What are Fungi ?
Fungi are a fascinating and diverse group of eukaryotic organisms that play essential roles across nearly all ecosystems and profoundly impact human life. Often overlooked or mistaken for plants, fungi belong to their own distinct biological kingdom.
What Are Fungi?
Fungi are defined by several key characteristics that set them apart from plants, animals, and other kingdoms:
Eukaryotic Organisms.
Lack Chlorophyll and are non-photosynthetic. Unlike plants, they do not convert sunlight, water, and carbon dioxide into oxygen and sugar.
Heterotrophic. This means they cannot produce their own food and must feed on preformed organic material. Instead of ingesting and then digesting like animals, fungi first digest and then ingest, producing exoenzymes to break down organic matter externally before absorbing the nutrients.
Possess rigid cell walls containing chitin. Chitin is also found in the exoskeletons of insects and arachnids. This chemical composition differs from plant cell walls, which are primarily composed of cellulose.
Their plasma membranes contain ergosterol as the principal sterol, unlike mammalian cell membranes which contain cholesterol. This unique sterol is a target for many antifungal agents.
Synthesize lysine by the L-α-adipic acid biosynthetic pathway (AAA pathway), a feature that distinguishes them from bacteria and other microorganisms that use the meso-α,ε-diaminopimelic acid pathway (DAP pathway).
Are nonvascular organisms, meaning they lack specialized transport tissues.
Generally not motile, although a few, like chytrids, have a motile phase.
Mostly reproduce by spores, which can be wind-disseminated. Both sexual (meiotic) and asexual (mitotic) spores may be produced.
Typically store their food as glycogen, similar to animals, rather than starch like plants.
Fungi exhibit diverse morphologies:
They can be unicellular yeasts, which reproduce by budding. Yeasts exist naturally on fruits and vegetables.
They can be multicellular molds that produce filaments called hyphae. These hyphae can be produced in specialized spore-forming structures known as fruiting bodies.
Some fungi are dimorphic, meaning they can exist as both a mold form (typically at room temperature in the lab) and a yeast form or spherules (in tissue or at 37°C). This dimorphism is crucial for some pathogenic fungi to cause disease.
When Did Fungi Appear?
Fungi have an ancient lineage, with fossil evidence indicating their existence in the Precambrian and Devonian eras. Phylogenetically, fungi are not closely related to plants; instead, they are now understood to have diverged from the animal lineage. Recent DNA evidence suggests that fungi and animals diverged from their last common ancestor, a unicellular flagellated organism, approximately a billion years ago.
Where Can We Find Fungi?
Fungi are ubiquitous, found in nearly every habitat. Most species live on land, primarily in soil or plant material, rather than in marine or freshwater environments. They occupy many different environmental niches, and it is believed that a high number of fungal species have yet to be discovered.
In human environments, allergenic mold spores are common throughout the US, with counts highest in summer and early fall. Indoors, common mold sources include damp spaces, baths, showers, crawlspaces, and basements.
Why Are Fungi Important?
Fungi are profoundly important due to their diverse roles and interactions:
They are a crucial component of nature, breaking down and recycling nutrients from dead plants and other organic matter.
They form symbiotic relationships with plants and animals.
They are a vital part of the food web as decomposers and, in some cases, pathogens.
They are essential in both grassland and forest ecosystems.
Many plants rely on fungi for survival, and herbivores like cows depend on fungi in their gut to digest grass.
Fungi affect human lives in numerous ways, from causing diseases to providing valuable industrial products.
It is estimated that approximately 1.5 million fungal species exist, but only about 75,000 to 69,000 species have been identified and named, meaning a vast majority remain unknown to science.
How Fungi Interact in Nature
Fungi derive their nutrition from organic material, whether living or dead. This shapes their ecological interactions:
Decomposers and Saprophytes: Along with bacteria, fungi are the primary decomposers and recyclers in the environment. They break down dead animal and vegetable material into simpler compounds, recycling carbon, nitrogen, and essential mineral nutrients, which then become available to other organisms. They are especially important in breaking down wood components like cellulose and lignin. Only a few materials, such as certain pesticides and plastics, resist fungal degradation.
Parasites (Pathogens): Some fungi derive nutrition from living organisms, harming their hosts.
Plant Parasites: Fungi are the major cause of plant diseases, accounting for about 90% of them. This results in billions of dollars in crop and forest losses annually. Examples include rust and smut fungi, black stem rust of wheat (Puccinia graminis), powdery mildew of grasses (Erysiphe graminis), ergot (Claviceps purpurea), and corn smut (Ustilago maydis). Important forest pathogens in North America include chestnut blight (Cryphonectria parasitica), Dutch elm disease (Ophiostoma ulmi), and white pine blister rust (Cronartium ribicola). The severity of a plant disease is determined by the interaction of the host, pathogen, and environment—known as the disease triangle.
