Living Next Door to Algae

What is a lichen? Lichens are not plants. Not fungi. Answering this question is not exactly easy. An icon for symbioses examples in biology, lichens are a whole neighborhood of organisms who thrive on living together, and because of this have a crazy survivability. They can be found everywhere including ice-free terrestrial, semiaquatic, and tidal ecosystems and, on every continent. They can survive well below freezing and without water. In nature, when you are a good neighbor, you create resiliency and something more beautiful than you could alone.

Lichen on a tree in the Rocky Mountains.
Lichen on roots of mangroves in Honduras.
Lichen spotted on rocks in the Austrian Alps.

Community roles

Scientists have long recognized the fundamental partnership that produces lichens: a fungus joins with algae or cyanobacteria in a relationship that benefits both individuals. Every lichen has a mycobiont (fungus) and a photobiont (photosynthetic organism).

In 2016, a study found that some lichens across six continents also contain basidiomycete yeasts, single-celled fungus. This is pretty neat because we now know that a lichen can be made of 2+ different organisms representing three different kingdoms of life (fungi, plants, bacteria) in one tight knit system. Understanding lichens involves both considering the whole organism, and also the interactions between the component organisms who cooperate to make lichens work.

The housing

Fungus, and in some cases yeast, provides structure and insulation allowing lichens to survive in extreme climates like arid deserts and freezing tundra. Fungi have a network of fibers that help to anchor the lichen to different structures (e.g. trees, rocks, soil, cement).

The food

Lichens, generally speaking, are fungi that have discovered farming! Their inhabitants, algae and/or cyanobacteria, produce food and energy for the fungus. This allows the fungus to grow on various surfaces rather than relying on a living or decaying host.

Algae and cyanobacteria can live without the mycobiont, but the mycobiont cannot live without the photobiont. Just how we cannot live without food, the mycobiont needs food. Because of this, some have argued the validity this as a symbiotic relationship and that it is more of a parasitic relationship. But whether it’s a fair trade or a strict landlord-tenant agreement, the result is a neighborhood that survives where no individual could stand alone.

The protection

Basidiomycete yeasts, the single-celled fungi, can produce chemicals that help lichens ward off predators and repel microbes. Recently researchers have claimed these yeasts provide protection and defense mechanisms against environmental threats.

More to Lichen

Our broad definition above encompasses a tremendous amount of diversity (more that 15,000 species!), but often lichens are categorized as crustose, foliose and fruticose.

Examples of three types of lichens. | Drawing by Lynn Heller

While lichens are incredibly resilient, they also have their sensitivities. They are widely known and valued as indicators for environmental quality and climate change because they respond, react, and adapt to shifts in their surroundings. Lacking defensive tissues (like a skin or cuticle), they absorb moisture, gases, and minerals directly from the air, revealing information about the environment.

Because they absorb everything, including harmful chemicals, they act as “scrubbers” that can improve air quality. This also means they are valuable biomonitoring tools. Scientists can evaluate air pollution in an area by extracting toxins from their fibers. While some species show resilience against these toxins, less tolerant species can tell us about the condition of an area based on their presence (or lack thereof). By looking at which neighbors have stayed and which have moved out, we can better understand the state of the surrounding environment.

An Artful Action

Explore the symbiotic relationship between art and science! Check out this blog showcasing lichen through 500 years of botanical art. Lichens make a great subject because of their range in color and textures. Go on a neighborhood walk and see how many different ‘communities’ of lichen you can find on a single tree or rock. Try to find one of each: a crust, a leaf, and a shrub.