Fungi

Dr. Becki Brunelli

Fungi

Learning Objectives

By the end of this section, you will be able to:

List the characteristics of fungi
Describe fungal parasites and pathogens of plants and infections in humans
Describe the importance of fungi to the environment
Summarize the beneficial role of fungi in food and beverage preparation and in the chemical and pharmaceutical industry

Part a shows a cluster of mushrooms with bell-like domes attached to slender stalks. Part b shows a yellowish-orange fungus that grows in a cluster and is lobe-shaped. Part c is an electron micrograph that shows a long, slender stalk that branches into long chains of spores that look like a string of beads.

Figure 10.19 The (a) familiar mushroom is only one type of fungus. The brightly colored fruiting bodies of this (b) coral fungus are displayed. This (c) electron micrograph shows the spore-bearing structures of Aspergillus, a type of toxic fungi found mostly in soil and plants. (credit a: modification of work by Chris Wee; credit b: modification of work by Cory Zanker; credit c: modification of work by Janice Haney Carr, Robert Simmons, CDC; scale-bar data from Matt Russell)

The kingdom Fungi is a diverse group of eukaryotic organisms (containing a true nucleus and many membrane-bound organelles), ranging from your garden-variety mushrooms to yeasts and molds (Figure 10.19). While scientists have identified about 135,000 species of fungi, this is only a fraction of the more than 1.5 million species of fungus likely present on Earth. They were once thought to be closer to plants, but DNA evidence shows they’re actually more akin to animals. Unlike plants, fungi don’t do photosynthesis; they get their nutrients from breaking down organic matter. Reproduction varies, with some fungi reproducing asexually and others both asexually and sexually. They’re crucial in ecosystems as decomposers and and participate in the cycling of nutrients by breaking down organic materials into simple molecules. Fungi have a knack for forming beneficial relationships with other organisms.

But fungi aren’t all friendly; they can cause devastating diseases in plants and animals, like the Dutch elm disease that wiped out a significant number of elm trees across the US in the 1900s. In humans, treating fungal infections is tricky because standard antibiotics don’t work on them. On the flip side, fungi have their perks in the commercial world—think yeasts in your bread and beer, and even antibiotics like penicillin.

Cell Structure and Function

Fungi are complex eukaryotes with a membrane-bound nucleus, mitochondria, and internal membrane systems such as the endoplasmic reticulum and Golgi apparatus. Unlike plants, they lack chloroplasts and thus don’t photosynthesize. Despite the absence of chlorophyll, fungi can be vividly colored, as seen in the poisonous Amanita muscaria with its iconic red cap and white patches (Figure 10.20). These pigments are part of the cell wall and serve protective functions, including shielding against UV radiation and, in some cases, being toxic.

The photo shows two large mushrooms, each with a wide white base and a bright red cap. The caps are dotted with small white protrusions.

Figure 10.20 The poisonous Amanita muscaria is native to the temperate and boreal regions of North America. (credit: Christine Majul)

Growth and Reproduction

The fungal body, known as a thallus, can exist in either a unicellular or multicellular form, with some fungi even switching between the two in response to environmental conditions. Unicellular fungi like yeasts (e.g., Saccharomyces cerevisiae used in baking) are quite common. However, most fungi are multicellular and consist of hyphae, thread-like structures that form a web-like mass called mycelium (Figure 10.21). Mycelium can grow in various environments, including soil, decaying matter, or even living tissue. While individual hyphae are microscopic, mycelia can span vast areas—like the Armillaria ostoyae, which covers over 2,000 acres in Oregon and is thousands of years old. Hyphae are usually divided by end walls known as septa, which have tiny holes allowing for quick nutrient transfer.Fungi flourish in moist, slightly acidic conditions and have varying oxygen requirements—some need it to survive, while others can’t tolerate it at all. Yeasts are more versatile, thriving in both aerobic and anaerobic conditions. In terms of reproduction, fungi can be either sexual or asexual, producing spores that are carried by wind or animals. These spores are designed for maximum dispersal; for instance, the giant puffball mushroom releases trillions of spores to increase the odds of landing in a growth-supporting environment (Figure 10.22).

