Bound water behavior in Cetraria aculeata thalli during freezing

Abstract

Lichens are extremophilic organisms surviving in extremely low temperature and dehydration; however, the molecular mechanism of frost and dehydration resistance is not yet fully understood. Low temperature behavior of Cetraria aculeata thalli at the hydration level between ∆m/m0 = 0.039 and ∆m/m0 = 0.964, defined as a ratio of mass of water adsorbed to the thalli, ∆m, to dry mass of the thalli, m0, and interpreted as water content, %WC, in a fractional form (∆m/m0 = %WC/100) was investigated using nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC). Proton free induction decays were decomposed into components: coming from solid matrix of thallus and exponentially decaying from loosely bound and from tightly bound water. At low hydration levels the loosely bound water fraction (cooperatively freezing water) is detected down to − 15.6 °C. With the decreasing temperature loosely bound water fraction is gradually transferred to the tightly bound water pool (non-freezing water) which could play significant role in freezing resistance mechanism. At higher hydration levels DSC scans show the presence of cooperative water freezing in thallus. The lowest hydration level at which the cooperative freezing of water bound in C. aculeata thallus is detected by DSC is equal to 0.453. DSC courses recorded after 120 min incubation of C. aculeata thallus in − 20 °C suggest the lowest hydration level in which ice formation could be observed is decreased to 0.167, as a result of water molecule diffusion to places where ice microcrystallites were formed. Supercooled water behavior in C. aculeata thalli reveals a composed behavior. Two processes take place: supercooled liquid water mobility gradually decrease with the decreased temperature and the diffusion of supercooled water molecules to ice microcrystallites and, thus the solid phase growth.