The Lowly Lichen
by Stephen James O'Meara
Lichens are some of the oldest plantlike organisms on Earth. Their fossils can be traced back 400 million years, and some microfossil evidence suggests that their ancestors date back 2.2 to 2.7 billion years.
While most living creatures are a single organism—an ant, a fish, a tree—a lichen is a combination of two organisms (a fungus and an alga or cyanobacterium) living so close together that it's hard to differentiate between the two. In fact, many lichen species include a fungus that cannot survive on its own.
Most of the lichen is composed of fungal filaments. These filaments rely on algal or cyanobacterium cells to take care of photosynthesis, the production of sugars necessary for life. The filaments simply suck sugars out of the photosynthesizing cells. But the alga or cyanobacterium does not go unrewarded for its sugar production. These photosynthesizers depend on a symbiotic fungus to supply them with water and minerals, as well as shelter in which to grow.
The partnership appears to be not only mutually beneficial, but also highly successful. Lichens enjoy long lives—up to 1,000 years—and can survive in extreme environments. There are some 13,500 to 17,000 species of lichens, extending from the tropics to the polar regions. They will grow on almost any stable and reasonably well-lit surface, including soil, rock, and the sides of trees. Because lichens are highly intolerant of atmospheric pollution (particularly sulfur dioxide), you won't find many in the city. In fact, their presence is an excellent bio-indicator of air quality and cleanliness.
Now a trivia question: Who first suggested that a lichen is a symbiotic relationship between a fungus and an alga?
The answer: Beatrix Potter, the author best known for her books about Peter Rabbit. When not writing children's stories, Potter enjoyed studying and drawing lichens.
- alga: Singular form of algae, which is any of a large group of simple living things that grow in water. Algae were once considered to be plants, but they do not have true roots, stems, or leaves.
- filament: A fine wire or thread. In this case, a shape.
- fungus: One of a group of organisms that are neither plants nor animals and that have no flowers, leaves, or green coloring. Mushrooms, molds, and mildew are fungi.
- photosynthesis: The process plants use to make food.
- symbiotic: Adjective form of symbiosis, which is the relationship of two different organisms that live in close association. Symbiosis is often to the advantage of both organisms.
- As you read in Chapter 1, algae are single-celled organisms that can make their own food. How might an alga benefit if it lives with a fungus to form lichen?
[anno: The fungus can supply water and minerals to the alga, and the fungus provides shelter.]
- The article says that the fungus and the alga have a symbiotic relationship. Symbiosis means that two different organisms live in close association. This symbiosis, or symbiotic relationship, often benefits both organisms. There are many examples of symbiotic relationships in nature. Find an example of a symbiotic relationship between a plant and an animal, between two plants, or between two animals. Write a short paragraph about how the relationship benefits both organisms.
[anno: Answers will vary. Students should correctly identify two organisms that have a symbiotic relationship and explain how the relationship benefits both organisms.]