The photo depicts a light brown fungus, growing in a Petri dish. The fungus, which is about 8 centimeters in diameter, has the appearance of wrinkled round skin surrounded by powdery residue. A hub-like indentation exists at the center of the fungus. Extending from this hub are folds that resemble spokes on a wheel.

Figure 10.21 The mycelium of the fungus Neotestudina rosati can be pathogenic to humans. The fungus enters through a cut or scrape and develops into a mycetoma, a chronic subcutaneous infection. (credit: CDC)

Part a is a photo of a puffball mushroom, which is round and white. Part b is an illustration of a puffball mushroom releasing spores through its exploded top.

Figure 10.22 The (a) giant puffball mushroom releases (b) a cloud of spores when it reaches maturity. (credit a: modification of work by Roger Griffith; credit b: modification of work by Pearson Scott Foresman, donated to the Wikimedia Foundation)

How Fungi Obtain Nutrition

Like animals, fungi are heterotrophs, relying on complex organic compounds for both carbon and nitrogen rather than fixing these elements from the atmosphere. However, their method of nutrient acquisition is unique: unlike most animals, their digestion comes before ingestion. Fungi secrete exoenzymes into their environment to break down complex molecules, absorbing the smaller resultant compounds through their expansive mycelial networks. Fungi are mostly saprobes, organisms that derive nutrients from decaying organic matter, efficiently breaking down complex substances like cellulose and lignin into absorbable glucose. This decomposer role is ecologically crucial, recycling nutrients back into ecosystems and even offering promise for bioremediation applications, such as breaking down pollutants like diesel oil or absorbing heavy metals.

Plant Parasites and Pathogens

The production of enough good-quality crops is essential to our existence. Plant diseases have ruined crops, bringing widespread famine. Most plant pathogens are fungi that cause tissue decay and eventual death of the host (Figure 10.23). In addition to destroying plant tissue directly, some plant pathogens spoil crops by producing potent toxins. Fungi are also responsible for food spoilage and the rotting of stored crops. For example, the fungus Claviceps purpurea causes ergot, a disease of cereal crops (especially of rye). Although the fungus reduces the yield of cereals, the effects of the ergot’s alkaloid toxins on humans and animals are of much greater significance: In animals, the disease is referred to as ergotism. The most common signs and symptoms are convulsions, hallucination, gangrene, and loss of milk in cattle. The active ingredient of ergot is lysergic acid, which is a precursor of the drug LSD. Smuts, rusts, and powdery or downy mildew are other examples of common fungal pathogens that affect crops.

Parts a, b, c, and d show fungal parasites on grapefruit, grapes, a zinnia, and a sheaf of barley, respectively.

Figure 10.23 Some fungal pathogens include (a) green mold on grapefruit, (b) fungus on grapes, (c) powdery mildew on a zinnia, and (d) stem rust on a sheaf of barley. Notice the brownish color of the fungus in (b) Botrytis cinerea, also referred to as the “noble rot,” which grows on grapes and other fruit. Controlled infection of grapes by Botrytis is used to produce strong and much-prized dessert wines. (credit a: modification of work by Scott Bauer, USDA ARS; credit b: modification of work by Stephen Ausmus, USDA ARS; credit c: modification of work by David Marshall, USDA ARS; credit d: modification of work by Joseph Smilanick, USDA ARS)

Aflatoxins are toxic and carcinogenic compounds released by fungi of the genus Aspergillus. Periodically, harvests of nuts and grains are tainted by aflatoxins, leading to massive recall of produce, sometimes ruining producers, and causing food shortages in developing countries.

