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Over 800 lichen secondary metabolites are known, some unique to lichens and some produced also by plants and nonlichen fungi. Most are weak phenolic acid derivatives of the acetate-polymalonate pathway, including depsides, depsidones, simple phenolic acids, aliphatic acids, dibenzofurans, esters, chromones, xanthones, anthraquinones and naphthoquinones. A smaller number are derivatives of the mevalonic acid pathway, including carotenoids and triterpenoids, and the shikimic acid pathway, including yellow-pigmented pulvinic acid derivatives.

Most lichen compounds are fungal in origin, and their biosynthesis is analogous to processes that go on in nonlichen fungi. Photobionts are apparently not required for synthesis, but most compounds are produced only by lichenized mycobionts, and some isolated mycobionts produce compounds different from the symbiotic fungi, all of which suggests that the presence of photobionts creates conditions necessary for proper synthesis of many compounds.

Lichen products are typically deposited as water-insoluble crystals on the outer surfaces of fungal hyphae, which can be extracted in organic solvents, in some cases without harming the metabolic function of the symbionts. Some compounds are found only in cortical tissues above the photobiont layer, while others are restricted to internal medullary tissues, or to specific vegetative or reproductive structures. Concentrations vary within thalli, especially in tissues of different age, but usually range from 1-5% thallus dry wt. Unusually high concentrations (up to 30% dry wt.) have been measured in some cases.

Lichen compounds are commonly identified by spot tests, thin-layer chromatography, and high performance liquid chromatography. Lichen chemistry has long been used in lichen taxonomy as an aid to identification of groups, and abundant chemical data have been collected for lichens. Indeed, more than one-third of all described lichens have been characterized chemically, making them one of the best-studied groups of fungi in this regard.

What do lichen compounds do? Given the energetic investment made in their production and the fact that they are unique to lichenized fungi, most lichenologists assume they are adaptive in some way.

Various adaptive functions have been proposed for these compounds, the most commonly discussed of which are:

(1) Regulators of internal water relations.

(2) Regulators of photobiont metabolism. 

(3) Mineralization of essential elements.

(4) Light-screening protection from harmful UV radiation.

(5) Allelopathic agents.

(6) Defense from pathogens and predators. 

(7) Stress-induced response.

 

 

 

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