Drought-induced structural alterations at the mycobiont photobiont interface in a range of foliose macrolichens

Cryotechniques, such as low temperature scanning electron microscopy (LTSEM) and freeze-substitution for transmission electron microscopy (TEM), were applied to two cyanobacterial and three green algal macrolichens in order to locate free water and to visualize drought-induced structural alterations at the mycobiont-photobiont interface. The following species were examined: Peltigera canina/Nostoc punctiforme, Sticta sylvatica/Nostoc sp. (both Peltigerales), Parmelia sulcata/Trebouxia impressa, Hypogymnia physodes/Trebouxia sp. (both Lecanorales), and Xanthoria parietina/Trebouxia arboricola (Teloschistales). In all species free water was confined to the symplast and the apoplast. No intercellular water reservoirs were found in the gas-filled thallus interior. Thalline fluctuations in water content reflect fluctuations in apoplastic and symplastic water. All the taxonomically diverse lichen photobionts have access to water and dissolved nutrients via the fungal apoplast only. Drought stress (i.e., water content 20%/dw and below) caused dramatic shrinkage and deformation in all cell types. At any level of hydration the fungal and algal protoplast maintained close contact with the cell wall. This applied to the cyanobacterial photobionts and their murein sacculus and gelatinous sheath too. Although the cytoplasm of both partners was strongly condensed in desiccated lichens the cellular membrane systems, usually negatively contrasted, were very well preserved. The significance of these data is discussed with regard to the functioning of the symbiotic relationship.