What killed this tree?
Dutch elm trees used to be common, beautiful trees in the United States. Now most of them, like this tree, have been killed by Dutch elm disease. Dutch elm disease is caused by a fungus. This is just one example of how fungi can interact with other species.
Symbiotic Relationships with Fungi
Fungi don't live in isolation. They often interact with other species. In fact, fungi can be dependent on another organism for survival. When two species live close together and form a relationship, it is called symbiosis. Symbiosis can be beneficial to one or both organisms, or sometimes one organism hurts the other. Some of the partners in these relationships include plants, algae, insects, and even humans.
Fungi and Plants
If it were not for fungi, many plants would go hungry. In the soil, fungi grow closely around the roots of plants, and they begin to help each other. The plant roots together with the special root-dwelling fungi are called mycorrhizae (Figure below).
As plants and fungi form a close relationship, the plant and the fungus “feed” one another. The plant provides sugars to the fungus that the plant makes through photosynthesis, which the fungus cannot do. The fungus then provides minerals and water to the roots of the plant. Since the plant and the fungus are helping each other out, this is a mutualistic relationship, a type of symbiosis known as mutualism. In a mutualistic relationship, both organisms benefit.
These roots (brown) and the mycorrhizae (white) help to feed one another.
Have you ever seen an organism called a lichen? Lichens are crusty, hard growths that you might find on trees, logs, walls, and rocks (Figure below). Although lichens may not be the prettiest organisms in nature, they are unique. A lichen is really two organisms, sometimes referred to as a composite organism, that live very closely together: a fungus and a bacterium or an alga. The cells from the alga or bacterium live inside the fungus. Besides providing a home, the fungus also provides nutrients. In turn, the bacterium or the alga provides food to the fungus by performing photosynthesis. A lichen is also an example of a mutualistic relationship. Because lichens can grow on rocks, these organisms are some of the earliest life forms in new ecosystems.
This tree is covered in lichen, a symbiotic relationship between a fungus and a bacterium or an alga.
Many insects have a symbiotic relationship with certain types of fungi:
- Ants and termites grow fungi in underground “fungus gardens” that they create. When the ants or termites have eaten a big meal of wood or leaves, they also eat some fungi from their gardens. The fungi help them digest the wood or leaves.
- Ambrosia beetles live in the bark of trees. Like ants and termites, they grow fungi inside the bark of trees and use it to help digest their food.
Fungi as Parasites
Although lots of symbiotic relationships help both organisms, sometimes one of the organisms is harmed. When that happens, the organism that benefits, and is not harmed, is called a parasite. This type of relationship is known as parasitism.
Examples of parasitic fungi include the following:
- Beginning in 1950, Dutch Elm trees in the United States began to die. Since then, most of these trees have been eliminated. The disease was caused by a fungus that acted as a parasite. The fungus that killed the trees was carried by beetles to the trees.
- Some parasitic fungi cause human diseases such as athlete’s foot and ringworm. These fungi feed on the outer layer of warm, moist skin.
lichen: Organism that results from a mutualistic relationship between a fungus and a cyanobacterium or green alga.
mutualistic relationship: Relationship in which both organisms benefit from the association.
mycorrhizae: A mutualistic relationship between fungi and plant roots.
parasite: Relationship between two organisms in which one benefits, and the other is harmed.
symbiosis: Relationship that develops when two organisms live closely together.
- Fungi can form a mutualistic relationship with photosynthetic organisms, including plants, bacteria, and algae.
- Fungi can also be parasites of trees and people.
Use the resources below to answer the questions that follow.
- What controls hyphae growth?
- What is an arbuscle? How does it form? What is its function?
- What nutrient does this fungi help make available to the plant?
- What biomolecules does the plant form with this nutrient?
- Why are these biomolecules crucial to the plants survival?
- Why was it harder for plants to obtain phosphorus when they moved from an aquatic to a terrestrial environment? How did fungi help this situation?
- How old is the fossil evidence for this association between plant and fungi?
- What other types of nutrients do fungi help plants obtain?
- What other beneficial effects of this symbiosis have been shown?
- Why might a lichen be considered a mutualistic relationship?
- Give an example of a fungal parasite.