4. Leopold Park 🍂


Below you will find the audio and text of the walk !
Have a good adventure !

PS: The name of the man who opened the doors to the discipline of genetics is Gregor Mendel, not George Mendel (which is stated in the audio above).

What happens when human exceptionalism and the idea of individualism limited by a few (physical) boundaries is turned upside down by scientific discoveries ? 

Biology has always had immense power to ferment moral notions about life on Earth and has done so since the imperialism of the 18ᵉ century (Haraway, 2010). 

Lichens are among those organisms that have shaken our beliefs not only about the idea of individuality, but also about sexual reproduction and heteronormative dogmas (Griffiths, 2015). The concept of symbiosis elicited by lichens has long been polemical and has stimulated many existential questions. Lichens thus continually challenge us to question and re-examine our relationship to the world around us. 

“Lichens tell us things about life, they inform us” 

Trevor Goward in the book Entlangled Life: How Fungi make our worlds, change our minds and shape our future by Merlin Sheldrake (2020, p.89) 

I would like to introduce you to a few things that symbiosis tells us about and the role that lichens have played in the discovery of this concept. 

Entangled, a lichen is not one, nor two, but multiple. Lichens are composed of two types of fungi, algae, and many bacteria. In addition to this assemblage of several species, lichens are sometimes attacked by lichenicolous fungi. The photos below show different lichens (yellow, Xanthoria parietina and blue Physcia tenella) being attacked by lichenicolous fungi, creating a pink and/or orange colour. The photos were taken by the author (2020) and are under a CC BY-SA 4.0 license.

How did the idea of symbiosis come about?

It was in 1869 (three years after Ernst Haeckel coined the term ecology) that a Swiss botanist, Simon Schwendener, published an article introducing the double hypothesis of lichen.

In his article, Schwendener introduces the notion, radical at the time, that the lichen is not one organism but many. According to Schwendener, in the lichen, the alga is the slave of the fungus and is forced to serve it. This theory reflected the ideological and political ideas at the time. This hypothesis was refuted by lichenologists – people who study lichens – and was seen as shocking. One could not imagine two organisms living in such a close relationship and creating such a different form. 

Moreover, based on the idea of evolution by natural selection developed by Charles Darwin, in 1859, new species were created by the divergence of species from each other. Evolution was seen as coming from competition (an interpretation of the people at the time, because Darwin’s writings do not ONLY speak of competition) instead of cooperation. Schwendener’s idea amounted to creating links between different evolutionary lineages (see photo of phylogenetic tree), which did not fit into the understanding of taxonomy at the time. The idea of the emergence of new species by the convergence of several species seemed impossible. 

Tree of life, illustration created by Ernst Haeckel in 1874. Humans (Menselen) are at the top of the tree and bacteria (Amoeba) are at the bottom. The idea of symbiosis challenges these categories and links species that seemed to have nothing to do with each other. Image found on Wikipedia in the Public Domain.

It was in 1877 that the German botanist Albert Frank introduced the idea of symbiosis as the close relationship between a species of algae and a type of fungus forming a lichen. Later, the term symbiosis would be used to describe a spectrum of interactions ranging from mutualism to parasitism (see content for Square Ambiorix for more information on these terms). 

The lichen became a symbol of symbiosis and a biological principle when scientists discovered that symbiosis is not the exception, but the rule. Examples include the complex relationships in corals, or bacteria and/or other organisms in the functioning processes of living organisms. 

As a result, the process of evolution could no longer be seen as a race to compete – a theory that had stemmed from the misinterpretation of Darwinian thinking – but as the collaboration between organisms of several taxonomic groups. This forced many scientists to rethink their ideologies and the processes of evolution.

Where does diversity on earth come from? 

In Europe and the United States in the 1990s, evolutionary theories were influenced by the discoveries of Gregor Mendel, who opened the door to genetics. The main explanation for the variation and diversity of organisms on Earth was related to genetics (e.g. genetic mutations). Natural selection acts over time on the phenotypes – the observable traits of an organism, such as blue eyes in humans – that are most adapted to the environment. This is the main idea of what is called the modern synthesis of evolution.

