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What are Lichens?
'Lichens are a case of fungi
that have discovered agriculture'
Trevor Goward, lichenologist.
Introduction
There are estimated to be over 18,000 different forms of lichens distributed across the world. In
temperate Europe there are over 1,700. They occur in all ecosystems and biomes of the world,
from high mountaintops, to dry dusty deserts, to the freezing arctic tundra. However, in just the
same way that you can walk down the same street each day without noticing many of the things it
contains, such as the man at the bus stop or the new shop window display, so you can walk the
same country path daily yet miss the lichens as you go by.
You would probably only realize lichens were there if someone pointed them out to you, and thus
drew your attention to them. Even if someone did that, you might still be unaware of what lichens
actually are and what they do. This guide intends to teach you about lichens; to help you find them,
and to show you why they are important.
Lichens are fascinating organisms, which play an important but often unnoticed role in the habitats
and ecosystems in which they live. There are many different kinds. And they are found on a variety
of different surfaces; including soil, trees, rock, and even on glass. There are a wide variety, with
them being adapted to the specific characteristics and climatic features of the habitats in which they
occur.
What are lichens?
Lichens are special because rather than being a single organism in their own right, they are instead
a combination of two separate and very different forms of life. Although they exist as a single en-
tity, they are actually a complicated partnership between two organisms. Biologically such a part-
nership is known as a symbiosis. This is a type of biological interaction in which both participating
partners benefit. The main partner in the relationship is a fungi, and the second and lesser, but still
important partner is an algae.
Most organisms live alone, independently, and somewhat self-sustaining. Although they might in -
teract with other organisms during their daily life, this is not essential for the organisms immediate
survival. Lichens, instead are a unique co-operative venture. Each partner carries out essential tasks
allowing both partners to survive as a single whole. This allows them to live in areas either could
not survive alone. It is best to think of lichens as being a fungal-algal combination; and as a form
of life in its own right. Lichens are not exactly 'species', they are a way of living.
It is easy to overlook lichens on a pavement as you walk on by.
Fungi, or what we commonly and colloquially think of as 'mushrooms', are a form of life which live
and are sustained by dead substances. They obtain all the nutrition they need from dead material.
And they are tough. They are able to live in very hostile conditions. They can live where it is cold.
They can live where it is dry. They have tough bodies which can withstand whatever the environ -
ment throws at them. But they do have one problem; and that is obtaining the energy they need to
survive. They are not able to photosynthesise like green plants, so can't utilise sunlight to gain en-
ergy. How can they solve this problem?
Algae, on the other hand, might be considered as being a bit soft. These are more plant like in that
they can make their own energy from the sun through photosynthesis. However, although they are
able to do this, they are delicate and sensitive organisms. They are susceptible to drying out. They
can't tolerate or handle difficult conditions. They require constant moisture, relative warmth, and
environmental stability. What can algae do when these are not available?
Sometime in the evolutionary past fungi and algae came together, bringing with them the best qual -
ities that each of them had to form a partnership. The fungal partner was tough and able to
provide protection to the delicate algae. In return the algae could produce food to sustain both it-
self and the fungus. Thus both became 'winners' and were able to colonise and live in habitats pre -
viously no other life form could survive in. However, although ultimately both type of organisms
gained from this arrangement, this was not some kind of collaborative get-together. It seems the
fungi is the dominant partner in the relationship, and has more or less 'kid-napped' the algae for its
own ends. However, maybe the algae should not be seen as a too unwilling partner as they gained
from the relationship likewise.
What lichens are not.....
For lay people it can be difficult knowing the difference between lichens, moss and algae. This is
understandable. They all do look pretty similar. They are all 'plant' like. They are all green and ap-
pear to grow in similar ways. They don't move. Superficially they are just the green mouldy stuff
you find at the bottom of trees, or on rooftops, no? However, this is a misconception and lichens,
mosses and algae are very different organisms from each other.
