Ajsylu F. Khajrullina

The lipid composition of the mycobint and photobiont symbiotic partners of lichenized ascomycetes varies greatly. The aim of this study was to compare the profile of the major sterols in two closely related lichens from the genus Lobaria with different photobionts. The three-component lichen Lobaria pulmonaria has two photobionts. While the main photobiont is the chlorophycean alga Symbiochloris reticulata, this lichen contains small amounts of the cyanobacterium Nostoc. By contrast, the cyanobacterium Nostoc is the main photobiont in Lobaria retigera. Relatively loosely bound sterols were extracted using a chloroform–methanol mixture, and subsequently, more tightly bound sterols by alkaline saponification. The initial chloroform–methanol extraction step indicated that ergosterol is the principal sterol in both species, with phytosterols constituting a minor fraction. However, the addition of an alkaline saponification step to the standard protocol of sterol extraction greatly increases the release of tightly bound phytosterols, such as campesterol, stigmasterol, and β-sitosterol from L. pulmonaria, but not from L. retigera. Therefore, the mycobionts and Nostoc mainly possess sterols extractable by the standard mixture of chloroform/methanol, while the chlorophycean algal photobiont contains tightly bound sterols. This observation could be important when studying the roles of sterols in the stress tolerance of lichens.

Extremophile organisms can survive under extreme conditions through tolerance mechanisms. Such organisms include lichens, which are symbiotic associations of fungi and algae and/or cyanobacteria. Among other factors, the high stress tolerance of lichens can be attributed to their ability to synthesize a wide range of metabolites, including chemically diverse lipid compounds. Despite their obvious relevance, the biochemical mechanisms of stress tolerance in lichens that are mediated by changes in the lipid profile remain understudied. Peltigerous lichens constitute a separate division of lichens characterized by high growth rates and metabolic activity indices. Taking into account the temperate climate in which these lichens grow, it can be assumed that they may be highly sensitive to temperature fluctuations. These factors led to the choice of Peltigera canina, epigeic lichen, as the subject matter of this study. The present work examined stress-induced changes in the redox status of P. canina, as well as changes in its lipid composition at an elevated temperature (40 °С). The exposure of hydrated lichen thalli to an unfavorable temperature led to an increase in the level of hydrogen peroxide, phenoloxidase activity, and lipid peroxidation, which constitutes an important part of stress response in lichen. The stress-induced changes in the redox status of lichen thalli stimulated an increase in lipophilic antioxidant agents: a dramatic increase in the level of α-tocopherol and an increase in carotenoids, specifically β-carotene. Thus, the accumulation of lipophilic antioxidants constitutes an important part of the lipid-mediated stress response of P. canina to temperature elevation.