Meanwhile in Russia, researchers were studying lichens and their symbioses. According to them, competition and genetics could not be the only source of diversity. Cooperation in nature was emphasised through symbiosis which could and does create new forms of lives. 

This idea of symbiosis as a source of variation came to Europe in the 1970s thanks to a woman, the American biologist Lynn Margulis. 

The discovery of the Russians and Lynn Margulis 

Living organisms are separated into three areas : bacteria, archaea (unicellular organisms that have a different membrane from bacteria) and eukaryotes. Animals, fungi and plants are eukaryotes. The cells of eukaryotes are larger than those of bacteria and archaea and have specialized structures (such as the nucleus or mitochondria). Plants have other specialized cells such as the chloroplast (where photosynthesis takes place). 

Phylogenetic tree with three main groups – Archaea, Bacteria, Eukaryotes. Credit: Phylogenetic tree of the living world from Woese et al, 1990 on Wikimedia, Public Domain. 

The Russians’ discovery that Lynn Margulis brought to Europe, is that the first eukaryotic cell forming the body of animals today would have been created through symbiosis. Indeed, an eukaryote, with a very simple structure, would have engulfed a bacterium. 

This engulfed bacterium would have remained in the host and over time this symbiosis would have become permanent – surely, because it was beneficial to both cells – allowing the evolution of multicellular organisms. The bacterium would be the descendant form of the mitochondria (see cell diagram) which is the part of the eukaryotic cell allowing it to produce energy. The same process would have happened for the chloroplast in plants with a photosynthetic bacterium that came to be enclosed by a eukaryote.

Diagram of a eukaryotic cell with the different organelles (parts). The mitochondrion (top) is an ancient bacterium engulfed by the single eukaryotic cell. Credit: Cell by OpenStax found on Wikimedia. CC BY 4.0. 

This theory, which emphasises the importance of symbiosis in evolution, was initially denied for several years before being confirmed by scientific evidence in 1970. Lynn Margulis said that “the first eukaryotic cells were analogous to lichens”. This idea of endosymbiosis – symbiosis that lasts and creates new forms – has revolutionized our understanding of the natural world and has called into question the time required for the creation of new forms. Rapidly, through symbiosis, new adaptations and life forms can be created.

Today, several authors (Haraway, 2010; Gilbert, 2012) argue that every organism is a lichen, meaning symbiotic. For example, a squid (Euprymna scolopes) that lives in coastal and shallow waters in the Hawaiian archipelago, depends on its symbiosis with bacteria (Vibrio fischeri) that it acquires at birth to develop a luminescent organ in its head. This organ is essential to the squid in order to prevent the light of the stars and the moon from producing a shade. The luminescent organ of the squid thus enables it to hide from predators. This symbiosis can actually be observed everywhere. Every tree depends on its fungal network to absorb nutrients from the soil through the roots. Cows could not digest the grass they eat without the anaerobic bacteria in their rumen.

Euprymna scolopes, a Hawaiian bobtail squid that, thanks to its symbiosis with Vibrio fischeri bacteria develops a luminous organ that allows it to hide from its predators (as it prevents the creation of a shadow in the moonlight). Credit: Hawaiian Bobtail squid by Chris Frazee and Margaret McFall-Ngai on Wikimedia. CC BY 4.0. 

But then, who is the individual ? Where is the limit if every organism depends on others ?

I’ll leave you to ponder that question ! 

Are symbiotic relationships threatened ?

Symbiosis relationships are threatened in a changing world. A good example is that of corals. These organisms, similar to lichens, are symbiotic. With the acidification of the ocean due to global warming, some bacteria leave the symbiosis or become parasites, bleaching the coral and leaving it dead.

Coral bleaching due to the loss of its symbiont from climate change. Credit: Bleached coral, Acoropora sp. by Vardhan Patankar on Wikimedia. CC BY-SA 4.0. 

Lichens are changing our perceptions of the world. Recently, David Griffiths wrote an article called “Queer Theory for Lichens” where he questions our ideas of individuality through the findings on lichens. 