Mosses and liverworts are known biologically as bryophytes. These are non-vascular plants. This
means they do not possess the system of inner tubes allowing nutrients or water to flow between
different parts of the plant. There is no xylem or phloem; the main nutrient and water carrying
structures of plants. Instead substances move around by simply flowing! Despite maybe being a
rather simple form of life mosses have shown that simplicity is a virtue and are extremely success -
ful. Mosses occur all over the world and are extremely common.
Not lichens: Left, moss on a rooftop.
Right, algal bloom.
Algae are a kingdom of simple single celled plant like organisms. They are able to photosynthesise
as they possess the cell organelles known as chloroplasts, the cell bodies capable of capturing en -
ergy from light and turning it into sugars. They are different from lichens in that they are capable
of living independently without a fungal partner. Like mosses, despite a simple way of living they
are extremely abundant and successful.
Not lichens: Common Feather-moss
The history of our understanding of lichens
Lichen biology has been relatively neglected compared to other aspects of botany. One possible
reason being the difficulty scientists initially had in understanding what lichens actually were. Biolo -
gists could make more progress by concentrating on other simpler topics..... It took much research
to finally determine what was actually happening in lichens.
Lichens were first noted by the Greek Theophrastus, a pupil of Plato, in the third century before
Christ. Later they were described in a Greek herbal book 'Matera medica' in AD 68. The French
naturalist Joseph Pitton de Tournefort was the first to place them in their own grouping, thinking
they were a type of moss. However, placing this difficult and incomprehensible group of organisms
in their own group was simply a tactic to avoid the problem of establishing what they actually really
were. Later scientists subdivided lichens according to their structure; mainly on the basis of the
shape of the thallus, or main body, of the lichen 'plant'. Linnaeus, the great biological taxonomist,
considered there to be eight sub groupings of lichens and classified them according to their growth
form.
Lichen research only really began with the work of the Swedish biologist Erik Acharius, who came
from Wadstena near Lake Malar. He is considered the father of lichenology, and published a num-
ber of volumes classifying lichens at the beginning of the 19th century. His best work was the 1810
volume 'Lichenographica Universalis', in which he classified lichens according to their reproductive
structures rather than simply the shape. This proved the pre-cursor to further study of lichens,
which now moved away from simply listing lichen species, or rather distinct types, to more detailed
study of their ecology and biology. Further advances were made by the Italian Giuseppe de Notaris
and the French Tulasne brothers during the mid-19th century.
Initially the algae present in lichens were misidentified as buds of the hyphal filaments and misla-
belled with the term 'gonidia'. These gonidia were thought to play a part in lichen reproduction, be -
ing released from the hyphal tips to sprout into new lichen elsewhere. This idea remained remark-
ably persistent once suggested, with some authors still backing this idea into the 1950's.
A major advance was the proposition that lichens were a symbiosis. This idea was conceived by the
Swiss Botanist Simon Schwendener in 1867 with his publication of 'Ueber den Bau des
Flechtenthallus' which roughly translates 'On the composition of the lichen thallus'. His ideas be-
came known as the 'Dual Hypothesis of Lichen'. At the time this idea was considered most contro -
versial, and he was even ridiculed for his ideas, and they took a long time to become accepted.
However, research involving growing new lichens from spores on agar culture, initially obtained
from Collema types of lichens, showed he was right. Later more research was conducted by Bonnier
in the 1880’s, who tried to grow spores of many other species. The first modern studies were com-
pleted in the 1950s by Ahmadjian. He identified the separate partners involved in the symbiosis
and studied the conditions required for growth to occur. This work began to elucidate the complic -
ated nature of the lichen association, work that continues today.
Eventually Schwendener's idea ushered in modern thinking on lichens. Throughout the 20 th cen-
tury new and more complicated aspects of lichen biology were examined. The advent of genetic
techniques has allowed the exact taxonomic relationships of lichens to be deciphered for the first
time.
