You Scratch my Back, I’ll Scratch Yours

An Introduction to Mutualism

As I mentioned last week, I’ll be talking about symbiosis for the next twelve stories and starting with the most recognizable form: mutualism.

Nothing in biology makes sense except in light of evolution.

Theodosius Dobzhansky

All interactions in nature, including all three forms of symbiosis, need to be understood in an evolutionary context. Mutualism is the easiest to understand in this regard because both organisms involved derive some kind of fitness benefit from the relationship. This means that both are more likely to survive and reproduce.

However, before I continue, I want to make it clear that in all of the examples of mutualism I am going to share in the next few weeks, none of the organisms involved have come to any sort of ‘agreement’ about their relationship. I have written previously about the problems inherent in treating wildlife like people. Even in cases where such thinking does no or little harm, it clouds our understanding of the natural world. The organisms in these mutualistic relationships did not ‘decide’ to work together. They simply did, and continued to do so because the relationship provided an evolutionary benefit.

Because mutualistic symbiosis benefits both participants, it tends to increase their spread and success. In fact, the first example of mutualism that I will share with you is so successful that it exists almost everywhere and is so ubiquitous that most people do not even recognize it, even though they pass by it every day.

This is lichen. While it resembles moss, and often grows alongside it, lichen is not a plant. It is a symbiotic compound organism. Lichen is comprised of photosynthetic algae or cyanobacteria and fungus. Green algae are closely related to land plants, but cyanobacteria are not. Rather, they are a form of photosynthetic bacterium unrelated to eukaryotic organisms (those whose cells contain a nucleus). Fungi, while they resemble plants, are actually more closely related to animals, because they do not produce their own food.

What benefit does each organism in lichen receive from this relationship? Of course, I’ll tell you in a second, but I’d like you to think about it a bit and try and figure it out for yourselves. There’s at least one hint in the information above.

Ready? If you found the hint about food sources, good for you. Fungi can’t produce their own food (most are detritovores, meaning they consume decaying matter), but algae and cyanobacteria can in the same way that land plants do: photosynthesis. Photosynthesis is the process of taking in carbon dioxide and water and converting them into sugars using the energy from sunlight (oxygen is produced as a byproduct). By coexisting with a photosynthetic organism, the fungus part of lichen has an easy source of food not available to its non-symbiotic fungal relatives.

But what about the algae and cyanobacteria? What are they getting out of the deal? The vast majority of organisms belonging to those groups live in aquatic environments (both freshwater and marine). While some are colonial, many exist as single cells. This makes them vulnerable to predation. By fixing themselves to a fungus, they lose their dependency on water and can live in a variety of areas that would normally be hostile to them. The fungus helps collect water and nutrients from the environment and being part of a multi-cellular organism also reduces the risk of predation for a normally single-celled algae or bacterium.

Lichens are one of the most successful symbiotic relationships on the planet. They are everywhere, covering an estimated 6% of Earth’s land surface, and can survive in a variety of extreme environments, including arctic and hot desert conditions, which neither of the component organisms could live in. It grows easily on trees, rocks, and almost any other solid surface.

Lichen is often classified based on its shape or growth form. Some form a thin, crust-like layer over the surface where they grow, such as the picture above. These are known as crustose lichen. Leafy forms, such as the one pictured below, are called fruticose. There are also foliose, or shrub-like, leprose, or powdery, and gelatinous lichens.

Lichen is often beneath our notice. But it is precisely that innocuous nature that fascinates me so much. I decided to look into its evolutionary history. The fossil record for lichen is poor, since they do not fossilize easily except in amber (fossilized tree sap). Fossil lichens date back at least 400 million years and there is evidence that the process of forming the symbiotic relationship evolved multiple times, indicating a highly successful evolutionary strategy.

I hope this example shows you just how powerful mutualism can be in the natural world. The relationship does not have to be grand or inspiring, and oftentimes it isn’t. The organisms involved derive simple evolutionary benefits (like nutrition, structural support, or protection) that would not be available to them otherwise.

For next week, I’ll share a story about a much more well known example of symbiosis. Until then, the next time you are outside, take a look at the trees and rocks around you. Most will be covered with lichen. Think about how special that seemingly insignificant growth truly is and how, in some ways, its success dwarfs our own despite our vast intelligence and technological advances.