1. Ambiorix Square🍃

What is a lichen?

You will find below the audio and the text of the walk !
Have a good adventure !

PS: Small correction of the audio, the rhizines are represented by the letter “e” and not the letter “d” on the diagram (see below in the text).

The problem was that we didn’t know who we were talking about when we said “we”

Adrienne Rich, American feminist poet, essayist and theorist

I would like to take you through a world of questions, entanglements, and connections. A world of mystery and imagination. A world where little is known. Through this adventure and journey of lichen discovery, I would like to imagine with you other stories about us, our perceptions, our beliefs and our futures. I would like to open a door to another world, a world where we create spaces – like cities – by considering other organisms, like lichens. 

Lichens (pronounced L-Y-K-E-N), as we will see, push us to these reflections and point out to the fact that our bodies do not end at our skin. Lichens invite us to consider new behaviours and new sensitivities.

I suggest you follow me for a few hours of discovery of an organism and another way of living and feeling the world. For this, I ask you to listen, to perceive, to open your senses.

Imagine fungal hyphae. 🍄

Credit: Mycelium by Kirill Ignatyev on Flickr, CC BY-NA 2.0

Those long, white, earthy filaments – the hyphae – form the structure of fungi. Assembled together in a decentralised and anarchic fashion, hyphae form the mycelium. The mycelium is not a thing, it is a process. You can access incredible videos of the movements in the hyphae here. Water and nutrients flow through the networks of mycelia which can be stimulated electrically. Some fungi therefore conduct electricity along their hyphae, similar to the impulses of nerve cells in animals.

Despite their delicacy, hyphae have incredible strength; they push the earth, but also sometimes cement (like Coprinus comatus). Imagine these hyphae in the earth, under your feet, in this bubbling ecosystem, surrounding the roots of plants. Indeed, tree roots depend on these mycelia for their nutrient supply. I suggest you watch this video by Merlin Sheldrake which is a laser analysis of the relationship between the hyphae of a fungus and a plant. The hyphae also form the structure of the mushroom we eat. And, according to some estimates, if you take a piece of mycelium found in a teaspoon of soil and stretch it out, it could be anywhere from a few hundred metres to kilometres long (Sheldrake, 2020).  

Now imagine an exchange between the hyphae of fungi and algae. Not the algae you might see on the beach, but small cells.

Credit: Relationship between algae and mushroom hyphae. Photo appeared in a University of Michigan paper here.

In the lichen, the hyphae are not alone, they are entangled in and with alga cells or cyanobacteria. These photosynthetic structures – algae and cyanobacteria – use the energy of the sun as well as carbon dioxide (CO2) to produce their energy. This cooperation between photosynthetic and fungal forms creates the lichen. This relationship allows both organisms to live in places where they could not have lived alone.  

Lichens are worlds, they are ecosystems. They are inhabited by hundreds if not thousands of other species including fungi and a myriad of bacteria (Pingle, 2017). Lichens are thus emergent phenomena: they are more than the sum of their parts.  

So how do we define these organisms that form an assemblage of many species, that are not “one” but are at the same time. Where is the boundary of the lichen? What is a lichen?

A lichen is a symbiotic organism. This means that it is an intimate relationship between different species where one organism is considered the host and the other is the symbiont. Symbiosis is a spectrum that contains several kinds of relationships between living organisms, including mutualism on the one hand and parasitism on the other.

Mutualism is a relationship where both organisms benefit from the association. For example, humans depend on their microbiome – which is the community of microbes and their genetic material – to digest food. Some research also shows that the microbiome in our gut can affect our immune system and moods. On the other hand, the microbes, through the symbiosis, live in an environment where they have access to food and are protected. Both organisms (human and microbe) benefit from this relationship.

In a parasitic relationship, one organism benefits from the relationship while the other organism loses. The SARS-CoV-2 virus is a parasite for humans and generates a disease that we all know by now: the coronavirus. The virus, on the other hand, finds a comfortable environment in humans where it can reproduce.

Well, back to our lichens….

Crédit: Cyanobactérie de James Golden, Université de San Diego Californie accédé en Flickr, CC BY-NA-SA 2.0

A lichen is a stable and lasting mutualistic association between fungi and an alga or cyanobacterium.