In Britain the first and possibly still the most important society dedicated to lichens was established
in 1958. The British Lichen Society aimed to collate information about lichens and provide a forum
for people interested in them to meet and share ideas and information. It remains very popular
with both professional lichen biologists and amateurs alike, and remains the leading international
organization committed to lichen study and conservation.
The Swiss Botanist Simon Schwendener,
who developed the lichen as symbionts idea.
Artificial symbiosis
After Schwendenener's initial idea suggesting duality, an obvious step was first to isolate the separ -
ate partners in the lichen association, to grow them on independently, and then later to attempt to
reintegrate them again into a lichenous association. This proved a considerable challenge. Isolating
the separate components was relatively simple and was quickly achieved. As was growing the separ-
ate algae and fungi; this was done in special cultures which provided the ideal conditions for such
growth. However, recombining the separate partners proved much more difficult. This has only
been achieved by a small handful of workers in only a few species.
The attempts to isolate and then re-integrate the individual organisms in symbiosis are most fam-
ously described by Ahmadjian in his book 'Lichen Symbiosis'. Here he describes the methods ne-
cessary for effective isolation of algal and fungal partners. Being in symbiosis significantly enhances
fungal success. On their own fungi grow slowly, forming only small isolated colonies on culture
substrates. None of the characteristic lichen growth forms, colouration, or properties develop.
There is no distinct differentiation of the tissues into different structures or functions. As might be
expected, the effects on the algae are not as dramatic when isolated, but these are often found in a
free living form anyway, and being relatively simple forms of life would not develop any complex
form anyway. It appears that for the fungi symbiosis is essential for them to fully exhibit all their
biological features and when removed from such an environment they are severely limited.
Reformation of the symbiosis is extremely difficult to recreate in laboratory settings. Those who
have had success have shown that it is promoted when nutrients are in short supply. There appears
to be no incentive in forming a symbiotic arrangement when nutrients are readily available. Also
most success has been seen where water conditions fluctuate considerably between dryness and re-
peated wetting. These factors indicate that this symbiosis in nature is probably promoted by unpre-
dictable and unstable conditions; those in which lichens thrive the best.
The substrate upon which symbiosis is attempted also affects success. It might be thought that an
artificial culture providing suitable conditions might be best. However, symbiosis appears to occur
most effectively on natural substrates such as rock or wood.
THE LICHEN PARTNERSHIP
As already discussed, although forming a single complete entity, lichens are composed of two indi-
vidual organisms which combine together in partnership. This is an association between a fungus
and algae. Biologically this association is known as a symbiosis because both partners in the rela-
tionship benefit by living together. This form of cooperation allows each partner to thrive in habit-
ats which individually neither could inhabit otherwise. Each partner in the relationship plays an es-
sential and necessary role aiding the survival of both.
The fungal partner in lichen is called the 'mycobiont'. Myco is the Greek word for fungus. Biont
being the term used to denote a living thing. The algal partner is called the 'photobiont' as it is that
which provides the photosynthesising ability. As it is the fungi which forms the outer external sur-
face of the lichen this partner is termed the 'exhabitant', while the internal living algae are termed
'inhabitant'.
The fungal partner
Many people consider fungi as being 'plants', probably because they appear to have rather plant like
characteristics such as living in or on the ground and not moving around much. However, in fact
they are a different type of organism altogether. Unlike plants, they do not possess cellulose within
the cell walls, an essential characteristic of plant matter. They also do not contain plastids, the small
inner-cell structures performing important metabolic tasks. These include chloroplasts, which are
like little factories engaged in photosynthesis, and which plants have in abundance. Structurally
fungi are very different to plants and thus are currently placed in a Kingdom of their own; the My-
cetae. It is best to consider them as being neither plant nor animal, but as something biologically
in-between.
Fungi obtain their energy through breaking down dead material. They are decomposers. As they do
not possess chloroplasts they obviously can't photosynthesise. This means they have to find a way
to obtain the nutrients they require, and utilising dead material is the obvious option. Fungi are
tough. The strong cell walls means they can live in inhospitable habitats without problem.
