How dry is dry? Molecular mobility in relation to thallus water content in a lichen.

Lichens can withstand extreme desiccation to water contents (WCs) to or below 0.1 g H2O g-1 dry weight (DW) and in the desiccated state are among the most extremotolerant organisms known. Desiccation tolerant life-forms such seeds, mosses and lichens are able to survive "vitrification", this is the transition to a "glassy state" of their cytoplasm, which causes the metabolism to cease. However, our understanding of the mechanisms of desiccation tolerance is compromised by the poor knowledge of which reactions occur in the "dry" state. Using Flavoparmelia caperata as a model lichen, we studied at which WCs vitrification occurs upon desiccation. Molecular mobility was assessed by dynamic mechanical thermal analysis (DMTA), and the de- and re-epoxidation of the xanthophyll cycle pigments, measured by HPLC, was used as a proxy to assess enzyme activity. We conclude that at 20°C vitrification occurred between 0.12 and 0.08 g H2O g-1 DW and that enzymes are active in a "rubbery" (0.17 g H2O g-1 DW) but not in the glassy state (0.03 g H2O g-1 DW). These new insights into the biophysical and biochemical processes related to molecular mobility in the "dry" state may support future studies into the mechanisms of desiccation tolerance.