By Andy Winfield
I’ve recently started noticing lichen. Now I can’t stop noticing it; it’s everywhere, living on, and in multiple surfaces. Walking around Bristol it’s on the harbour walls, the loch gates, on holiday it’s on the rocks of the cliffs and hanging from trees. Lichen covers around 8% of the planet’s surface area, and so could be argued that it’s one of the most successful collaborations in the natural world. When you do start noticing, you want to keep noticing, and get in closer with a little hand lens, a looking glass.
A conservative estimate for the number of species of lichen is 15-20,000, and they grow everywhere. They’re at the top of Mount Everest, in the arctic, in desert regions; there’s even a species that grows on the shells of barnacles. So with a life form that is in most places, under our feet and above our heads, how have I not seen how magnificent they are before?! There is a lot to know about lichen, and they’re all so different that there is very little conformity among species, so I’ll try and cover the basics here; but lichen basics are different to all other basics.
You may know that lichen is a collaboration; it’s part fungi part algae (or sometimes cyanobacteria). We now know, however, that there are often more than one species of fungi or algae; in her wonderful book Lichenpedia, Kay Hurley describes lichen as “not dual organisms, nor even triple, they are actually tiny ecosystems”, and lichenologist Toby Spribille says that “we have yet to find any lichen that matches the traditional definition of one fungus and one alga”. The same lichenologist found that there is almost always yeast cells present in of a lichen, and many species have bacteria. Curiouser and curiouser.
So how can we identify a lichen? There are 3 main forms of growth (or thallus) of lichen, which are always formed by the fungal partner. The first is crustose, firmly attached and hard to remove; as its name suggests, it’s like a crust on the surface, sometimes even like a shadow or chewing gum stain on a rock. Foliose, a flattened leaf like thallus that spreads over the surface in a creeping kind of way. Finally fruticose, its branching structure can hang from trees or grow up like a tiny shrub; it is fruticose lichen that forms part of the diet of reindeer.
So how do these bands of distinct cells live, and how do they create these structures that are different from all other organisms? The main partner in the lichen plc is fungi; they form the thallus and cradle the alga/cyanobacteria cells. These algal cells photosynthesise producing food for all parties, all the while enveloped in a fungal safety net enabling the resistance of extreme conditions. This protective fungal screen means that lichen can grow in the harshest conditions, with one species recorded as photosynthesising in temperatures of –15 degrees Celsius, and in deserts they’re protected from desiccating completely. As well as determining the shape and structure of team lichen, the fungi can also absorb water and minerals. Another fungal role is to secure the structures to whichever surface it is growing on, and it does this using rhizines, small filaments with no other job but anchoring, and they do it very well. It only takes a glance at a crustose lichen on a boulder to know that rhizines are very good at their job.
So how do lichen become more lichen? As with everything to do with lichen, it’s complicated. Some rely on fragmentation, bits breaking off and blowing onto new surfaces. Soredia are small bundles of cells, all the necessary cells of a lichen to begin a new one somewhere else; they’re created and packed off to a new surface when a wind, rain, a creature removes them. Isidia are similar to soredia, but are growths that break off rather than bundles that fall out of the lichen. Sexual reproduction is limited in lichen as only a fungal partner can produce spores; these spores then have to land on compatible algal cells to create a new lichen.
Lichen is everywhere. Just there, photosynthesising, being a conglomeration of cells that plays its part in the carbon cycle, and we haven’t even mentioned cultural uses from food and dyes, longevity, their still ambiguous history on earth. If you walk around our harbourside here in Bristol and look at the ground, you will see lichen. You’ll see it on stone, on the wood of the loch gates. I recently went to Cornwall where the granite cliffs are painted with the mapwork of lichen. These tiny, beautiful little wonderlands.