Lichens are not plants, they are fungi from a special group that is very easy to associate with algae and form what we call lichens. There are very different types, but characteristically they are an obligate symbiotic association of a fungus with a unicellular alga that occurs as a colony within the fungus. Lichens similar to today’s have existed on the planet for more than 300 million years, but large lichens are much more modern. These frequently accompany forests or grow forming mats in the tundra or protective crusts in other inhospitable environments.
When we say that lichens are the product of an obligate symbiosis, I mean that this fungus cannot live if there is no algae or bacteria of that type, which is the type that suits it, for symbioticize. Why has evolution facilitated the appearance of this being a mixture of two? Research continues to better understand its evolutionary success, but it will surely be because it serves many purposes. It is very useful for many insects, it is used to refresh the bark of trees or to colonize rocks. These symbiotic organisms, lichens, do not have roots, they do not get very deep into the bark, they only remain on it and do not make any difference to the tree, but they provide greater freshness and greater humidity in its bark. And on the rocks, lichens can penetrate half of their body and that helps to break the rock, they make small cracks in which soil is formed, with that little bit of soil mosses and some small plants can begin to develop, which It is the beginning of a more complex system. And over the passage of thousands of years, soil can form, so lichens also collaborate in the formation of soil on rocks. But what they do collaborate on in much shorter periods of time is to make the rock a habitable system with great biodiversity.
Lichens, thanks to the participation of the chlorophyllic symbiont, are fungi that become photosynthetic. That is, they become oxygen producers. This is a kind of marvel of evolution.
Each species of lichen is a different species of fungus, however, the unicellular algae that are part of 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 can be the same or different in each species.
I have explained to you so far that lichens are organisms composed of two living beings: fungus and unicellular algae. But it’s not always like this. There are some lichens that, instead of algae, are symbiotic by cyanobacteria, which are bacteria that can carry out photosynthesis.
There are lichens almost everywhere on Earth. But its paradise is the polar areas. And that is because there are almost no other chlorophyllous beings that survive there, for example in maritime Antarctica there are only 2 species of vascular plants and around 300 species of lichens. There they form miniature forests covering the soil and rocks, the little soil that remains, areas that remain uncovered by snow for a long time or that are not even covered by snow. Lichens resist low temperatures very well and even tolerate the water inside their organism freezing. When that happens, the lichen enters a lethargic state and nothing happens to it. When the thaw arrives, when the temperature rises, they are no longer covered in snow and light reaches them, the water becomes liquid again and the lichen revives.
But lichens also live, and sometimes very well, in almost any environment hostile to other living beings, for example in deserts. In the Atacama or Kalahari deserts there are enormous lichen mats. In our temperate worlds they live within forests or on their periphery and coexist with other living beings, in particular they coexist very well with plants and insects. They also live on bare rocks which they cover, providing them with beautiful colors.
The only places where they do not live are polluted environments. They do not live in places heavily intervened by people, they do not live in cultivated soils. But it is very important to note that they do not live in cities or in the surroundings of industrial hubs due to pollution. When in temperate environments one does not see lichens one must ask what is happening there.
There was a very famous Finnish lichenologist who lived for a long time in Paris, William Nylander, who already in 1866 noticed the lack of lichens in Paris, in the Luxembourg gardens, and realized that it was due to the urban environment that was becoming inhospitable and dry. We are talking about the industrial revolution. And his studies on the disappearance of lichens in contaminated environments are the basis of subsequent work on the use of lichens as bioindicators of environmental quality.
Ana María 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. Among her lines of research are the biomonitoring of atmospheric pollution through lichens and the evolution of lichens and their relatives.
Question sent via email bySonia Inaraja
Coordination and writing:Victoria Toro
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