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1. INTRODUCTION TO
LICHENS

2. What are Lichens?
Lichens are 'dual organisms.' Every lichen is a partnership between members of two different
kingdoms which live together in a special, mutually beneficial relationship (a symbiosis). Each
lichen is made up of a fungus (usually an ascomycete) and an alga (green or blue-green).
• There are almost 20,000 lichens, each involving a different fungus, but the same algal partner can
be found in many different lichens, so many fewer algae are involved.
• The body of the lichen is built up by tough fungal hyphae, and the algae live inside that framework.
• The fungus protects the alga from the harsh world outside, and provides it with water and mineral
nutrients. The alga makes its own food by photosynthesis, and leaks some of this food, which is
then absorbed by the fungus (which cannot make its own food).
• This partnership is so tough and self-reliant that lichens can grow in places like bare rock in deserts,
where nothing else can survive. When it is too dry, too hot, or too cold, lichens go into a state of
suspended animation until conditions improve. Since the algae make up only about 5% of each
lichen, and are out of action for much of the time, you can imagine that lichens grow very slowly --
only a few ] per year. They make up for this by living for centuries, or in a few cases, millennia.
• Lichens have only one serious weakness -- they must absorb their mineral nutrients from the rain.
So if the air is polluted with sulphur dioxide, this dissolves in the rain and is absorbed by the
lichens which often die as a result.
• The lichen is a symbiosis between two different species: (1) a host fungus (i.e. mycobiont) and (2)
endosymbiotic algae or cyanobacteria (photobiont). The host fungus provides the algae with
protection from dessication, while the algae/cyanobacteria provides the host with carbohydrates
produced from photosynthesis. The relationship is mutually beneficial (i.e. symbiotic), as both
parties stand to gain from the association.

3. crustose
foliose
fruticose

4. Growth And Development In Lichens
• Like all living things lichens need nutrients
and energy to grow.
• Lichens will and do grow on just about
everything, natural or manmade. Different
species of lichens prefer, or only grow on
different substrates. Thus some species
will be found on smooth barked trees,
some on rough barked and some on only
one species of tree. Also some lichens
grow on basic rocks while others only
grow on acidic rocks and some have
particular mineral requirements,
thus Acarospora sinopica only grows on
rocks with a high iron content. When
young and very small they grow slowly,
then once they are reasonably well
established they grow much more quickly,
obviously when they are dying, for what
ever reason they grow more slowly again,
or not at all.

5. upper cortex
algal layer
all lichens
contain algae
medulla or
cyanobacteria
lower cortex

6. Reproduction In Lichen
• Lichens have two fundamentally different sorts of
reproductive bodies. These are 1) spore forming bodies
and 2) vegetative reproductive bodies.
• The other main group of fungal partners are
Basidiomycetes. Basidiomycetes are relatively rare as
lichen partners, and one of the ways they differ from
Ascomycetes is that they produce their spores on a
basidium, a special structure which normally holds four
spores at its top.
• Spores come in a great variety of sizes, shapes and
forms. They are for instance, much larger in the
genus Pertusaria than in the genus Acarospora. Many
are simply a single whole spore while in others the spore
may be divided up into 2 or more subsections. These
spores are all microscopic in size and float easily in the
air, this enables them to disperse widely when they are
released. Quite how these spores meet up with the
correct algal partners to form new Lichens is something
nobody really understands, though as most of the algae
occur in the wild on their own it may be that it is simply
a matter of chance that some will come to rest on the
right algae.
•

7. • Apothecia normally look like exposed disks
which may be raised above the surface of the
thallus, level with it, or sunk below the surface.
In some genera/species the apothecia look
more like slits, i.e. Graphus sp.. The Asci are held
within a mass of special sterile hairs called
paraphyses. These paraphyses are composed of
fungal hyphae and often have coloured tips.
These then give the Apothecia their coloured
centres. Differences in the colours of Apothecia
can be important in identification.
• Perithecia here the spore bearing body is not
open as apothecia but is in a chamber with only
a small opening to the outside world, this
opening is called an 'ostiole'. Perithecia are
often buried almost completely in the substrate
on which the lichen is growing. Both these
structures release ascospores - spores produced
within an ascus (a sack).

8. • Pycnidia are reproductive bodies which
release conidiospores - spores produced
from the end or side of special hyphal
filaments called conidia.
• Vegetative Reproductive Bodies -
Vegetative reproduction is important to
many lichens and has the advantage of
dispersing both partners at the same time.

9. Anatomy Of Lichens
• Most commonly, photobionts are located in a
layer within the fungal tissue. The layer is
generally oriented in a manner that maximises
photosynthesis, and is protected from rapid
changes in water availability. Each cell or group
of cells of the photobiont is usually wrapped by
hyphae, and in some cases penetrated by a
haustorium. Moribund cells may be digested by
the fungus, but for the most part, the
photobiont remains healthy during the
functional period of the symbiosis. The
increased size of cells of the photobiont
indicates that reproduction is regulated by the
symbiosis.
• The primary photobiont is commonly a green
alga. This symbiont is found within a layer below
the surface of the lichen. Cyanobacteria may
also be held in small eruptions of or under the
surface called cephalopodan. Cyan bacteria can
fix atmospheric nitrogen, and thus complement
the primary activities of the photobiotic, energy
fixation.
• In fruiticoselichens, the central core of stems
may be hollow. The core may have hyphae
oriented in a woven pattern, and the hyphae
may be thick-walled and multi-layered.

