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What
are Lichens?
Lichens present a very intriguing problem
for people whose job is to name different kinds of organisms. This is because
a lichen is not a separate organism in the sense of being one type of
individual. It is actually a close partnership between a fungus and an alga.
(Algae are very simple plants).
The two types of organisms in the
partnership are so closely interwoven that they appear as a single
individual. This individual looks entirely different to either of the partner
organisms making up the structure. Lichens are distinctive and they form many
different, recognizable types. Many of these have been given specific names
of their own, despite the fact that eachlichen is already a mixture of
different species.
There are more than 1,700 species of lichen
in Britain. Approximately 18,000 species of lichen have been described and
identified worldwide. The algal partners in lichens can be found living on
their own in nature, as free-living species in their own right. The fungal
partners in British lichens are recognizable Ascomycetes or Basidiomycetes. However, they have come
to need the right kind of algal partner in order to survive. Unlike other
fungi or indeed their algal partner, they cannot survive on their own.
Of the more than 1500 *genera of algae worldwide, relatively few make
suitable lichen partners. In Britain, only three genera, Trebouxia, Trentepohlia
and Nostoc are common fungal partners. Interestingly, the same alga
can combine with different fungi to produce entirely different lichens. The
same fungus can also form different lichens depending on the type of alga
which it associates with. Most lichens contain only one kind of alga, but
some may contain two. Identifying the type of alga in a lichen may be difficult,
as they frequently look different to the free-living forms.
The fungal partner forms the main body of
the lichen, with the algal cells either scattered among the fungal hyphae, or arranged in a layer just below the
upper surface of the lichen.
Lichens colonize some of the most
inhospitable habitats on earth. They can survive in extremely cold areas such
as on high mountains and in regions such as the arctic. They may be virtually
the only plant form surviving in some of these areas and can be vitally
important sources of food for animals. They are also found throughout less
extreme climates, inhabiting just about any solid surface. This can range
from rocks on sea shores, to walls, trees and concrete. A few are unattached
and blow about freely.
Lichens are so enormously successful and
widespread because of their unusual partnership. The algal cells, through the
process of photosynthesis, provide the fungus with some of the organic
nutrients which it needs. In lichens where the partner is a species of Nostoc,
organic nitrogen is also supplied to the fungus, because Nostoc is
able to fix atmospheric nitrogen. In return, the water, nutrients and gases
absorbed from the environment by the fungus are shared with the algae. The
fungus also plays a vital role in providing a physical structure to shelter
the algae from excess sunlight and in particular, water loss.
There is uncertainty over the exact nature
of the relationship between the fungus and the alga. Some people think the
fungus may be a type of weak parasite, which doesn't kill all of the algal
cells or, that it keeps the alga imprisoned as a kind of slave.
Alternatively, it may be a type of relationship called a 'symbiosis' where
both partners benefit. Whatever the exact nature of the relationship, it
undoubtedly results in an 'organism' which is capable of surviving in places
where neither the fungal partner, nor the algal one, could survive on their
own.
Lichens have a variety of different growth
forms. The simplest lichens are crusts of loosley mixed fungal hyphae and
algae. Others are more complex, with leafy or shrubby forms like miniature
trees, also having specialised structures to attach them to a surface.
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Crustose
(or
Crustaceous) lichens are, as their name suggests, encrusting forms which
spread over and into the surface of their habitat. They cannot be removed
from the surface without crumbling away.
Foliose lichens are lichens with
leafy lobes, which spread out in a horizontal layer over the surface. They
are attached by root-like threads and can be easily removed with a knife.
Fruticose lichens are shrubby
forms with many branches. They can be removed from the surface by hand.
There are many gradations of form in
between these three main groups and a lichen may not always fit clearly into
one or other of these artificial categories.
Lichens reproduce either by tiny parts of
the lichen breaking off and growing somewhere else, or by the fungal partner
producing spores. Lichens may have powdery masses on their surface. These are
the tiny bits of the lichen body which will be shed to form new lichens. The
individual bits are called soredia and they contain both the fungus and the
algal partner together.
In most cases, fungal spores are either
produced in apothecia or perithecia on the surface of the lichen. The spores
come only from the fungal partner and do not contain any algal cells. They
may germinate after being shed from the fruiting body, but they will only be
able to form a new lichen if they happen to make contact with a suitable
algal partner. Without the alga, the germinating spore will die, as the
fungus cannot survive on its own.
The fruiting bodies of most lichens are
unusual in that they may continue to produce spores at intervals for several
years. The fruiting structures of individual fungi in contrast usually last
for a relatively short period.
Lichens grow relatively slowly. The actual
growth rate depends both on the species and on the environmental conditions
around it. The smaller encrusting lichens may grow as little as 1mm a year!
Larger forms may grow up to 1cm per year.
This slow growth rate has been used to
develop a method of dating surfaces on which lichens are growing. The method,
known as lichenometry, has been used in places such as the arctic, where
lichens grow very slowly and can live for very long times. The method works
by using a series of photographs over a period of time, to work out the
growth rate of the particular lichen. From the size of the lichen, it is then
possible to calculate how long it has been growing there. Using this method,
some individual lichen colonies have been estimated to be 9000 years old. If
this is so, then these particular lichens may well have been alive while
people were still in the Stone Age and woolly mammoths roamed!
Lichens absorb water and minerals from
rainwater and directly from the atmosphere, over their entire surface area.
This makes them extremely sensitive to atmospheric pollution. As a result,
there are usually very few lichens around industrial centres and towns.
Different lichen species vary in their
tolerance to pollution and therefore make very good biological indicators of
levels of atmospheric pollution.
A walk around your local churchyard can
often reveal a lot about air quality in your area. Churchyards are usually
relatively undisturbed areas, with stone headstones which provide a good
substrate for lichens. A good look at these lichens will give an indication
of how good the air quality is locally.
Lichens have had a wide variety of uses
over the ages. Before the advent of modern dyes they were extremely important
sources of dyes for clothing. Different lichens yielded different dye colours
and they could be mixed to produce a wide variety of colours.
Lichens also have an interesting chemistry
and produce a large number of acids, many of them found only in lichens. The
litmus dye used so widely as an acid/alkaline indicator in chemistry comes
from lichens. Some species also have antibiotic properties. Some of the
lichen acids are utilized in drugs that can be more effective than
penicillin.
One of the more bizarre uses of lichens
from the past is as packing material for ancient Egyptian mummies!
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