Animal and Human Parasites and Pathogens

Fungi can affect animals, including humans, in several ways. Fungi attack animals directly by colonizing and destroying tissues. Humans and other animals can be poisoned by eating toxic mushrooms or foods contaminated by fungi. In addition, individuals who display hypersensitivity to molds and spores develop strong and dangerous allergic reactions. Fungal infections are generally very difficult to treat because, unlike bacteria, fungi are eukaryotes. Antibiotics only target prokaryotic cells, whereas compounds that kill fungi also adversely affect the eukaryotic animal host.

Many fungal infections (mycoses) are superficial and termed cutaneous (meaning “skin”) mycoses. They are usually visible on the skin of the animal. Fungi that cause the superficial mycoses of the epidermis, hair, and nails rarely spread to the underlying tissue (Figure 10.24). These fungi are often misnamed “dermatophytes” from the Greek dermis skin and phyte plant, but they are not plants. Dermatophytes are also called “ringworms” because of the red ring that they cause on skin (although the ring is caused by fungi, not a worm). These fungi secrete extracellular enzymes that break down keratin (a protein found in hair, skin, and nails), causing a number of conditions such as athlete’s foot, jock itch, and other cutaneous fungal infections. These conditions are usually treated with over-the-counter topical creams and powders, and are easily cleared. More persistent, superficial mycoses may require prescription oral medications.

Part a is a photo of a red, ring-shaped skin lesion. Part b is a light micrograph of long, thread-like mycelia and small, oval sporangia. Part c is a chest X-ray of a person with a fungal infection. There are diffuse, scattered light areas where the infiltration of fluid has replaced the air in the microscopic air sacs.

Figure 10.25 (a) Ringworm presents as a red ring on the skin. (b) Trichophyton violaceum is a fungus that causes superficial mycoses on the scalp. (c) Histoplasma capsulatum, seen in this X-ray as speckling of light areas in the lung, is a species of Ascomycota that infects airways and causes symptoms similar to the flu. (credit a, b: modification of work by Dr. Lucille K. Georg, CDC; credit c: modification of work by M Renz, CDC; scale-bar data from Matt Russell)

Systemic mycoses spread to internal organs, most commonly entering the body through the respiratory system. For example, coccidioidomycosis (valley fever) is commonly found in the southwestern United States, where the fungus resides in the dust. Once inhaled, the spores develop in the lungs and cause signs and symptoms similar to those of tuberculosis. Histoplasmosis (Figure 10.24c) is caused by the dimorphic fungus Histoplasma capsulatum; it causes pulmonary infections and, in rare cases, swelling of the membranes of the brain and spinal cord. Treatment of many fungal diseases requires the use of antifungal medications that have serious side effects.

Opportunistic mycoses are fungal infections that are either common in all environments or part of the normal biota. They affect mainly individuals who have a compromised immune system. Patients in the late stages of AIDS suffer from opportunistic mycoses, such as Pneumocystis, which can be life threatening. The yeast Candida spp., which is a common member of the natural biota, can grow unchecked if the pH, the immune defenses, or the normal population of bacteria is altered, causing yeast infections of the vagina or mouth (oral thrush).

Beneficial Fungi

Fungi play a crucial role in the balance of ecosystems. They colonize most habitats on Earth, preferring dark, moist conditions. They can thrive in seemingly hostile environments, such as the tundra, thanks to a most successful symbiosis with photosynthetic organisms, like lichens. Fungi are not obvious in the way that large animals or tall trees are. Yet, like bacteria, they are major decomposers of nature. With their versatile metabolism, fungi break down organic matter that is insoluble and would not be recycled otherwise.

