Since the first scientific descriptions, carried out by the Swiss botanist Simon Schwendener, and the contribution of the German botanist Albert Bernhard Frank with the term ‘symbiosis’, both at the end of the 14th century, lichens have aroused the fascination of biologists and they have frequently been designated as the most refined form of symbiosis.
In general, a lichen has always been described as a living being formed by the cooperative union of two different organisms: a fungus, called a mycobiont , and an algae or a cyanobacterium, called a photobiont .
The body of the lichen is mainly formed by a fungus. The mycobiont is the organism that provides the structure, determines the morphology, supports the lichen in the substrate and facilitates its reproduction.
The fungi that form the mycobiont of a lichen do not belong to a specific group; on the contrary, different lichens can present structural fungi from different groups. There are over 18,000 species of lichen-forming fungi, most of which belong to the Ascomycota group, the same group as truffles, mold or brewer’s yeast. Although there are some species that have mycobionts from the Basidiomycota group, the same group to which common mushrooms such as champignons belong.
However, lichen fungi have evolved this symbiosis in such an intimate way that they are totally dependent on it and unable to live alone in the natural environment. Some have been isolated and cultivated under laboratory conditions, but they are difficult to maintain, and when they survive, their morphology is profoundly different from that of the lichen.
The most important difference between lichen fungi and free-living fungi is their food source. Free-living fungi are saprophytic, consuming organic matter from the environment and decomposing it. Lichen fungi can also perform this function, however, it has been observed that they can be sustained without the contribution of organic matter, thanks to the presence of the photobiont.
As was the case with the fungi that make up the mycobiont, photobionts can belong to very different groups. The most common is that they form part of the green algae genera Trebouxia, Coccomyxa, Trentepohlia or Phycopeltis , or cyanobacteria of the genus Nostoc or Scytonema , although there are also lichens with algae of the Xanthophyceae group — yellow- green algae —. Some genera of lichens, such as Lobaria, have several different photobionts.
However, unlike mycobionts, lichen photobionts lack such deep symbiotic dependency, and can be naturally free-living. However, the lichen symbiosis provides them with very significant advantages. On the one hand, the body of the fungus acts as a protection system against organisms that feed on the photobiont, and at the same time as a screen that attenuates sunlight, making it more tolerable. In addition, it helps to retain water , essential for the algae, and through its hyphae more easily obtain the mineral salts that the photobiont requires for its subsistence.
The photobiont, for its part, provides the lichen with food , in the form of glucose that it produces thanks to photosynthesis. When the lichen has a combination of cyanobacteria and green algae, the former can capture and fix atmospheric nitrogen, thus facilitating the nutrition of the latter.
Traditionally, one of the peculiarities of lichens was the enormous difficulty in producing them in the laboratory. Photobionts, and, with more difficulty, also mycobiont fungi, could be cultivated separately without problem. But by uniting them, the symbiosis did not occur , and, therefore, the lichen was not formed. It was as if a link in the process was missing.
And indeed, it was. A discovery made in 2016 showed that lichens have a third component that had not been observed before: a yeast from the Cystobasidiomycetes group .
This new component is another fungus, this time unicellular, and different from the mycobiont, which had gone unnoticed until genetic analyzes were carried out that revealed its presence. Although it was originally described in a lichen of the genus Bryoria , these new components have since been found in many other species. Some studies suggest that this third participant, the yeast, is in charge of keeping the union stable .
In addition, it has been observed that some bacteria also participate in the formation, maintenance and metabolism of lichens. Today lichens are no longer seen as living beings made up of two components in symbiosis, but rather as authentic complex ecosystems .
The new discoveries have shed a new level of understanding on lichens. The variation of their symbiotic system is enormous. One or several photosynthetic species from different groups, yeasts, bacteria , and more recently the participation of protozoa and even viruses , all of them embedded in the protective body of a mycobiont fungus, can coexist in the same lichen.
Lichens have evolved as open systems, as authentic miniature ecosystems. They present a dominant fungus, dependent on the association, within which multiple different species coexist and interact. Organisms that do not need the lichen to exist, but benefit from it.
Some authors propose to redefine the term ‘lichen’ as “a self- sustaining ecosystem , formed by the interaction of a fungus and an extracellular arrangement of one or more photosynthetic partners and an indeterminate number of other microorganisms”.
Understanding all the physiological processes and ecological relationships that operate in this ecosystem is still a scientific challenge that requires much research.
Hawksworth, D. L. et al. 2020. Lichens redefined as complex ecosystems. New Phytologist, 227(5), 1281-1283. DOI: 10.1111/nph.16630
Morillas, L. et al. 2022. Lichen as Multipartner Symbiotic Relationships. Encyclopedia, 2(3), 1421-1431. DOI: 10.3390/encyclopedia2030096
Spribille, T. et al. 2016. Basidiomycete yeasts in the cortex of ascomycete macrolichens. Science, 353(6298), 488. DOI: 10.1126/science.aaf8287
Vernon Ahmadjian. 1967. The Lichen Symbiosis. Blaisdell.