Lesson Overview 21.4 Fungi

THINK ABOUT IT What is the largest organism in this photo?

THINK ABOUT IT You might think it’s the tree, but in fact it’s a fungus. The only trace of the fungus is the ring of mushrooms, but the mushrooms are just the reproductive structures of a much larger organism. Most of the mass of the fungus is underground, spanning at least the width of the ring of mushrooms, and extending more than 2 meters into the ground! Hundreds of years ago, some cultures believed these rings of mushrooms marked spots where fairies danced. Today people still call them fairy rings.

What Are Fungi? What are the basic characteristics of fungi? Fungi are… Heterotrophic eukaryotes with cell walls that contain chitin.

What Are Fungi? Many fungi grow from the ground and this once led scientists to classify them as nonphotosynthetic plants. However, fungi aren’t plants. Instead of carrying out photosynthesis, fungi produce enzymes that digest food outside their bodies. Then they absorb the small molecules released by the enzymes. Many fungi feed by absorbing nutrients from decaying matter in the soil. Others live as parasites, absorbing nutrients from their hosts.

What Are Fungi? The cell walls of fungi are composed of chitin, a polymer made of modified sugars that is also found in the external skeletons of insects. The presence of chitin is one of several features that show fungi are more closely related to animals than to plants.

Structure and Function There are two general growth patterns among fungi. Yeasts are tiny fungi that live most of their lives as single cells. Mushrooms and other fungi, on the other hand, grow much larger. Their bodies are made up of cells that form long, slender branching filaments called hyphae.

Structure and Function In most fungi, cross walls divide the hyphae into compartments resembling cells, each containing one or two nuclei. In the cross walls, there are openings through which cytoplasm and organelles can move.

Structure and Function The body of a mushroom is actually the fruiting body, the reproductive structure of the fungus. The fruiting body grows from the mycelium, the mass of branching hyphae below the soil. Clusters of mushrooms are often part of the same mycelium, which means they are part of the same organism.

Structure and Function Some mycelia live for many years and grow very large. The mycelium of the soil fungus in a fairy ring has grown so large that it has used up all of the nutrients near its center. It grows and produces fruiting bodies—the mushrooms—only at its edges, where it comes in contact with fresh soil and abundant nutrients.

Reproduction Fungi can reproduce asexually, primarily by releasing spores that are adapted to travel through air and water. Breaking off a hypha or budding off a cell can also serve as asexual reproduction.

Reproduction Most fungi can also reproduce sexually. The life cycle of the bread mold Rhizopus stolonifer is shown.

Reproduction Sexual reproduction in fungi often involves two different mating types. The gametes of both mating types are about the same size and are not usually called male and female. One mating type is called “+” (plus) and the other “–” (minus). Hyphae of opposite mating types fuse together, bringing + and – nuclei together in the same cell.

Reproduction The + and – nuclei form pairs that divide as the mycelium grows. Many of the paired nuclei fuse to form diploid zygote nuclei, which go through meiosis to make haploid spores within a zygospore.

Reproduction The zygospore germinates and a sporangium emerges. The sporangium reproduces asexually, releasing haploid spores produced by meiosis. Each spore has a different combination of parental genes, and each can make a new mycelium.

Diversity of Fungi More than 100,000 species of fungi are known. Biologists have placed fungi into several distinct groups. The major groups of fungi differ from one another in their reproductive structures.

Diversity of Fungi

The Ecology of Fungi How do fungi affect homeostasis in other organisms and the environment? DECOMPOSITION Fungi are champions of decomposition. Many species help ecosystems maintain homeostasis by breaking down dead organisms and recycling essential elements and nutrients. PARASITISM Parasitic fungi can cause serious diseases in plants and animals by disrupting homeostasis. MUTUALISM Some fungi form mutualistic associations with photosynthetic organisms in which both partners benefit.

Decomposition Many fungi feed by releasing digestive enzymes that break down leaves, fruit, and other organic material into simple molecules. These molecules then diffuse into the fungus. Many organisms remove important trace elements and nutrients from the soil. Fungi recycle these essential elements and nutrients. If these materials were not returned, the soil would quickly be depleted.

Parasitism – Plant Diseases A number of parasitic fungi cause diseases that threaten food crops. Corn smut, for example, destroys corn kernels. Some mildews, which infect a wide variety of plants, are also fungi.

Parasitism – Animal Diseases Fungal diseases also affect insects, frogs, and mammals. For example, the Cordyceps fungus infects grasshoppers in rain forests in Costa Rica. Microscopic spores become lodged in the grasshopper, where they germinate and produce enzymes that slowly penetrate the insect’s external skeleton. The spores multiply in the insect’s body, digesting all its cells and tissues until the insect dies. Hyphae develop, cloaking the decaying exoskeleton in a web of fungal material. Reproductive structures, which will produce more spores and spread the infection, then emerge from the grasshopper’s remains.

Parasitism – Animal Diseases Parasitic fungi can also infect humans. The fungus that causes athlete’s foot forms a mycelium in the outer layers of the skin, which produces a red, inflamed sore from which the spores can easily spread from person to person. The yeast Candida albicans is often responsible for vaginal yeast infections and for infections of the mouth called thrush.

Mutualism – Lichens A lichen is a symbiotic association between a fungus and a photosynthetic organism. The photosynthetic organism is either a green alga or a cyanobacterium, or both. The protective upper surface of a lichen is made up of densely packed fungal hyphae. Below this are layers of green algae or cyanobacteria and loosely woven hyphae. The bottom layer contains small projections that attach the lichen to a rock or tree.

Mutualism – Lichens Lichens are extremely resistant to drought and cold. Therefore, they can grow in places where few other organisms can survive—on dry bare rock in deserts and on the tops of mountains. Lichens are able to survive in these harsh environments because the green algae or cyanobacteria carry out photosynthesis, providing the fungus with a source of energy, while the fungus provides the green algae or cyanobacteria with water and minerals. The densely packed hyphae protect the delicate green cells from intense sunlight.

Mutualism – Lichens Lichens are often the first organisms to enter barren environments, gradually breaking down the rocks on which they grow. In this way, lichens help in the early stages of soil formation. Lichens are also remarkably sensitive to air pollution: They are among the first organisms to be affected when air quality deteriorates.

Mutualism – Mycorrhizae Fungi also form mutualistic relationships with plant roots. These symbiotic associations of plant roots and fungi are called mycorrhizae. Researchers estimate that 80 to 90 percent of all plant species form mycorrhizae with fungi. The hyphae collect water and minerals and bring them to the roots, greatly increasing the effective surface area of the root system. In addition, the fungi release enzymes that free nutrients in the soil. The plants, in turn, provide the fungi with the products of photosynthesis.

Mycorrhizae The presence of mycorrhizae is essential for the growth of many plants. The seeds of orchids, for example, cannot germinate in the absence of mycorrhizal fungi. Many trees are unable to survive without fungal symbionts.

Mycorrhizae This graph illustrates the growth rates of three species of trees with and without mycorrhizae.

Mycorrhizae The roots of plants are plugged into mycorrhizal networks that connect many plants. In an experiment using isotopes to trace the movement of carbon, ecologist Suzanne Simard found that mycorrhizal fungi transferred carbon from paper birch trees growing in the sun to Douglas fir trees growing in the shade nearby. As a result, the sun-starved fir trees thrived, basically by being “fed” carbon from the birches. Simard’s findings suggest that plants—and their associated fungi—may be evolving as part of an ecological partnership.