature onUmbilicaria aprinacollected from Schirmacher Oasis, East Antarctica, was determined over a wide range of hydration using proton free induction decays, proton nuclear magnetic resonance (NMR) spectra and differential scanning calorimetry methods. The proton NMR line is a superposition of the broad component from the solid matrix of the thallus and a narrower component from the averaged bound water pool. Proton free induction decays may be resolved into three components: a solid component well described by the Abragam function and two exponentially decaying components from water loosely bound and water tightly bound in the thallus. With decreased temperature the loosely bound water pool (freezing water) is transferred to the tightly bound water pool (non-freezing water), and vanishes below -40°C. Bound water freezing and melting temperatures decrease with the decrease of hydration level, suggesting that heterogeneous ice nucleation is responsible for water freezing. The onset of bound water freezing temperature isc.10°C lower than the melting temperature. TheU. aprinathalli do not reveal the ability to stimulated ice nucleation at higher temperature. Freeze-thaw cycles showed that forn> 5 cycles no substantial change occurs in the difference between melting and freezing temperatures.