Cyanobacteria look like algae but are actually single-celled or colonial bacteria (see photo).

Credit: Cross-section of the thallus of Nefornus lichen found on Wikimedia, CC BY-SA 4.0.

As you can see in the diagram above, the algal cells are entangled in the fungal hyphae (part b). The diagram represents a cross section of the thallus. The thallus is the visible surface of the lichen and consists of several layers of cells (from different organisms). The thallus is a word I suggest you remember, as it is essential for naming the lichen.

The upper and lower parts of the diagram represent the cortices (a, upper cortex and d, lower cortex) that protect lichens from their environment. These structures are primarily formed of fungal (mushroom) tissues. At the bottom of the upper cortex is the layer of algae or cyanobacteria when they are in symbiosis with the fungi (represented by the green circles in part b). Then comes the medulla, it is the layer of mushroom hyphae (part c on the diagram). The part e represents the rhizines, which lichens use to fix themselves to the surface on which they are found.  

With the naked eye, we can only see the thallus as well as the lower and upper cortex. If you have access to a microscope, I suggest you take a piece of thallus and cut it in half. You will then be able to see the green layer, the algae, as well as the white medulla. 

Now that we know the anatomy of the lichen, let’s move on to their functions!

Algae are photosynthetic organisms. Hence, lichens capture the energy from the sun, and with water and carbon dioxide (CO2) from the air, they transform it into carbohydrates. Carbohydrates are sugars that provide energy to the lichens. These sugars are partly absorbed by the fungal cells. 

The algae are called photobionts. These photobionts can be either an alga or a cyanobacteria. Both forms can live without being associated with fungi, but their symbiotic association with the fungus changes them radically (structurally and morphologically). 

Notes: Photosynthetic organisms are autotrophic – they produce their own energy source. We humans get our energy from the food we eat, because we cannot photosynthesise – we are heterotrophs.

The fungus, on the other hand, provides a protected environment and gathers moisture, which prevents the algal cells from drying out. The fungus also provides the algae or cyanobacteria with nutrients. Lichens are always named after the dominant fungus. This will become important when we start learning the names of lichens.

Note: Normally, we think of plants as photosynthetic organisms. However, some animals produce their energy from the sun’s energy. Have you ever heard of Elysia chlorotica? This sea slug is found on the east coast of the United States and has the ability to sequester a chloroplast – the organelle where the process of photosynthesis takes place – from its prey, an alga. This non-permanent symbiosis allows the sea slug to survive for up to 12 months thanks to the energy supply that the algae provides through photosynthesis.

Credit: Elysia chlorotica by Patrick Krug accessed on Flickr, CC BY 2.0.

Activity 🌸

On the tree in front of you, can you tell the difference between lichens and mosses? 

Look at the pictures below to help you. The green, fluffy organisms are the mosses. 

It’s quite common to confuse these two organisms despite the fact that they are completely different. A moss is a plant, completely photosynthetic. As we said, a lichen is a symbiosis and is more of a fungus than a plant. Indeed, a lichen is composed of 95% fungus and the rest, algae.

The forms of lichens are very varied. 

Can you already differentiate between different morphological forms  ?
Different colours ?

If you are already more experienced, you can look at the identification key available here and try to identify the different species. 

Otherwise, check out the identification guide I have created for urban lichens. It contains the description of 28 urban lichens found on the tree bark.


  • The content of this walk is very much inspired by the book of Merlin Sheldrake: 

Sheldrake, M. (2020). Entangled life: how fungi make our worlds, change our minds & shape our futures. Random House.

You can also find amazing videos about fungi on his YouTube account here.

  • Pringle, A. (2017). Establishing new worlds: the lichens of Petersham. Arts of Living on a Damaged Planet.

All the pictures in the diaporama were taken by the author and are under a CC BY-SA 4.0 license.

That’s the end of this first stop, we meet at the next stop – in front of number 57 Avenue Michelangelo. The exact location can be found on the map below! 👇🏾👇🏾

If you don’t continue the ride, can you give us feedback on your experience here ? It will help us improve! 

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