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How Lichens Adapt To Protect Themselves From Harsh Conditions

A microscope snapshot of the lichen Aspicilia blastidiata. Credit: Aleksandr Paukov/ UrFU

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Lichens growing on substrates with different chemical properties have a different spectrum of secondary metabolites. This is probably how lichens adapt to unfavorable conditions - low pH of the substrate or toxic elements. Biologists of the Ural Federal University who studied more than 740 species of lichens discovered this new property of these organisms. Samples were collected from rocks and trees (spruce, pine, birch, alder, aspen, poplar) of the Middle Urals. The results were published in the journals Frontiers in Forest and Global Change and Diversity.

According to biologists from the UrFU, secondary metabolites help lichens adapt. These are peculiar "biochemical tools" with which the organism defends itself and survives under stressful conditions. Both animals and plants have primary metabolites, while plants have secondary metabolites only. In other words, to escape, unlike an animal which can escape, a plant produces a certain set of secondary metabolites which endow it with protective properties (taste, smell, color) and allow it to survive in drought, high temperatures, the spread of infections, etc.

In lichens, secondary metabolites accumulate in large amounts. They do not participate in metabolic processes, and the role of many in lichen life is not clear. The patterns of their formation are only beginning to be studied.

"Lichens contain 800-1050 secondary metabolites or "lichen acids," including phenolic compounds, steroids, terpenoids, etc. Thus, some secondary metabolites give lichens a higher resistance to acidity. Others endow lichens with resistance to high concentrations of heavy metals in the substrate. At the same time, some have antimicrobial properties, which also help in survival, especially for slow-growing lichens. "Lichen acids" are also potential medicines for humans, possessing antitumor, anti-inflammatory, and many other properties," explains study co-author Aleksandr Paukov, Associate Professor at the Department of Biodiversity and Bioecology at UrFU.

The study showed that lichens growing on serpentinite adapt to high concentrations of toxic metals (they bind them with secondary metabolites and convert them into an insoluble form). Lichens growing on willow have relatively poor secondary chemistry, because willow bark contains a lot of salicylic acid (the tree uses this acid to protect against bacteria).

In total, the species studied contained 76 metabolites, reported biologists of the UrFU. Atranorin turned out to be the most common of all metabolites. For humans, this substance has anti-inflammatory, antibacterial, antifungal, cytotoxic, antioxidant, antiviral and immunomodulatory properties. The lichen, on the other hand, uses atranorin as a light filter, protecting it from excessive solar radiation.

"We also found specimens that are 'lazy' and do not produce secondary metabolites. For example, lichens on aspen produce few secondary metabolites, because the aspen bark provides them with normal vital functions. We also discovered an interesting fact - lichens have substances that they do not use in any way. We have yet to find out what they are produced for," explains Aleksandr Paukov.

The next stage of research is to test the results in the laboratory. It is planned to "clean" the lichens from secondary metabolites, place them in appropriate conditions, and see if they will build up their protective properties again.

Lichens are a symbiosis of a fungus, unicellular algae, and/or cyanobacteria. Lichens are the first to settle in barren places and form the primary soil. The practical use of lichens is extremely diverse. They are food for animals (deer, red deer, roe deer, elk), serve as a raw material for the production of dyes, as an odor fixative in perfumery. In Japan, the leafy lichen Umbilicaria edible is a delicacy. In some countries, bread is baked with the addition of lichens. These "organisms" are also used in folk medicine for the treatment of skin diseases, lung, heart, etc. Lichens are also used in environmental monitoring: due to their high sensitivity to chemical pollutants lichens do not grow in polluted areas.

Reference: Paukov A, Teptina A, Ermoshin A, Kruglova E, Shabardina L. The role of secondary metabolites and bark chemistry in shaping diversity and abundance of epiphytic lichens. Front. for. glob. change. 2022;5. doi: 10.3389/ffgc.2022.828211

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