Although we don’t often stop to notice them, lichens are nearly everywhere on Earth. Lichens are very hardy. They can survive extreme environments that are too harsh for other organisms. As a result, lichens live on every continent, even frosty Antarctica. In addition, they are the dominant organism on about eight percent of Earth’s surface. Worldwide, there are about 15,000 species of lichens.
Lichens grow on just about any surface. Hikers commonly see lichens growing on tree bark. They are also found growing on soil or rocks. Lichens can grow on human-created structures too, such as wooden fences, bricks, or cemetery headstones. In fact, any structure that has been standing for a while is likely to host some lichens.
Both people and animals use lichens. People have used lichens as both food and medicine, as well as a source of fabric dye. Lichens also provide about two-thirds of food supply used by caribou in the far north.
Although each lichen has its own scientific name, which means it is recognized as an organism, each lichen is actually made up of two different organisms living together.
One part of a lichen is called a fungus. Fungi include the familiar mushroom as well as microscopic molds and yeasts. Since people often see mushrooms growing on lawns or in forests, many conclude that fungi are plants. However, they are not. Many plants use a chemical called chlorophyll to help them make food. Unlike plants, fungi cannot create their own food.
Just like people and animals do, fungi rely on other organisms to provide them food. Fungi eat decaying organic material. For this reason, fungi serve an important role in helping organic matter to decompose.
The other organism that makes up a lichen is called an alga. Algae are commonly found in water, either freshwater or saltwater. They are neither fungi nor plants. Algae may be colored green, brown, red, or gold. Although they are usually in some type of water, they can survive on land when they partner with a fungus, within a lichen.
Unlike fungi, algae can make their own food through a process called photosynthesis. Within a lichen, the algae create the food that feeds the fungus, allowing the lichen to grow and spread. Just as people grow vegetables in farms or gardens, the fungus grows the algae it uses for food. For this reason, the Canadian lichenologist Trevor Goward says that lichens are “fungi that discovered agriculture.”
What Is Symbiosis?
The partnership between a fungus and an alga within a lichen is called symbiosis. In a symbiotic relationship, two different organisms live closely with one another. There are three different types of symbiotic relationships in nature.
In the first type, both members of the relationship benefit. This is called mutualism. A well-known example of mutualism is the relationship between sea anemones and clownfish. Sea anemones live attached to rocks and coral and cannot swim. They must wait until a fish swims close enough to catch with their stinging tentacles. However, they don’t eat clownfish, and these fish are not hurt by the anemone’s sting as they swim among its tentacles. Both creatures live together. The anemone protects the clownfish from predators as well as provides food from any fish the anemone cannot finish eating. Clownfish help the sea anemone by acting as bait, attracting bigger, predator fish that may become the anemone’s next meal.
A second type of symbiosis is called commensalism. In this relationship, one organism receives benefits from another. However, the other creature is neither harmed nor benefitted. For example, remoras are fish that hitch rides with sharks, turtles, or other aquatic animals. Remoras have a flat oval disk on top of their heads that allow them to attach to a host without harming it. Each host provides both protection for the remora, as well as providing leftovers from its meals.
The third type of symbiosis is called parasitism. In a parasitic relationship, one organism benefits while the other one is harmed. For example, tiny insects called fleas live on both mammals and birds, surviving by consuming the host’s blood. In the plant kingdom, the familiar holiday plant called mistletoe attaches itself to a host tree or shrub from which it absorbs water and nutrients. If the host becomes infested with too many mistletoe plants, sometimes it can weaken and die.
Biologists debate whether the relationship between the fungus and algae in a lichen is an example of mutualism or parasitism. Those who think that the relationship is mutual say that when they team together, a fungus and algae in a lichen may be stronger than they are individually—remember, they are very hardy. A lichen may be able to survive in conditions that neither the fungus or nor algae could survive alone.
Other biologists say that the lichen is actually an example of a parasitic relationship. They say that within a lichen, the alga is simply a captive of the fungus. An alga can make its own food through photosynthesis while the fungus cannot. The fungus enslaves the alga in order to feed on nutrients it produces.