Amorphous in their beauty, beautiful in their amorphousness, lichens run Mars-red and sun-yellow. Sometimes moldy, sometimes mossy, they bloom in delicate curls of green and gray. Those rusty stains on the sidewalk by your house, pale leaflets creeping up the tree in your yard, puckered cups between the slats of your fence? Once you know what to look for, they’re everywhere. But the public spotlight on lichens is recent compared to other species. In fact, lichens only entered the academic scene in the late nineteenth century, thanks to Simon Schwenderer, the son of a Swiss farmer. On a crisp morning in 1867, the fresh-faced Schwenderer, as the newly minted director of the botanical gardens in Basel, Switzerland, proposed a hypothesis that would change the biological world.
Schwenderer’s hypothesis, also called the dual hypothesis of lichens, proposed that lichens were not a single organism; instead, they were highly organized structures composed of an alga and a fungus. While Schwenderer first presented the hypothesis at the Swiss Natural History society, Schwenderer reiterated the theory until his death in 1919 in a stubborn effort to convince his peers.
To understand the divisive nature of Schwenderer’s hypothesis, it is important to understand just what his proposal entailed. Instead of a single, autonomous organism, a lichen was a holobiont made up of two symbionts, named eponymously after the close relationship between the alga and the fungus. The heart of this relationship was found in the lichen’s primary structure, the thallus. A cross-section of a lichen thallus looks similar to a textbook depiction of skin, the layers of tissue stacked like an overstuffed sub. In practice, however, the thallus functions more like your vital organs, only tightly packed in a few paper-thin sheets. At the bottom of the thallus lies the lower cortex, a fungal layer out of which grow root-like protrusions that anchor the lichen to the closest surface. Past the lower cortex rises the medulla, a loose, tangled layer of branching fungal nodes. Finally, tucked between the medulla and the upper cortex, Schwenderer’s critical symbiont: the algal layer. This thin rind—a few strands of hyphae wrapped around a scattering of algal cells—was the epicenter of lichen symbiosis.
Schwenderer’s hypothesis stated that these hyphae provided structure so algal cells could stabilize and grow. This partnership allowed the lichen to benefit both from the incredible hardiness of fungi and the sugars provided by photosynthesizing algae. Schwenderer proposed a partnership between organisms without competition—an intertwining, an intimacy. Accepting this fact would shatter a fundamental truth: that all organisms were autonomous, fighting each other in the race for life.
It took more than ten years of heated debate over symbiont relationships before Schwenderer’s hypothesis became law. Whereas the two kinds of relationships we now group under the umbrella of symbiosis—parasitism and mutualism—were familiar to Schwenderer’s contemporaries, grasping symbiosis remained difficult. After all, parasitism was only a step above base animal competition: if a louse couldn’t compete with its host for nutrients, it would steal them instead. Likewise, while mutualism seemed altruistic at first blush, self-interest ultimately drove it.
But symbiosis, as defined by lichens, seemed to transcend the grabby-hands egocentrism that academics ascribed to living things. Instead, it toed a fragile line between neutrality and dependency. Although the fungal component—the mycobiont—constitutes the majority of the thallus, it relied on the algal component, or the photobiont, for survival, breaking the confines of host–parasite relationships. Without the nutrients provided by algae, the mycobiont would starve, unable to glean enough chemical energy from the rock or bark it had attached itself to. Conversely, the fungal presence barely affected the photobiont; in fact, while lichen mycobionts cannot survive on their own, lichen algae often grow on their own in the wild.
Notwithstanding the decided lack of consensus on the purpose of lichen symbiosis, the sheer volume of research supporting Scwenderer’s findings meant that, by the 1910s, most academics accepted the dual hypothesis as canon. Now, lichens are textbook examples of symbiotic relationships: the alga is the breadwinner; the fungus, the hearth. But diminishing the complexity of the lichen does a disservice to Schwenderer’s research and the enormity of its implications—that organisms can no longer be expected to act autonomously, but more importantly, that tolerance—alliance—is not quite so human as assumed.
Even now, lichens continue to rock the academic boat. In 2016, a group of researchers found a third symbiont, a yeast—a discovery that had the potential to destroy the dual-organism rule that had been accepted as canon for so long.
