Research Projects - Chemical ecology:
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.