Importance to Ecosystems

Fungi play a pivotal role in ecosystems as key decomposers, facilitating the cycling of essential nutrients like carbon, nitrogen, and phosphorus. Their unique ability to break down complex organic materials such as cellulose and lignin makes them invaluable in the recycling of plant matter. They also engage in various forms of symbiosis, most notably mycorrhizal associations with 80-90% of plant species. In these relationships, fungi extend their hyphal networks into the soil to transport water and minerals to plants, receiving photosynthetic products in return. These symbiotic connections can manifest in multiple ways, from fungi enveloping plant roots to specialized structures like arbuscles for nutrient exchange. Orchids, which lack nutrient-rich seeds, are particularly dependent on such partnerships for their lifecycle.

Lichens, a symbiotic partnership between a fungus and a photosynthetic organism like an alga or cyanobacterium, are key pioneers in barren environments like glacial areas, facilitating soil formation by breaking down rocks. They are also prevalent on tree trunks and rocks in mature habitats, serving as an important food source for caribou. The fungal partner offers protection and mineral supply, while the photosynthetic component contributes carbohydrates and, in some cases, fixed nitrogen. Together, they form a thallus (body of the lichen), providing mutual benefits that include protection from environmental extremes.

Fungi form mutualistic relationships with various arthropods, such as Basidiomycota species with scale insects and leaf-cutting ants in Central and South America. The scale insects facilitate nutrient transfer from plants to the fungi, which in turn offer protection. Meanwhile, leaf-cutting ants cultivate fungi in specialized gardens using cut leaf disks; these fungi digest cellulose into simpler sugars that nourish the ants. This symbiosis benefits both parties: fungi get a consistent leaf supply and reduced competition, while ants gain a food source.

Importance to Humans

Although we often think of fungi as organisms that cause diseases and rot food, fungi are important to human life on many levels. As we have seen, they influence the well-being of human populations on a large scale because they help nutrients cycle in ecosystems. They have other ecosystem roles as well. For example, as animal pathogens, fungi help to control the population of damaging pests. These fungi are very specific to the insects they attack and do not infect other animals or plants. The potential to use fungi as microbial insecticides is being investigated, with several species already on the market. For example, the fungus Beauveria bassiana is a pesticide that is currently being tested as a possible biological control for the recent spread of emerald ash borer. It has been released in Michigan, Illinois, Indiana, Ohio, West Virginia, and Maryland.

We also eat some types of fungi. Mushrooms figure prominently in the human diet. Morels, shiitake mushrooms, chanterelles, and truffles are considered delicacies (Figure 10.25). The humble meadow mushroom, Agaricus campestris, appears in many dishes. Molds of the genus Penicillium ripen many cheeses. They originate in the natural environment such as the caves of Roquefort, France, where wheels of sheep milk cheese are stacked to capture the molds responsible for the blue veins and pungent taste of the cheese.

The photo shows a mushroom with a convoluted black cap.

Figure 10.25 The morel mushroom is an ascomycete that is much appreciated for its delicate taste. (credit: Jason Hollinger)

Fermentation—of grains to produce beer, and of fruits to produce wine—is an ancient art that humans in most cultures have practiced for millennia. Wild yeasts are acquired from the environment and used to ferment sugars into CO₂ and ethyl alcohol under anaerobic conditions. It is now possible to purchase isolated strains of wild yeasts from different wine-making regions. Pasteur was instrumental in developing a reliable strain of brewer’s yeast, Saccharomyces cerevisiae, for the French brewing industry in the late 1850s. It was one of the first examples of biotechnology patenting. Yeast is also used to make breads that rise. The carbon dioxide they produce is responsible for the bubbles produced in the dough that become the air pockets of the baked bread.

Many secondary metabolites of fungi are of great commercial importance. Antibiotics are naturally produced by fungi to kill or inhibit the growth of bacteria, and limit competition in the natural environment. Valuable drugs isolated from fungi include the immunosuppressant drug cyclosporine (which reduces the risk of rejection after organ transplant), the precursors of steroid hormones, and ergot alkaloids used to stop bleeding. In addition, as easily cultured eukaryotic organisms, some fungi are important model research organisms including the red bread mold Neurospora crassa and the yeast, S. cerevisiae.

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