Two organisms living together and cooperating: the lichen symbiosis
The resistance properties of the lichen come from the symbiosis between the alga and the fungus: the alga provides organic matter through its photosynthetic activity while the fungus provides water, minerals and protection. In addition to the pooling of their respective capacities, this cooperation makes it possible to create new capacities, such as the synthesis of defensive substances.
Think of the lichen as a greenhouse built by the fungus and housing the green algae. The fungus provides them with everything they need to produce sugars through photosynthesis: water (a little dew, rainwater, fog or snowflakes), minerals from the dust and vitamins from its production.
Sometimes the mushroom lives with cyanobacteria rather than green algae, which allows it to harvest both sugar and nitrogen fertiliser. But sometimes the greenhouse houses both green algae and cyanobacteria, but in this case the cyanobacteria are housed in separate rooms; each to their own! Cyanobacteria chambers are called "cephalodies".
Lichens are sometimes subjected to extremely unfavourable environmental conditions. Fortunately, the symbiotic association makes it possible to overcome these trials:
Is the lichen suddenly subjected to a dry spell? No problem, it slows down and stops all exchanges with the outside world. But everything will start working again in a few minutes at the slightest source of moisture. This is called "revivification": the lichen's thallus adjusts its degree of humidity to that of the surrounding atmosphere; when it is dry, the thallus is dry; when it is wet, the thallus is wet. The lichen has a little sponge-like quality§
The sun is too aggressive and risks damaging the algae? The fungus then unrolls its production on the roof to protect it from the sun; some lichens are even capable of tanning! Also, during periods of extreme cold, the greenhouse is kept frost-free because the fungus has included anti-freeze in its survival equipment.
Even when attacked by micro-organisms or herbivores, the lichen can defend itself. It is able to synthesise antibiotic or anti-herbivore substances and thus repel its predators!
In the end, you don't need much in the lichen greenhouse and you are very thrifty. Indeed, the mushroom has a very efficient water policy: just what is needed and no losses on the way. There is no question of wasting food either, the growth of the algae is perfectly regulated by our gardener. This thrifty lifestyle allows the lichen to live for a very long time.
The cooperation of the algae and the fungus maximises the chances of survival by providing protection and resources. Living is good, but it is also necessary to perpetuate one's species and therefore to reproduce, under all conditions. Here again, lichens have developed different strategies, which are described in detail in the next section: "Diversified modes of reproduction to maximise chances".
For 150 years, scientists have considered lichens to be the result of a symbiosis between an organism capable of photosynthesis (algae and/or cyanobacteria) and a fungus. But a study published in 2016 by researchers from the University of Montana revealed the presence of new organisms in lichens: yeasts! Yeasts are microscopic fungi consisting of a single cell.
The researchers analysed the DNA present in the outer layer called "cortex" of two species of lichens. The results reveal not only the presence of DNA from the algae and the fungus but also from a yeast. They also found that most of the lichens studied are associated with a yeast of a different species each time. Finally, it is this yeast that partly determines the phenotype of the lichen. This discovery makes it possible, for example, to understand why two lichens made up of the same alga and the same fungus can have a different colour. However, the role of these yeasts remains to be clarified.
* Spribille et al, " Basidiomycete yeasts in the cortex of ascomycete macrolichens ", Science, Vol. 353, no 6298, 2016, p. 488-492.