The study originally aimed to identify the cause of different levels of acid in two species of lichens. While differences in mycobiont and photobiont species provided some explanation, they weren’t nearly sufficient to explain the discordance, so the researchers examined wild specimens and found strains of basidiomycete yeasts within the cortexes of both lichens. The yeasts provided the missing piece of the acid-level puzzle. More intriguingly, however, they lived entirely within the lichen cortex.
But later studies into the nature of yeast–lichen relationships cast the structural importance of yeasts in doubt. A researcher at the Estonia University of Life found that photobionts were highly specialized toward their fungal hosts—essentially, they chose a select few species of mycobiont partners—in comparison to the yeasts, who formed relationships much more freely. Since more intimate symbiotic relationships had previously correlated with a higher level of specialization, the relative imprecision with which yeasts selected their fungal hosts signaled that yeasts were not as significant to lichen structure as previous studies suggested.
The question of what level of interdependency constitutes symbiosis is not a new one—in fact, scientists began contesting it from the moment the term “symbiosis” was introduced in the late 1870s—but advances in lichen science only throw the concept further into confusion. Recently, a group of researchers from the University of Chile stumbled upon an unexpected member of the lichen holobiont as they examined the capacity of bacteria to capture nitrogen in cyanolichens, lichens that use photosynthesizing bacteria as their main photobiont, as opposed to algae.
The group contrasted two genera of cyanolichens, Peltigera and Cladonia, throughout the study, but since morphological classification is difficult when it comes to lichens, they used genetic sequencing to identify the different strains of bacteria. Cladonia lichens had a much more diverse community of bacteria compared to Peltigera, leading the researchers to believe that Cladonia lichens were “less selective in recruiting nitrogen-fixing bacteria” because of their photobiont’s lack of nitrogen-fixation abilities.
In other words, the study suggests that, in addition to having a mycobiont and a primary, photosynthesizing photobiont, some lichens have an additional bacterial symbiont. This distinction between a bacterial community on the lichen and a bacterial community as a symbiont means that the bacterial community actively contributes to the lichen’s overall health—once again challenging the traditional bi-symbiont lichen structure. However, the symbiont in this situation is not one strain of bacteria: it is a community of bacteria—a microecosystem.
Take a walk outside and you will spot a lichen. They spread over trees, stones, mulch; they grow on apartment buildings and federally protected monuments. Leave your garbage bin or your wheelbarrow alone for long enough and they will bloom; let your car sit for a few months and they will bloom on that, too. They can drift through the extreme temperatures and radiation of outer space, return to Earth, and grow again. They’re everywhere, the tardigrades of the macro-organismic world.
Despite this, lichens have never been grown in a lab. While all of the elements of a lichen can be grown from seed or spore, replicating the spark that fuses them together into one indestructible morphon has proved impossible. We don’t know why they form. We can’t even predict when or where they will. Their relationships with the world of rocks and clouds remains just as inscrutable as the fungus-alga relationship was in 1877. Indeed, this “symbiosis” with the abiotic world may be as nebulous and earth-shattering a concept to us as a tepidly tolerant relationship between organisms was to Schwenderer’s audience at the Swiss Natural History Society.
But, in the same way that biologists in the late nineteenth century already had half the answer to the question of symbiosis—however small-minded their use of parasitism and mutualism seem to us—we have the pieces to the lichen puzzle already. “An ecosystem,” writes National Geographic, “is a geographic area where plants, animals, and other organisms, as well as weather and landscape, work together to form a bubble of life.” And what is a lichen but a small planet filled with life? An interaction between bacterial colonies, algal chloroplasts, fungal hyphae, yeast cells, rocks, sun, wind? And—perhaps—something intangible, something not so different from ourselves?
After all, have you ever considered your own autonomy? Billions of bacteria crawl over your skin at any moment, and the microbiome in your gut is unlike that of any other human on Earth. You are a feeding ground the size of the planet for a dust mite. Despite the improbability of your existence, you mean the world to a fragile web of organisms whom you sustain and who sustain you. Symbiosis may be too small a word for the intricacies you host. So, when you next walk by a lichen, take a moment to wonder at the miracle of two ecosystems—two universes—passing each other on this strange and marvelous planet.
“A Glimpse of the Universe: A History of Lichens and Ourselves” is the second of six pieces in ROOTED, which won the Gold Medal Portfolio Award for writing sponsored by the New York Times in the 2023 Scholastic Arts & Writing Awards.