By Kirsty Jackson (@kjjscience)
Have you ever been for a walk, or sat in the park, and wondered what that weird stuff growing on the side of the trees, on walls, or even the very bench you are sitting on is? Chances are that what you are looking at is Lichen. Lichens can be a range of colours from green, grey and even orange. They are often overlooked as insignificant, thought of as a nuisance or even mistaken for moss but if you look a little bit closer you’ll see they are something quite remarkable.
Lichens are not a plant. Lichens are in fact made up of both a fungus (mycobiont) and a photobiont (either algae or cyanobacteria). They form a stable, self-supporting association, or mutualistic symbiosis with each other, where the photobiont lives within the fungus. It’s for that reason that Lichens do not have a taxonomic group of their own like mosses or ferns do. Lichens are polyphyletic, meaning that the lichen lifestyle has evolved more than once and are named after the fungal symbiont. Approximately one fifth of all fungi become “lichenised” and roughly 1700 different lichens are found in the UK and Ireland.
Lichens grow in leaf-like structures called thalli. If you section through a thallus you can see different layers within it (a bit like a trifle). On top you have an upper cortex made up of fungal cells that protect the photobiont layer below. The photobiont layer sits on top of a loose layer called the medulla, made up of fungal hyphae. Then there is a lower cortex layer which also has the root-like structures called rhizinae that help the lichen attach to the surface it is growing on (see below).
Variations to this layering system can also be observed but this is the general model. Often reproductive structures can be seen upon the surface or protruding from the thalli. Lichen reproduction could take a whole blog post to itself, but I should bring to your attention that they used both asexual and sexual forms of reproduction.
Evidence for human uses of lichens can be found throughout history. The most abundant use of lichens was for wool and silk dyeing. A red/purple dye called orchil or cudbear is known to have been used by the Egyptians. Orchil is made from species of the Ochrileichia, Roccella, Lasallia and Umbilicaria lichens. It is even mentioned in the Old Testament. The lichen was steeped in ammonia, or more commonly urine, for several weeks in a closed container. When dried the resulting paste could then be used for dyeing of materials. In the UK browns and brown-red shades of wool were also obtained by lichen dyeing. Crottle or crotal was made alternate layers of wool and lichen being boiled in a pot of peaty water until the wool was the right colour. The lichens used for crottle were Parmelia ompalodes and P. saxatilis. The dye from the lichens did not run (unlike our modern synthetic dyes) and is said to give the wool a more pleasurable texture and aroma. Lichens have also been used as food being regularly eaten by nomadic tribes living in Artic areas. In the UK lichen has only been eaten when food rations were low and “rock tripe” Lasallia pustulata was used as a survival food.
Lichens also provide a kind of aesthetic feel to our surroundings, a hint that something has stood for a long time. Try to imagine, for example, a church yard with no lichen and it just isn’t quite the same. People cover new builds with yogurt to encourage the growth of lichens and mosses, getting that aged look in a relatively short amount of time. Lichens are thought to be slow growers, but growth rates amongst the group vary widely. One feature of lichens that enables them to stand the test of time is their ability to enter a form of suspended animation. They can tolerate being dried out for more than a year and in their dry state can survive extreme temperatures, both hot and cold. When they find water again, they reanimate and can continue to grow as before.
Like many other things, lichens were also used in traditional medicine as they were thought to contain antibiotic and anti-cancer properties. Modern research is currently trying to establish if there is any science behind these claims. We are not the only creatures that use lichen; reindeer eat it, birds use it to decorate their nests, peppered moths and other insects use it as camouflage and lots more. I could go on to fill a whole book about lichens so I think I will leave it here. My plea to you from the lichens is: next time you go out for a stroll, spend a few moments having a closer look at the lichen that you find instead of just passing them by, you might see something that you’d have otherwise missed.
About the author: Kirsty Jackson is a fellow PhD student at the John Innes Centre, Norwich. She is studying rhizobial and mycorrhizal symbioses with legumes and loves all things fungi! When she isn’t in the lab she is involved with organising science outreach events. She is an author for the new John Innes SVC group blog. Follow her on twitter (@kjjscience).
Oliver Gilbert (2000) “Lichens” Harper Collins