Lichens: so small, yet so complex! Photo: www.scienceminusdetails.com
Lichens are composite organisms often found growing on rocks and trees. Some species are commonly found in gardens, especially on well-established trees, and the advice to the home gardener has always been to ignore them: they’re harmless to plants and also add a bit of interest as well as (to a very minor degree) helping to fertilize the plants they settle on.
Lichens aren’t plants, though, but “composite organisms.” They were long thought to be simple combinations of one algae and one fungus. By joining their forces in symbiosis, each organism helping the other, the new composite organism develops properties different from those of its component organisms.
Most algae, for example, cannot live outside an aquatic environment, but, sheltered within the cells of a fungus, they can now carry out their photosynthesis on a tree trunk, branch or rock. And the carbohydrates they produce feed the fungus … and some of the nitrogen they capture from the air drips down to the plant roots below. Everybody’s happy!
This symbiotic one algae/one fungi relationship has been known since the 1800s, but it turns out it’s more complicated than that. Scientists are finding lichens often have a third partner or a fourth or even a fifth!
There are often two fungi involved: an ascomycete fungus and a basidiomycete yeast. And in some cases, there is a second basidiomycete involved in the partnership. And other lichens have been found where there is one fungus and two algae species … and sometimes three. With some 17,000 identified lichen species in the world (and probably at least as many unidentified ones), the possibilities seem almost endless!
I suppose symbiosis is rather like cooking: you need just the right ingredients to make things turn out!