Lichens are not plants, but fungi from a special group that are very easily associated with algae and form what we call lichens. There are very different types, but what is characteristic is that it is an obligate symbiotic connection between a fungus and a single-celled alga that occurs as a colony within the fungus. Lichens similar to today’s have been on the planet for more than 300 million years, but large lichens are much more modern. They often accompany forests or form mats in the tundra or protective crusts in other inhospitable environments.
When we say that lichens are the product of an obligatory symbiosis, I mean that this fungus cannot live if there are no algae or bacteria of this species suitable for symbiosis. Why has evolution made it seem like this is a mix of two? Researchers are increasingly understanding its evolutionary success, but that will certainly be the case because it serves many purposes. It is very useful for many insects, it is used to refresh the bark of trees or colonize rocks. These symbiotic organisms, lichens, have no roots, they do not penetrate very deeply into the bark, they just stay on it and make no difference to the tree, but provide more freshness and more moisture in its bark. And on the rocks, lichens can penetrate half of their bodies and help break up the rock. They make small cracks in which soil forms. With this small piece of soil, mosses and some small plants can develop, which is the beginning of a more complex system. And over the course of thousands of years, soil can form, so that lichens also play a role in the formation of soil on rocks. However, what they achieve together in a much shorter time is to turn the rock into a habitable system with great biodiversity.
Thanks to the participation of the chlorophyll symbiont, lichens are fungi that are photosynthetic. This means they become oxygen producers. This is some kind of miracle of evolution.
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Each species of lichen is a different type of fungus, but the single-celled algae that make up these lichens belong to a few groups that are very similar to each other. Each species of lichen is a different fungus, but the algae that form lichens may be the same or different in each species.
So far I have explained to you that lichens are organisms made up of two living things: fungi and unicellular algae. But that’s not always the case. There are some lichens that, instead of algae, form a symbiosis with cyanobacteria, which are bacteria that can carry out photosynthesis.
There are lichens almost everywhere on earth. But his paradise is the polar regions. And that’s because there are almost no other chlorophyllous creatures there; for example, in maritime Antarctica there are only two species of vascular plants and around 300 species of lichens. There they form miniature forests that cover the ground and rocks, the few remaining soils, areas that remain free of snow for a long time or are not covered with snow at all. Lichens resist low temperatures very well and even tolerate freezing of the water in their organism. When this happens, the lichen falls into a lethargic state and nothing happens to it. When the thaw comes, when the temperature rises, they are no longer covered with snow and light penetrates them, the water becomes liquid again and the lichens come to life again.
But lichens also live, and sometimes very well, in almost any environment that is hostile to other living beings, for example in deserts. There are huge mats of lichen in the Atacama or Kalahari Desert. In our temperate worlds, they live in forests or on their periphery and coexist with other living things, especially plants and insects. They also live on bare rocks that cover them, giving them beautiful colors.
The only places they don’t live are polluted environments. They do not live in places where humans have heavily intervened, they do not live on cultivated soils. However, it is very important to note that they do not live in cities or around industrial centers due to pollution. If you don’t see lichens in temperate climates, you have to wonder what’s happening there.
There was a very famous Finnish lichenologist who lived in Paris for a long time, William Nylander, who noticed the lack of lichens in the Parisian gardens of Luxembourg as early as 1866 and realized that this was due to the urban environment becoming increasingly inhospitable and dry. We are talking about the industrial revolution. And his studies of the disappearance of lichens in contaminated environments form the basis for later work on the use of lichens as bioindicators of environmental quality.
Ana Maria Crespo de Las Casas She is a professor at the Complutense University of Madrid and a member of the Royal Academy of Sciences of Spain. His research interests include biomonitoring of air pollution caused by lichens and the evolution of lichens and their relatives.
Question emailed from Sonia Inaraja
Coordination and writing:Victoria Toro
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