mutualistic relationship between a fungus and a cyanobacterium or green alga
mutualistic relationship between a plant and a fungus that grows in or on its roots
Fungi lack chlorophyll, so they cannot make food by photosynthesis as plants can. Instead, they are heterotrophs, like animals. But they don’t have a mouth or teeth. So how do fungi “eat”? They get their nutrition by absorbing organic compounds from other organisms. The other organisms may be dead or alive, depending on the fungus.
Fungi as Decomposers
Most fungi get organic compounds from dead organisms. They are decomposers called saprotrophs. A saprotroph feeds on any remaining organic matter after other decomposers do their work. Fungi use enzymes to digest organic remains and then absorb the resulting organic compounds. As decomposers, fungi are vital for the health of ecosystems. They break down nonliving organic matter and release the nutrients into the soil. Plants can then use the nutrients and pass them on to herbivores and other consumers.
Bacteria are also major decomposers, but they can grow and feed only on the exposed surfaces of organic matter. In contrast, fungi can use their hyphae to penetrate deep into organic matter. Fungi are also the only decomposers that can break down tough plant substances, including lignin (in wood) and cellulose (in plant cell walls). They have special enzymes to do this work. The enzymes are released by the tips of the hyphae. Because of these abilities, fungi are the primary decomposers in forests (see Figure below).
Forest Decomposers. These forest mushrooms may look fragile, but they do a powerful job. They decompose dead wood and other tough plant material.
Fruit and Vegetable Decomposition – Time Lapse
Symbiotic Relationships of Fungi
Not all fungi feed on dead organisms. Many are involved in symbiotic relationships, including parasitism and mutualism.
Fungi as Parasites
In a parasitic relationship, the parasite benefits while the host is harmed. Parasitic fungi live in or on other organisms and get their nutrients from them. Fungi have special structures for penetrating a host. They also produce enzymes that break down the host’s tissues.
Parasitic fungi often cause illness and may eventually kill their host. They are the major cause of disease in agricultural plants. Fungi also parasitize animals, such as the insect pictured in Figure below. Fungi even parasitize humans. Did you ever have athelete’s foot? If so, you were the host of a parasitic fungus. You can read more about fungi and human disease in the last lesson of this chapter.
Parasitic Fungus and Insect Host. The white parasitic fungus named Cordyceps is shown here growing on its host—a dark brown moth.
Mutualism in Fungi
Fungi have several mutualistic relationships with other organisms. In mutualism, both organisms benefit from the relationship. Two common mutualistic relationships involving fungi are mycorrhiza and lichen.
A mycorrhiza is a mutualistic relationship between a fungus and a plant. The fungus grows in or on the plant roots. The fungus benefits from the easy access to food made by the plant. The plant benefits because the fungus puts out mycelia that help absorb water and nutrients. Scientists think that a symbiotic relationship such as this may have allowed plants to first colonize the land.
A lichen is a mutualistic relationship between a fungus and a photosynthetic organism. The other organism is usually a cyanobacterium or green alga. The fungus grows around the bacterial or algal cells. The fungus benefits from the constant supply of food produced by the photosynthesizer. The photosynthesizer benefits from the water and nutrients absorbed by the fungus.Figure below shows lichen growing on a rock.
Lichen Growing on Rock. Unlike plants, lichen can grow on bare rocks because they don’t have roots. That’s why lichens are often pioneer species in primary ecological succession. How do lichen get water and nutrients without roots?
Some fungi have mutualistic relationships with insects. For example:
Leafcutter ants grow fungi on beds of leaves in their nests. The fungi get a protected place to live. The ants feed the fungi to their larvae.
Leafcutter Ants Video – The First Agriculture Note: This video has a brief mention of 50 million years.
Ambrosia beetles bore holes in tree bark and “plant” fungal spores in the holes. The holes in the bark give the fungi an ideal place to grow. The beetles harvest fungi from their “garden.”
Human Uses of Fungi
Whenever you eat pizza, you eat fungi, even if you don’t like your pizza with mushrooms. That’s because pizza dough contains yeast. Do you know other foods that are made with fungi?
Fungi for Food
Humans have collected and grown mushrooms for food for thousands of years.Figure below shows some of the many types of mushrooms that people eat. Yeasts are used in bread baking and brewing alcoholic beverages. Other fungi are used in fermenting a wide variety of foods, including soy sauce, tempeh, and cheeses. Blue cheese has its distinctive appearance and flavor because of the fungus growing though it (see Figure below).
These are just a few of the many species of edible mushrooms consumed by humans.
Blue Cheese. The dark blue strands running through this cheese are a fungus. In fact, this cheese is moldy! The fungus is Penicillium roqueforti, a type of mold.
Harmless fungi can be used to control pathogenic bacteria and insect pasts on crops. Fungi compete with bacteria for nutrients and space, and they parasitize insects that eat plants. Fungi reduce the need for pesticides and other toxic chemicals.
Other Uses of Fungi
Fungi are useful for many other reasons.
They are a major source of citric acid (vitamin C).
They produce antibiotics such as penicillin, which has saved countless lives.
They can be genetically engineered to produce insulin and other human hormones.
Most fungi are decomposers called saprotrophs. They feed on decaying organic matter and return nutrients to the soil for plants to use. Fungi are the only decomposers that can break down wood and the cellulose in plant cell walls, so they are the primary decomposers in forests.
Many fungi are involved in symbiotic relationships. Some are parasites. They are specialized to penetrate a host and break down the host’s tissues. Parasitic fungi often cause illness and may eventually kill their host. Two common mutualistic relationships involving fungi are mycorrhiza (fungi and plant roots) and lichen (fungi and either cyanobacteria or green algae). Some fungi also have mutualistic relationships with insects.
Humans use fungi for many purposes, including as food or in the preparation of food. Humans also use fungi for pest control. In addition, fungi can be used to produce citric acid, antibiotics, and human hormones. Fungi are model research organisms as well.
Lesson Review Questions
1. How do fungi obtain organic compounds from dead organisms?
2. Why are fungi the primary decomposers in forests?
3. How significant are fungi as plant parasites?
4. Describe an example of a mutualistic relationship between fungi and insects.
5. List several ways that humans use fungi.
6. Assume that you notice a fungus growing on a plant. What possible relationships might exist between the fungus and the plant? What type of evidence might help you identify which is the correct relationship?
7. Compare and contrast mycorrhiza and lichen.
8. Explain how fungi might have allowed early plants to colonize the land.
Points to Consider
You read in this lesson that many fungi are parasites, and they make their hosts sick. An example in humans is athlete’s foot.
Do you know any other human diseases caused by fungi?
Besides parasitism, how else might fungi make people sick?