10. • The thallus is commonly layered. The thallus may
be covered by or enmeshed in extracellular
matrix expressed by the fungus. For instance,
some crustose lichens have a polysaccharide
layer on the surface. The photobiont is located at
the base of the polysaccharide layer.
Polysaccharide layers may also be found within
the cortex of the thallus where their function
may be different. The thallus is commonly
interleaved by hyphal layers. Some thalli have
hydrophobic layers on the surface or within the
thallus. The hydrophobicity appears to be related
to the presence of hydrophobins expressed by
the fungus.Indeed, different hydrophobins act in
different parts of the thallus. Finally, the lower
layer of crustose lichens lack hydrophobic
materials, indicating a role in the uptake of water
and solutes to the tissue.
• The matted anatomy of most lichens is
particularly important for uptake and storage of
water. Though water can be taken up rapidly,
even from condensation at night, water is also
lost. Thus the anatomy is closely linked to the
functioning of the thallus. Water is necessary for
metabolic processes, and in the absence of
water, the lichen slows or stops its metabolic
processes.

11. Uses Of Lichens
• Because they are capable of colonizing bare
rocks and other mineral substrates, lichens are
important in soil formation during
some ecological successions. For example,
lichens are among the first organisms to
colonize sites as they are released from
glacial ice. In such situations lichens can be
important in the initial stages of nitrogen
accumulation and soil development during post-
glacial primary succession.
• Lichens are an important forage for some
species of animals. The best known example of
this relationship involves the northern species
of deer known as caribou or reindeer (Rangifer
tarandus) and the so-called reindeer lichens
(Cladina spp.) that are one of their most
important foods, especially during winter.

12. • Some species of lichens are very sensitive to air
pollutants. Consequently, urban environments
are often highly impoverished in lichen species.
Some ecologists have developed schemes by
which the intensity of air pollution can be
reliably assayed or monitored using the
biological responses of lichens in their
communities. Monitoring of air quality using
lichens can be based on the health and
productivity of these organisms in places
variously stressed by toxic pollution.
Alternatively, the chemical composition of
lichens may be assayed, because their tissues
can effectively take up and retain sulfur and
metals from the atmosphere.
• Some lichens are useful as a source of natural
dyes. Pigments of some of the more colorful
lichens, especially the orange, red, and brown
ones, can be extracted by boiling and used to
dye wool and other fibers. Other chemicals
extracted from lichens include litmus, which was
a commonly used acid-base indicator prior to
the invention of thepH meter.

13. • In addition, lichens add significantly
to the aesthetics of the ecosystems in
which they occur. The lovely orange
and yellow colors
of Caloplaca and Xanthoria lichens
add much to the ambience of rocky
seashores and tundras. And the
intricate webs of
filamentous Usnea lichens hanging in
profusion from tree branches give a
mysterious aspect to humid forests.
These and other, less charismatic
lichens are integral components of
their natural ecosystems. These
lichens are intrinsically important for
this reason, as well as for the
relatively minor benefits that they
provide to humans.

14. Ecology
• Lichens play an important role colonising
new surfaces. Among the metabolites
excreted by some lichens are acids. Acids
have the capacity to degrade the surfaces
on which they are located, thus releasing
minerals for uptake by the thallus. Acidic
digestion has the effect of causing the
slow disintegration of the surface,
especially of limestone and other
calcareous materials.
• Lichens grow extremely slowly. Any one
thallus may be many decades old. The
outer edge is probably the only active
component of the thallus, unless the
lichen has started to overgrow itself. The
inner part is commonly inactive.
• Lichens have the potential to withstand a
wide range of environments. Thus they
adapt rapidly to local and seasonal
changes in temperature and water
availability: they are found in bleak artic
and desert environments.

15. • The thallus has the capacity to cope with
the frequent aridity of the environment.
Foliose thalli will curl as the thallus dries,
and then flatten as it rehydrates.
Photosynthesis follows the pattern of
wetting and drying. While changes in form
enable a return from dehydration, the
presence of trehalose, and possibly a
range of polyols, is also important. These
compatible metabolites enable the
cytoplasm to desiccate, while protecting
the functionality of the enzymes.
• The slow rate of growth and the reliance
on minerals in rain or high humidity has
consequences for survival of lichens in
polluted environments. Lichens absorb all
minerals in rain, and the presence of
pollutants, including sulphur, will result in
the decline of the thallus. Because of their
sensitivity to pollutants, most lichens are
uncommon in areas affected by acid rain
and aerial pollutants. However, some
lichens grow on surfaces containing high
concentrations of metals, and must be
adapted to those metals: single pollutants
will select lichens that can tolerate the
pollutant. Changing pollution will remove
most. In cities, the pollution profile is
variable and changing over time. Thus
lichens are disappearing from cities.

16. Conclusion
• Lichens are slow growing
associations between fungi
and photosynthetic symbionts.
They are widespread, and
commonly found as primary
colonisers on soil-less
surfaces. They utilise rainfall
for the water and dissolved
minerals in the air. The
absence of lichens indicates a
polluted environment.