Although the fungi involved in lichen relationships mostly belong to the cup fungi, the Ascomy -
cetes phylum, a small number do contain fungi from the Basidiomycetes. These are mostly tropical
lichens such as those from the Dictyonema genus. The different species of fungi involved in the rela-
tionship are described later.
The algae partner
The other partner in the lichen relationship are algae. Algae are a primitive form of green plant.
They are typically placed with Protists, this is a rather mismatch group of simple primitive mostly
single celled organisms. Unlike fungi, algae are able to photosynthesise because they have chloro-
plasts, but unlike most other higher plants they reproduce in a simple manner using spores.
There are many species of algae worldwide, and over 1,500 genera. But only a small proportion,
roughly about 100 species, combine with fungi to form lichen relationships. Despite being able to
produce their own food, algae are not tough at all and can easily dry out. They lack the hard outer
cellular coating found in the fungi. This is a problem as for photosynthesis to occur conditions
have to be steady and constant and with reliable levels of moisture. This has meant that most algae
live in moist, wet places where they won't dry out and where conditions are stable. However, this is
rather limiting.
Cosmarium botrytis
is a type of algae, com-
monly found in freshwater.
In a lichen each algal cell has a great deal of contact with the fungal hyphae surrounding it; the
fungal filaments form enveloping membranes around algae enhancing surface area contact between
the two organisms.
There are three main types of algae that form lichenous associations; Trebouxia, Trentopohlia and
Cyanobacteria. Lets consider these separately:
Trebouxia are green algae belonging to the Order Trebouxiales. They are composed of a
single cell containing a single chloroplast. This is the most common type of algae found
within lichen. Originally it was believed a specific different algae species occurred in each
different species of lichen. However, it is now known that the same Trebouxia types of al-
gae can occur in multiple forms of lichen. They can also occur in free living form. The cells
are spherical in shape with the chloroplast being fairly central in the cell.
Trentepohlia is also a green algae, although actually in colouration it is more a reddish-or -
ange! Algae within this genus can live in a free form independently of lichen. This algae
forms the commonly seen orange ‘mould’ seen on urban walls or damp tree trunks. This al -
gae has a filamentous structure. There are about 40 species. The majority of Trentepohlia
algae are found in tropical areas rather than the temperature zones. In structure they are
either filamentous, rounded or barrel like. They often have thickened cell walls.
Cyanobacteria are blue-green algae within their own Phylum. They also can live in a free
form and when they do so they form a black jelly like algal layer. The algae have bead like
filaments which are often coiled. A typical lichen containing this type of algae are lichen
from the Collema genus, which have a characteristic jelly like appearance. The Nostoc and
Scytonema families or type of cyanobacteria are the ones most commonly seen in lichens.
An association with cyanobacteria provides a number of advantages. Although normally
the algal partner is simply a photobiont, when in association with a cyanobacteria other
biological processes can be performed. For example, cyanobacteria allow the lichen to
break down nitrogen directly from the atmosphere, thus making it available to the lichen
generally. A good example of this is seen in the Lung Lichen Lobaria pulmonaria. This ability
allows such lichen to live in poor habitats where nutrients are lacking.
Cyanobacteria can also facilitate the break down of sugars, which can then be made avail-
able to the fungal partner. This ability means that lichens can survive periods of harshness
without detriment and accounts for the apparent ability to survive in suspended animation
over long periods of time.
Although these are considered the three main types of algae seen in lichen symbioses, there are
others which can be involved. Algae from 26 different genera are known to take part in some form
of symbiosis with lichens.
About 70 percent of lichenous associations are with algae from the Trebouxia order and 10 percent
with those from Trentepohlia.
Even closely related lichen may contain different algal partners. For example, Lobaria laetevirens
lichen contains green algae, while the related species Lobaria scrobiculata contains blue-green cy-
anobacteria.
This microscopic
image shows the algae
present within a
lichenous structure.
