|What are Lichens?
- An astonishing partnership between two very
- Colonies which may be 9,000 years old
- Colourful dyes for clothes
- Packing for ancient Egyptian mummies!
- Pollution indicators
- High mountain dwellers and Arctic survivors
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 each lichen 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
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
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.