The Lichen Symbiosis Part 2

Lichens are a partnership. This can involve two or more partners in the one species. The partners are firstly mycobiont fungi often ascomycota, of which ascomycota forms the largest phylum of the kingdom fungi. Ascomycota contains 75% of all described fungi species and is the group that forms 95% of lichen species structures. This has resulted in one in five of all fungi becoming lichenised, in order to obtain different compounds etc. Ascomycota (see image 1 for a typical ascomycota fungi found in the UK) is also a paraphyletic group, consisting of all the descendants of a common ancestor, minus a few of the attached monophyletic groups. The other phyla of fungi that form lichens is basidiomycota (image 2 for a typical basidiomycota fungi found in the UK), and with ascomycota can form important components in the carbon and nitrogen cycles in some areas, so that the compounds the lichens produce, can be used by other species. In laboratory experiments, it has been found that some fungi can be facultative lichen partners, also having the ability to use nutrients from other sources, without their algae/cyanobacteria symbiont, however; most of the fungi cannot survive in the wild without their algal/cyanobacterial symbiont and are wholly dependent on them (obligate).

To complete the symbiotic relationship, prokaryotic or eukaryotic organisms come in to play; these are in the form of green algae a eukaryote, a division within the kingdom plantae or cyanobacteria, prokaryotic organisms, a phylum of the domain bacteria. Both types of organism are firstly autotrophs, taking simple elements within the environment and from them creating complex compounds. In addition, they are also phototrophs (photosynthetic), deriving their energy from the sun, in order to create complex compounds. It is also believed that most of the green algae occurring in lichens, can survive outside symbiosis, unlike the fungi. Both types provide the fungi with fixed carbon, with the cyanobacteria also fixing nitrogen, therefore, the lichens involved do not rely on atmospheric nitrogen deposition. However, green algae and cyanobacteria contribute very few species to the symbiosis, it is believed that there are only around 100 species across the globe, whereas, for each defined lichen species, there is a different species of fungi. This is because; a species of lichen is always named after the species of fungi in symbiosis. It is also known, that one species of fungi can also form different morphs of the lichen species, by associations with different species of green algae/cyanobacteria. In addition to this, a specific species of lichen can also have more than one species of algae often two. It has also been proved, that both green algae and cyanobacteria species, can be present in a single species of lichen, again forming a number of different morphs.

Principally, the algae provide the fungus with organic carbon and in the case of the cyanobacteria; it is likely nitrogen exchange takes place. In return, the algae/cyanobacteria are provided with secondary metabolites (that sustain an organism over the long-term in the environment, but are not essential at that point to sustain the organism’s life) i.e. sun-screens, herbivore deterrents and prevent dehydration damage.  Such chemicals can only be produced if the algae/cyanobacteria is present, therefore without the partner, the fungus has no chance of survival, as they do not produce the chemicals on their own. The algae/cyanobacteria also get a home, and a steady supply of minerals and water. This partnership has allowed the two organisms to survive in places that would be inhospitable to either partner on their own, and to a great deal of life on the planet.

I would like to thank Fay Newbery for her significant help towards creating this blog. As well as Dr Jonathan Mitchley for initial Identifications of lichens.

For British lichens, please take a look at The British Lichen Societies web page.

Or take a look at the following publications:

Nash III, H.T., 2010. Lichen Biology. 2nd ed. New York: Cambridge University Press

Gilbert, O., 2000. Lichens. London: HarperCollins

Purvis, P., 2000. Lichens. London: The Natural History Museum

Duncan, U. K. and James, P.W., 1970. Arbroath: T.buncle & CO. Ltd.