He writes: “There are no universal, transcendent traits that define the individual (human or otherwise); instead, the self or individual is always contingent and context-dependent”. Indeed, the human gut microbiome (communities of microbes) – on which we depend to digest our food – is highly dependent on the environment in which we live. The individual is thus dynamic and changes according to the context and the environment.

Griffiths goes further and assumes that this symbiotic view questions the way we see the body not as “clean, healthy, and pure or infected, sick, and impure” but as an assemblage of many species. Indeed, bacteria inhabit us and enable us to function.

These new ideas may have an effect on how we view ourselves and how we view infected bodies. Furthermore, in these times of environmental and social change, this new symbiotic approach to the world helps us realize and “feel” the connections and subtleties that we, humans, have with other organisms, without whom we cannot live. 

Finally, while from a certain perspective, the idea of “individual” is a concept and a category that helps humans, their behaviour and ideas, the notion of the organism boundaries is more complex. As one developmental biology researcher puts it, “we are all lichens”.

I hope this idea of symbiosis inspires you! 


We will introduce two new species, called Punctelia subrudecta, Flavorparmelia caperata and Evernia prunastri

Punctelia subrudecta is blue grey in colour and has small dots on its thallus. These dots are called pseudocyphellae. They are holes in the upper cortex, which let us see the underlying medulla (the part with the mushroom hyphae, more info here, at the beginning of this walk). Coming out of the medulla are soredia – clusters of algae and fungal hyphae that disperse and allow new lichens to settle. The soredia are coming out of the sorelia and are granular, like a small powder. This kind of lichen is called Punctelia for its small dots on the thallus. 

Credits: The pictures were taken by the author (2020) except the bluish one by Christian Thirion, used with permission. All photos on this blog and below are under CC BY-SA license. 

Flavoparmelia caperata is also a foliose lichen that is apple green when the thallus is wet and yellow-green when the thallus is dry. The thallus is quite matted and wrinkled and is covered with soredia (see the stop at Square de Meeûs for more details) like Punctelia subrudecta. These small granular clusters are located in the centre of the thallus. The lower surface of the thallus is black with simple rhizines (one filament) which are often well spaced. There are sometimes apothecia on the thallus (sexual reproductive structures like those of Xanthoria parietina) but these are quite rare. This species had disappeared with the increase of SO2 (a pollutant coming from industries) a few years ago but it now repopulates the cities. Credit: The second photo in the slideshow was taken by Jason Hollinger, found on Wikimedia with CC BY-SA 3.0 license. The other photos are taken by the author and are under a CC SA-BY 4.0 license.

Evernia prunastri is a foliose lichen that looks like a fruticose lichen. The characteristic of fruticose lichen is the fact that they are attached to one point called the holdfast. In Evernia prunastri, the lobes (the leaf-like parts) are flat and divided at the tips. The thallus of this lichen is covered with small granular clusters which are the means of reproduction of the lichen and are called soradia. The thallus is wrinkled and is paler on its lower surface. This lichen is difficult to see on the trunk, you have to look a little higher than eye level looking at the trunk from the opposite side of the lake. 

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


  • Haraway, D. J. (2016). Staying with the trouble: Making kin in the Chthulucene. Duke University Press.
  • Griffiths, D. (2015). Queer theory for lichens. UnderCurrents: Journal of Critical Environmental Studies19, 36-45.
  • Sheldrake, M. (2020). Entangled life: how fungi make our worlds, change our minds & shape our futures. Random House.
  • Gilbert, S. F., Sapp, J., & Tauber, A. I. (2012). A symbiotic view of life: we have never been individuals. The Quarterly review of biology87(4), 325-341.

For more information on Lynn Margulis (one of the rare women in science in her time), check out this article: Gray, M. W. (2017). Lynn Margulis and the endosymbiont hypothesis: 50 years later. Molecular biology of the cell, 28(10), 1285-1287

Let’s meet a little further on at the exit of the Parc Léopold towards the European Union buildings just in front of the new CityTree 👇🏾👇🏾

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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