A mutually beneficial partnership?
By living together algae and fungi can be very successful. Both partners can bring something to the
party, so to say. Within the lichen relationship the fungal partner can provide the protection from
drying out and thus the stability that algae need in order to perform photosynthesis. In return the
algae can provide nutrition to the starved fungi through photosynthesis. The relationship in which
algae and fungi combine to form lichen is therefore perfect and utilises the best properties of each
partner in the association.
However, the relationship appears to be steered by the fungi. These are the dominant partner.
Lichenous fungi living alone appear to coerce algae into a cooperative relationship by harvesting or
ingesting lichen cells. The hyphal filaments, the living strands of fungi, enclose algal cells in the
lichen arrangement. Additionally, the fungal strands form most of the biomass of the lichen. Al-
though now a somewhat symbiotic lichenous ways of living, probably originally fungi coerced algae
into the relationship. Algae involved in lichenous associations are often happily able to live inde-
pendently without fungal partners- but only if conditions are right. The algal partner forms very
much the minority partner in the association contributing much less biomass, not being involved in
the external surfaces of the lichen, and being deeply embedded within the lichens tissues.
Some even consider the lichen relationship to be somewhat parasitic in nature, with maybe the
fungal partner being a parasite of the algae. This makes sense when you consider that it is the
fungal partner that is the one that obtains and uses nutrients from the algae. You could consider
the fungi as 'farming' the algae. The fungi do seem to steer and control the relationship. For ex-
ample, they can alter the permeability of the algae cell walls to allow more or less photosynthetic
nutrients to leave. More importantly the fungal partners can hinder and control algae cell division
and thus reproduction; the be all and end all of all biological processes. And it appears to be the
fungi which initiate the relationship by stealing algae cells in the first place.
How exactly the fungi control the algae is of yet unclear. There is clearly some form of communic-
ation between the fungal and algal partners which is probably chemical in nature. That a lichenous
association is advantageous is shown by the fact that roughly about a quarter of all fungi species are
engaged in just such a combination.
Despite the dual nature of the lichen symbiosis the separate individual partners can live independ-
ently if grown on specially prepared cultures in laboratory settings. But in the wild, the fungus de -
pends on being in association with algae in order to survive. Although algae can live very well inde -
pendently, and in fact thrive if given suitable conditions, the fungal partner struggles alone. The
typical lichen thallus does not develop and fungal growth is slow and stunted when a fungus at -
tempt to grow independently.
What are the advantages of the symbiotic arrangement for the algae? The algae appears to receive
no nutritional benefit from the arrangement and nothing appears to be transferred from the fungus
to the algae. The sole advantage to the algae seems to be in protection from the environment.
The unique relationship between fungi and algae means that lichens can live in places and habitats
which otherwise neither could inhabit. It also means lichens can live in places where other forms
are life not as adaptable, or maybe resourceful, are able to exist. The way of living that lichens are
engaged in means they are excellent first colonisers, able to inhabit stony ground and be the first
organisms to invade a newly formed habitat such as a volcanic rock surface or newly surfaced is -
land.
The partnership may originally have begun as being parasitic in nature, but has evolved to become
more commensalistic. Both partners now benefit by living together. The path from parasitism to
symbiosis is a gradual continuum which lichens are well along the path to completing. Who knows
where this may lead? Eventually algae may become permanently embedded within their fungal
overlords?
PLEASE CITE AS:
Walker, MD. British Lichens. Biology, ecology and use as bioindic-
ators. Sicklebrook publishing, Sheffield, UK.
A full copy can be read for free!:
www.researchgate.net/publication/378184553_BRITISH_LICHEN
S_BIOLOGY_ECOLOGY_AND_USE_AS_BIOINDICATORS
Alternatively it can be purchased for a very reasonable amount at:
https://www.lulu.com/shop/mark-walker/british-lichens-biology-
ecology-and-use-as-bioindicators/paperback/pro-
duct-p66zjdq.html?page=1&pageSize=4