A
lichen is a fungus living with, but not apparently harming an alga
or cyanobacterium.
Scientists
argue about whether the fungus-alga relationship is symbiosis or ‘controlled
parasitism’.
Only
the fungal partner can reproduce sexually.
Over
40% of the world’s 30,000 species of ascomycete fungi are lichenised.
There
are approximately 1700 lichen species in Britain , of which about 1000 are very rare. There are only
seven species of algal partners.
Lichens colonize very harsh environments where few other organisms can survive – from hottest deserts to freezing polar regions.
Over
800 lichens have so far been recorded in the Antarctic, where there are only
two or three flowering plants and about 60 moss species.
The
name given to a lichen is the name of the fungus as it is not possible under
the International Code of Botanical Nomenclature to give a scientific name to a
dual organism.
Lichens
include the oldest living organisms on earth, with some individuals estimated
at 5000 years old.
80%
of the sugars produced by the algal component leak through its cell walls
compared with only two percent in a free-living alga.
The
fungus converts the sugars into rare substances whose functions are not clearly
known but include discouraging insects and molluscs from eating the lichen.
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It
is believed that if a fungal spore lands on a different lichen that contains a
suitable alga, it can take over the alga from the host.
Lichens
lack a waterproof epidermis and so are very susceptible to air-borne pollution.
Lichens
are able to absorb large quantities of heavy metals, sometimes up to 10% of
their dry weight. This makes them useful for monitoring environmental levels of
radionuclides.
Man
has used lichens for thousands of years to produce orange, brown and purple
dyes.
The
perfume industry uses large quantities of Oak Moss lichen as a scent fixative.
Their
most important modern use is in the manufacture of antibiotics.
The
number of lichenologists in Britain is declining, mainly owing to changes in university
curricula.
Reference
Dobson, F.S.(2000). Lichens An Illustrated Guide to British and Irish Species. 7th ed. Slough . Richmond Publ. Co.
David Walker
