One special feature of lichens is the diversity of chemical products they
produce. These products can be split up into intracellular and extracellular
products. Some of the types of intracellular products found in lichens are
carbohydrates, free amino acids, vitamins (generally in lower quantities
than in higher green plants) and sometimes carotenoids. Many of the carbohydrates
found in lichens are unique to lichens, or found only rarely in other plants.
For example, lichenin is the main component of the cell walls of hyphae
in lichens and is uncommon in other plants however, it is found in oat seeds
However, it is the extracellular products of lichens that are really exciting.
Most of these compounds are weak phenolic acids made only by lichenized
fungi. However, some compounds, such as endocrocin, parietin, other anthraquinones,
polyporic and thelephoric acids and terphenylquinones are also made in small
quantities by un-lichenized fungi.
These phenolic acids produced by lichens are related to tannins.
Cool Ways Lichens Use these Compounds:
Many of the extracellular compounds that lichens produce are “secondary” compounds,
meaning that they are not used directly in metabolism. Lichens produce over
600 of these secondary compounds and they can produce large quantities of
them – up to 5% of the lichen’s body weight (Brodo, 42). The
isolated fungal component can produce some of these compounds, but only
in tiny amounts.
Depsides, depsidones and fatty acids occur mostly in the medulla where
they coat hyphae with insoluble crystalline material that helps repel water
and maintain airspaces within the lichen. This is important for efficient
gas exchange for photosynthesis.
Substances deposited on the cortex of the lichen often act as sunscreens
or taste bad and act as deterrents to herbivory.
Other lichen substances function to inhibit growth of organisms that compete
for resources. For example, some of these compounds can inhibit the growth
of soil fungi or the germination of seeds of vascular plants. This can give
lichens an important edge because they are generally slow-growing.
Many of these unique lichen substances are used to identify lichens. Color
tests for this purpose were developed in 1870’s. The first chemicals
used were KOH and Ca(ClO)2, used individually or sequentially (K followed
by C). These two are still the simplest way to use chemicals to help with
lichen identification, and they often provide very useful information. Later
para-phenylenediamine was added as another useful reagent and microcrystal
tests were developed to help identify the lichen acids. Currently, people
also use more recent techniques such as thin layer chromatography and high
pressure liquid chromatography. Color tests are useful in identifying most
lichens, but are particularly necessary for identifying crusts, which often
do not have much visible morphological information that can be used for
Many extracellular lichen substances have characteristic colors,
which they impart to the lichen. For example, orange and yellow lichens
often contain anthraquinones, pulvic acid derivatives or usnic acid in
their cortex. In Caloplaca, the orange cortical pigment is parietin
(an anthraquinone), which reacts deep red/purple with KOH.
Extracellular lichen substances are sometimes categorized by the biosynthetic
pathways through which they are synthesized. Quinones are synthesized
in the acetate-malonate pathway.
Other compounds that C. coralloides has in small amounts are
emodin, fallacinal, parietinic acid and teloschistin.