What are lichens?

Lichens are not plants nor are they animals that behave like plants (looking at you, coral); in fact, lichens are the result of at least two different kingdoms of life working together. Lichens are made up of one or more types of fungi combined with a type of single-celled alga and/or a type of cyanobacterium. Algae are similar to plants but are not similar enough to actually be plants, and cyanobacteria are a type of bacteria that can make food from sunlight via photosynthesis.

The fungi create the structure of the lichen, collect minerals for it and store water within it, meanwhile the algae/cyanobacteria use photosynthesis to create food for the lichen. The minerals, water and food are shared between the different organisms that make up the lichen, in order to keep all of them alive. This is known as mutualistic symbiosis, a process where different species work together to solve their problems.

Because lichens are not just one organism, it can be hard to classify them. Scientists tend to cheat in this regard and just name the lichen after the main fungus that makes it up. This is not a great way of classifying lichen, as you can have two different lichens that are made up of different species of algae or cyanobacteria, which are scientifically classified as the same ‘species’ of lichen.

I say ‘species’ of lichen because, even if you did have a name for each and every type of lichen, lichen cannot be a proper biological species, as that would imply that they are all closely related.  Lichen of course cannot all be closely related, since the organisms that make them up come from completely different kingdoms of life.

A better way of classifying lichen may be to put them into groups based on their structure. When you do this, you end up with three different types of lichen.

Knowing all this you should not really liken lichen to a bit of mould, as it is much more impressive than that!

Where can you find lichens?

Lichens are very common and can be found all over the globe, covering about 7% of its surface. They will grow on anything sturdy enough to support them, which includes rocks and trees for the most part. The fact that they can live on rocks is incredibly important, as there is not much else that can, so lichen are usually the first organisms to arrive in a previously uninhabitable area. This gives lichens the title of ‘pioneer species’. Lichens can break down the minerals in rocks and, over time, convert the rocks into soil, which any plant can grow in. Once plants start growing in an area, they will attract animals, so you eventually end up with the ecosystems you see today. Without lichen, there may not be anything living on land!

Lichens are not too fussy about where these rocks are either. They can be in extremely hot environments like deserts, in extremely cold environments like in arctic ice and in low oxygen environments like on the tops of mountains. Organisms that can live in these extreme conditions are appropriately named as extremophiles, and they are the most likely kind of life that could survive on other planets. In 2005, two different lichens were sent into space and exposed to its extreme temperatures, lack of oxygen and high amounts of radiation for over two weeks. Upon analysing the lichen afterwards, scientists found that it had coped fine with the conditions, with none of it dying or losing any of its ability to photosynthesize.

Lichens do have one weakness though, sulphur dioxide. Normally sulphur dioxide is not a big issue for lichens, as it is only naturally released from inside the earth during volcanic eruptions. Unfortunately for lichens, we humans have a nasty habitat of digging stuff out of the ground and burning it to generate power. Sulphur dioxide is one of the main pollutants created by burning fossil fuels, so it is hard to find lichens in heavily industrialised areas.

This has meant that London historically has not been a very good place to find lichen. Back in the 1970s you would struggle to find any lichen at all in London, apart from one particularly hardy ‘species’ called Lecanora conizaeoides, which you could still only find in areas that were more sheltered from pollution.

This is a crustose / ‘crusty’ lichen, which is to be expected as crusty lichens are the most pollution resistant types of lichen. Different lichens being more or less resistant to pollution makes them a good bio-indicator, which means they are a natural way of assessing air quality. If there are no lichens present, the air quality is bad. If only crusty lichens are present, the air quality is okay. If all three types of lichen are present, the air quality is good.

Fortunately, we have cleaned up our act somewhat since the 1970s, so you can now find more than one type of lichen in London. The best places to find it are in the greener areas of London, growing on trees and old gravestones. See if you can spot any yourself and try to work out if they are crusty, leafy or shrubby lichens!

I hope you have enjoyed learning about lichens and have grown to appreciate those important examples of fantastic teamwork in the natural world!

Sources

https://blog.education.nationalgeographic.org/2016/07/25/lichen-the-threesome/

https://www.woodlandtrust.org.uk/blog/2019/04/what-is-lichen-seven-types-of-lichen-found-on-trees/

http://wales-lichens.org.uk/species-account/sticta-canariensis

https://www.youtube.com/watch?v=XQ_ZY57MY64

https://www.astrobio.net/extreme-life/lichen-cosmonauts/

http://www.obsessedbynature.com/blog/2015/01/10/lichens-in-londons-streets/

https://www.rbkc.gov.uk/pdf/Ecology_Lichen_moss_report_red_Jan_2012.pdf