Winter is a time when lichens become more noticeable, whether its on the branches and trunks of leafless trees, or on sea-cliffs; with fewer natural distractions about its a good opportunity to take a closer look. Lichens are fascinating; they are not a single organism, but actually a combination of a fungus and an alga (and/or cyanobacteria) living in partnership. The alga and/or cyanobacteria live inside the fungus, and in return for being given a home they provide the fungus with food that they make via photosynthesis. If you look closely (a hand lens or magnifying glass helps) they can be exquisitely beautiful. They come in all sort of colours and can be either branched, leafy or form a crust on the surface on which they live. Because lichens derive most of their nutrients from their symbiotic alga, they are pioneer organisms, growing in places like landslips and bare rock faces where nothing else can grow. Some lichens slowly decompose their substrate by secreting acids which chemically degrade the rock and then microscopic root-like hyphae grow from the base of the lichen and penetrate into the rock. Such "weathering" of the rock causes it to gradually turn into soil and so plants can then colonise the area. Lichens may be long lived, some are considered to be among the oldest living organisms (but lifespan is difficult to measure because what defines the "same" individual lichen is not precise) and can survive in some of the most extreme environments on Earth including Arctic tundra, deserts, and toxic slag heaps. I briefly touched upon some of the lichens that you find on sea-cliffs in my online course, "An Introduction to the Coastal Life of Britain" the other day and one of the participants asked if it were true that lichens are indicators of air quality. The short answer is yes, if you would like to find out more check out this great article by the Natural History Museum https://www.nhm.ac.uk/discover/nature-and-pollution-what-lichens-tell-us-about-toxic-air.html.
I took the top picture of lichens on a hedgerow when out on a local walk last week, and the bottom picture when I was on holiday on Orkney in the summer of 2019. The old farm buildings were very close to the sea and absolutely covered in what I believe to be Xanthoria parientina (but am happy to stand corrected if there's some lichen experts out there!).