Today’s Factismal: The technical term for things like lichens, which are made up of a fungus plus an algae, is composite organism.
Biology makes strange bedfellows. And perhaps the strangest is the common lichen. That’s because every lichen you see isn’t one thing; it is two: a fungus and an algae, each of which rely on the other to survive. Because the lichen cannot survive as just a fungus or an algae bu itself, it is known to biology wonks as a composite organism. (Some biologists have even suggested that humans are composite organisms, due to our reliance on gut bacteria for digestion and production of many vitamins!)
This partnership does amazing things for lichens. They are able to survive in a wide variety of environments, from dry deserts to wet rainforests to freezing arctic tundras to hot jungles. In a recent experiment, lichens were even shown to be able to survive on Mars! Despite their amazing range of habitats, lichens are remarkably sensitive to local environmental; conditions, making them valuable indicators of air quality.
The roots of their sensitivity to air quality lie in the way that lichens work. The fungus provides a structure for the lichen and forms a protective housing over the algae; in some cases, the fungus even penetrates the cell walls of the algae allowing for a more direct exchange of nutrients. The algae in turn provide sugars and other nutrients derived from photosynthesis. But in order to thrive, the lichen need water and dust; they grab these from the atmosphere.
That dependence on air-born nutrients causes lichens to take bizarre forms. Some are low and leafy, while others appear to have naked branches. Even those that just form crusts are bumpy. These adaptations give lichens more surface area, which means that they can absorb more nutrients.
As they absorb more nutrients, lichens concentrate any pollutants found in the atmosphere (e.g., lead, sulfur) and may end up with a high enough concentration to kill either the fungus or the algae. Interestingly, the amount of pollution needed to kill a lichen depends on the type of fungus, so scientists can evaluate air quality simply by counting the number of different lichen types.
If you’d like to help with their work, then why not go count the lichens in your neighborhood and report your results to the Great Smoky Mountains Institute at Tremont or the Natural History Museum of London?