LIKING LICHENS

Except in very polluted and disturbed areas, lichens are almost ubiquitous. They grow on all sorts of substrata – walls, roofs, trees, the ground, even in the microscopic spaces inside rocks. They are not single organisms but consist of algae or cyanobacteria (formerly called blue-green algae) living amongst the hyphae (filaments) of fungi, in a mutually beneficial relationship known as symbiosis: the algae/cyanobacteria gain from the protection provided by the micro-environment of the fungal filaments and fungal nutrients, whereas the fungi obtain carbohydrates produced by the photosynthesising algae or cyanobacteria. The “body” of the lichen is known as the thallus (plural thalli). They are not parasitical but epiphytic, living on the surface or hanging from their substratum, or sometimes endolithic (inside rocks). Their reproduction is complex and beyond the scope of what I can cover in this short news note. It may involve spores in fruiting bodies called ascocarps, pycnidia containing specialised hyphae, soredia or isidia with hyphae and algal cells, or fragments of thallus that act as propagules 

Lichens come in all sorts of colours, shapes and sizes: they may be flat but leafy (foliose), bushy like miniature leafless shrubs (fruticose), a flat, flaky crust (crustose) or flat and powdery (leprose).  There are micro-lichens and macro-lichens, the former have a very small thallus, often crustose, and the latter larger, often “leafy” or bushy.  Lichens growing on the ground are “terricolous”, on bark “corticolous” and on stone, brick, rocks, etc. “saxicolous”. They are even found on leaves, mosses or other lichens and are tolerant of climatic extremes from deserts to icy wastes, to salty shores, to contaminated mine waste. 

Lichens are thought to cover as much as 6% of the land surface of our planet and some 20,000 species have been recorded world-wide.

Because of their sensitivity to air pollution, lichens are useful indicators of air quality. Rather as sampling invertebrates in a stream can tell you how clean the water is from the species found, so the species mix of lichens in a defined area can tell you how clean the air is. In both cases, the organisms are there “sampling” their aquatic or aerial environment day in, day out. The assemblage of species tolerant of varying degrees of pollution tells you how clean their environment is. 

Another practical use for lichens is to indicate disturbance of the substratum on which they are growing. Lichens grow at a known rate and are very long-lived, so lichenologists studying them on dated substrata such as gravestones can extrapolate the data to undated substrata where the same species grow. This can be useful in determining environmental stability or disturbance over centuries or even thousands of years. 

Several lichens are brightly coloured and the pigments they contain have been used as dyes since prehistoric times. Hundreds of species are documented as being used in dyes (see photos below). 

Recently, it has even been suggested that lichens might be used in space travel. Work at the University of Crete has found that some can resist the kind of extremes found extraterrestrially – the endolithic ones might even spread across planets inside rocks. Most excitingly, because they produce hydrogen which is a key ingredient of rocket fuel, they might even “be a boon on long space journeys”. (Astrobiology, DOI: 10.1089/ast.2017.1789).

Betts Ecology always consider the occurrence and condition of lichens as part of our management and biodiversity protocols. They are fascinating organisms to study, can be found almost everywhere and studied in any season – ideal for hobbyists who want a challenging pastime!  

Photos © Betts Ecology.