Lichens are a group of organisms that can – just like fungi – not be counted among the plant kingdom, especially if blue-green algae (Anabaena, Nostoc, etc.) act as the ‘green’ partner of symbiosis (phycobionts). Phycobionts are usually green algae (Chlorophyceae), yellow-green algae (Xanthophyceae), and some other groups of algae. It is pointless to argue where to group them. It is the fact, that successful new life forms developed by the combining of algae and certain fungi (usually from the ascomycetes group, more rarely also from the basidiomycetes or zygomycetes) that is important. The classic proposition of the system theory that a system is far more than the sum of its parts applies here. Lichens, about 16,000 recent species of which exist, are therefore to be studied and analyzed just like any other class of organisms as a group of their own. A separate nomenclature exists in lichenology in order to characterize the structure and propagation of the single species and to observe their life cycles. A more detailed covering of this topic would go beyond the scope of our project. The following three problem areas are helpful in understanding the interactions of plant and fungus (also called phycobiont and mycobiont).
Phycobionts are without exception able to live without their symbiont. Blue-green algae change their phenotype only little in the symbiosis with their mycobionts. Symbiotic green algae that belong mostly to Chlorophyta of the genus Trebouxia are able to exist without their mycobiont, too. It may be noticed that the genus Trebouxia is rather common in its phycobiont state, but occurs in nature rarely as a free-living alga. No zoospores and gametes are produced as long as the symbiosis exists. Isolating studies showed that the ability to produce them is not lost, though. Algae living as phycobionts do rather – and just like their free-living counterparts – immediately produce the mentioned stages of propagation after isolation from their mycobiont. A number of algal species that are multicellular when living on their own, are single-celled as phycobionts.
The fungal mycelium of real lichens is, in contrast to the algae, unable to survive in nature without its phycobiont. The mycelium is usually not capable of producing fruiting bodies, though exceptions may exist. The mycelium of some few species may grow under laboratory conditions (enough nutriments, addition of algal extracts) on agar, but no lichen-specific thallus is produced, and the development of a fruiting body does not occur.
Lichens do characteristically live in an autotrophic way, since the phycobiont is photosynthetically active. Trebouxia secretes about eight percent of the carbohydrates produced by photosynthesis as a free-living alga, while Trebouxia cells isolated from lichens secrete up to 40 percent. The secreted compounds are mostly simple sugars (glucose, etc.), as well as sugar alcohols like ribit, erythrit, sorbit, etc. The increased rate of secretion is induced by the fungus that causes a reduction of the plasmalemma’s permeability and a change of the wall structure. Nitrogen-binding blue-green algae (like Nostoc) supply their partner in addition with reduced nitrogen compounds.
The commitment of the fungi seems to be larger. The different lichen species display different degrees of contact with the respective alga that mirror their evolutionary level. Numerous transitions between temporary associations of fungi and algae and permanent unions (the actual lichens) exist. The interactions can be classified as follows according to their increasing complexity:
Fungal hyphens and algae exist side to side but without a direct physical contact. The position of the alga has no influence on the orientation of the hyphens.
Single algae or groups of algae are loosely embraced by fungal hyphens.
Single algae or groups of algae are fitted closely by the fungal mycelium.
The hyphens differentiate and form specific clasping hyphens that enclose single algal cells or groups of them tightly.
Algae and fungi form tight contacts with each other. This may – especially in primitive lichens – lead to the development of haustoria.
Haustoria (sg. haustorium) are evaginations of the hyphen with which the fungus penetrates the cell lumen (in this case of the alga) through the cell wall. The cell content remains intact and enclosed by the plasmalemma. The contact areas of plasmalemma and haustorium are often changed preventing a super-proportional spread of the haustorium.
Microscopic preparations, especially that of higher developed forms, display an organization into zones. The surface is formed by an often scabby and pigmented cortex devoid of algae. It is followed (from outwards to the inner parts) by a core of loose mycelium. The advantages of this architecture can easily be recognized: the scabby cortex prevents a loss of water of the thallus and can at the same time store water very fast. The pigments and their positioning make sure that the algae are not exposed to too high but to still sufficient intensities of light.
Lichens can absorb large quantities of water. Their fresh weight is often a multiple of their dry weight. On the other hand are they rather unaffected by desiccation. They are able to survive longer periods of dryness without damage. The lichen thallus is species-specific. It is distinguished between crustose lichens-, foliose lichens, shrubby (fruticose) lichens and jelly lichens according to their morphology.
Lichens belong usually to the most unpretentious organisms. Many species are pioneers: they settle in places that provide no habitats for other organisms. They can be found in the northern parts of tundras (up to 86 degree N) as well as in the Antarctic (up to 80 degree S). They live on high mountains (nearly 5,000 m max.), in deserts, semideserts, in the tropics, and in temperate climates. Lichens thrive also on rocky subsoils, an ability that is hardly shared by plants and fungi. Fungi lack the organic substrate in such an environment, multi-celled plants cannot cope with the short periods of vegetation characteristic for extreme biotopes, and algae are too sensitive to desiccation.
These difficulties resemble the situations that must have predominated the earth’s surface before it was largely covered by vascular plants and their precursors.
The versatile nature of lichens is also displayed by the fact that they were able to survive in large parts of the earth even when they became inhabited by plants. Lichens adapted easily to the new conditions. They can often be found on plants (at the bark of trees, on leaves) where they lead an epiphytic life. They are rarely parasitic and are able to take up and store ions selectively. Species living on the bark of trees, for example, contain a higher percentage of silicates, phosphates, magnesium oxides, iron oxides, and aluminium oxides than the bark itself. An accumulation of heavy metal ions is not uncommon. The growth of lichens is inhibited even by small amounts of sulphur dioxide. The disappearance of lichens in towns and in woods growing in the vicinity of towns was a reliable indicator of the atmosphere’s rising sulphur dioxide concentration long before the discussion about the acid rain started.
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