This episode is sponsored by Squarespace.

Go to Squarespace.com/microcosmos to save 10% off your first purchase of a website or domain. A useful principle in the story of life is that you should never underestimate algae or cyanobacteria.

They will always manage to surprise you, and more importantly, to remind you that everything you have, ultimately comes down to them. Do you like oxygen? Well, take some time to thank a cyanobacterium.

If their ancestors hadn’t gone a little haywire with their photosynthesis and set off the Great Oxidation Event billions of years ago, we would not exist. But you don’t even have to look back into history to see the influence of algae and cyanobacteria. Today, even algae of the smallest size have a tremendous impact on our lives, both good and bad.

On one hand, they translate sunlight into food through photosynthesis. On the other hand, they bloom in waters and release toxins that can wreak damage on the organisms around them. But the real reason you should never underestimate algae or cyanobacteria is because at their core, they are organisms that seem to just always find a way.

They exist in super salty waters, in ponds, in hot springs. Though, these are of course aquatic organisms, and so you might think of their adaptations as restricted to watery environments. But what has made algae so capable of existing beyond those boundaries is their willingness to use other organisms as a means to their own survival.

At times, their symbiotic relationships sound absurd, like the species that makes a home out of sloth fur. But our favorite example of the creativity that algae and cyanobacteria approach their relationships with has to be lichen— that combination of fungus and algae, or fungus and cyanobacteria, that attaches to trees and rocks and anything else that sits around long enough. The concept of a lichen appears simple enough.

There’s a fungus, which has an incredible capacity for spreading and decomposing, but no means to make its own food. And there’s the algae or cyanobacteria, fully capable of making their own food, but in need of some help getting around on land. The result is called a dual organism, made up of the fungal mycobiont and the algal photobiont that takes on the spreading qualities of the first, and the colors of the second.

Fungi and algae can meet and work together in ways that actually don’t result in a lichen. The distinct structure that we associate with lichens comes down to the thallus, made up of distinct layers that reveal the different combinations of the organisms that make it. The thallus is what allows lichens to survive, keeping the organism alive in dry conditions so it can tolerate extremes in humidity and temperature.

The shape is generally a function of the fungus, which starts to weave its body into the appropriate layers as soon as it comes upon algae to team up with. At the edge is the cortex, where fungal cells are thicker and close in contact so they can form a protective structure. And beneath, it keeps a more fibrous and loose mass of hyphae, a scaffolding structure called the medulla that the photobionts layer themselves on, dividing and creating their own algal layer that’s large and green and positioned to access the sun.

That layer of cells tells us a little bit more about the organisms inside the lichen, though only under the right conditions. A dry lichen is typically gray. But with water, the fungal gray clears up, revealing the color of the photobionts inside the lichen.

In the case of algae photobionts, you’ll typically see a bright green color. But when there are cyanobacteria, the hues vary from dark green to black. Now lichens have to actually stick to the thing they are stuck to, and they have a few ways to anchor themselves onto surfaces.

Some have a structure called a holdfast that extends as part of the thallus, a bit like a peg that is very good at holding on to things like rock. Our lichen here though uses a different type of structure called rhizines. These are threads that come from the medulla or lower cortex, creeping out from the lichen like roots, only they do not have any capacity to draw in nutrients.

They are just anchors. Now from our perspective right now, there’s nothing controversial about this partnering of fungus and algae. We look in the lichen, we see signs of both organisms, and we boil it down to a simple exchange of protection for food.

This ancient relationship brought algae onto land and into places so different from their watery origins. But our understanding of lichens as not just one organism, but two is actually relatively recent, going back only to 1867, when a Swiss botanist named Simon Schwendener presented his work at the Swiss Natural History Society. He’d been looking at lichen under the microscope, just as we are now, when he considered the possibility that the mass of hyphae he was looking at belonged to a fungus, wrapped around algae.

His contemporary lichenologists did not agree. Sure, Schwendener was just one of multiple scientists who had noted a resemblance between the parts of lichen, and the bodies of fungi and algae. But that was just a resemblance, it seemed to the lichenologists at the time.

In their defense, they did not know that a dual organism could exists, They did not know that there was a symbiosis here How could they, when the word symbiosis hadn’t been invented yet? Schwendener had more reasons than just appearances though. One challenge that lichenologists repeatedly came across was that it was impossible to grow a new lichen from isolating either the spores or the green cells growing within the lichen.

But in 1877, a scientist named Albert Bernhard Frank came up with the term “symbiosis.” The organism that inspired him to come up with that word? Well of course., it was lichen. Frank was one of several biologists intrigued by the possibility of Schwendener’s hypothesis, this idea that organisms could live together, not just in an environment, but in one entity.

Now with time, lichenologists came around, and people only uncovered more examples and nuances around symbiosis. But that does not mean that we actually understand lichen. They remain challenging to cultivate in a lab, and we’re still understanding whether the symbiosis we’ve reduced to a simple meeting of two organisms might actually more complex—whether it can involve other species as well.

Perhaps there’s a way to describe lichens and the world inside of them that we just don’t know yet, a word that, like symbiosis, will reveal something new and remarkable to us about our world. The way we talk about science and knowledge sometimes makes it feel like we’re done, that science is a static set of all of the facts in the universe, because it is a lot easier to talk about the things that we know than the things that we don’t. Of course it’s hard to talk about the things that we don't know, because we don't know them.

But our world is always there, containing the information that we want in everyday occurrences that are sometimes hundreds of millions of years old, we humans just have to figure out ways to ask the questions that will actually give us answers. Thank you for coming on this journey with us as we explore the unseen world that surrounds us. And thanks to Squarespace for sponsoring this episode.

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