Animal and Human Parasites: Fungi can cause diseases in animals and humans, either through direct growth on the body (mycoses) or by the ingestion of toxins.
Mycoses: These are fungal infections where fungi grow directly on human tissue. They can be superficial (e.g., pityriasis versicolor), cutaneous (e.g., ringworm, athlete's foot caused by dermatophytes and yeasts), subcutaneous (e.g., sporotrichosis), or systemic (deep mycoses). Systemic mycoses, such as histoplasmosis (Histoplasma capsulatum), blastomycosis (Blastomyces dermatitidis), and coccidioidomycosis (Coccidioides immitis), are caused by dimorphic fungal pathogens that typically enter through the lungs. While many fungal pathogens require compromised host defenses (e.g., in AIDS patients, those on immunosuppressive drugs, or diabetics), dimorphic fungi can cause infections even in immunocompetent persons because they change form (from mycelium to yeast) to evade host defenses. The most common fungal infection in North America is yeast infection, often caused by Candida albicans, which is part of the body's normal flora but can cause disease when conditions are imbalanced.
Mycetismus: Refers to eating poisonous mushrooms. There is no easy way to distinguish edible from poisonous mushrooms; relying on "old wives' tales" is dangerous. The toxins in mushrooms appear to be waste products that happen to be poisonous to animals or humans. Major classes of mushroom toxins include cyclopeptides (found in "death angel" mushrooms like Amanita bisporigera), monomethyl hydrazine, coprine, muscarine, ibotenic acid/muscimol, psilocybin/psilocin, and general gastrointestinal irritants.
Mycotoxicosis: Involves the ingestion of toxins produced by molds growing on foods. For example, Aspergillus flavus and A. parasiticus can produce aflatoxins on peanuts, which are highly carcinogenic.
Mycoallergies: Are allergic reactions to fungi and their metabolites, commonly involving the inhalation of spores. The genus Alternaria is a main cause of inhalation allergy problems. Allergic fungal sinusitis is also recognized. While "black mold" (Stachybotrys chartarum) is often blamed for Sick Building Syndrome, scientific evidence directly linking it to the described symptoms is still lacking.
Mutualists: These fungi engage in reciprocally beneficial relationships with other living organisms, where both parties benefit.
Mycorrhizae: Are associations between a fungus and the roots of a plant. More than 90% of plants in nature have a mycorrhizal symbiont. The fungal hyphae extend into the soil, greatly increasing the surface area for the plant's absorption of nutrients, particularly phosphate, nitrogen, and potassium. In return, the fungus receives sugars from plant photosynthesis. These fungi are hypothesized to have been necessary for the movement of water plants onto land.
Lichens: Are dual organisms formed by a mutualistic relationship between a fungus (mycobiont) and an alga or cyanobacterium (photobiont). Neither typically survives on its own. Lichens can grow in very inhospitable environments, on surfaces like rocks and trees. They are sensitive to air pollution and thus serve as natural indicators of air quality. Ecologically, lichens are important for breaking down rocks into soil and are a food source for animals like caribou.
Endophytes: Are microscopic fungi that live almost entirely within the leaves, roots, and stems of apparently healthy host plants. The fungus receives sugars from the plant, and in turn, often produces bad-tasting toxins that deter herbivores from eating the plant. Their significance in nature is greatly underappreciated.
Commensal Fungi: These fungi use other organisms as a place to live but derive no nutrition from them.
How Fungi Benefit Humans
Fungi provide numerous direct benefits to humans:
Food and Beverages:
Many species of mushrooms are edible and enjoyed as food. They are primarily composed of water (69–90%), carbohydrates (16–85%), proteins (14–44%), and lipids (0.2–87%).
Yeasts have been used for baking and brewing for millennia. In baking, yeasts convert sugars to carbon dioxide, which causes bread dough to rise and gives baked bread its airy texture.
Molds are used to ferment certain gourmet cheeses, such as Roquefort, blue cheeses, Brie, and Camembert, often involving specific Penicillium species.
Medicine:
Fungi are a source of vital drugs, including antibiotics like penicillin and cephalosporin.
The immunosuppressive antirejection drug cyclosporine is produced by the ascomycete Tolypocladium inflatum.
Various fungi are commercially used to produce steroids, hormones, and even birth control pills.
Industrial Applications: Fungi are used in the commercial production of many valuable organic compounds, such as citric acid (used in soda pop, produced by Aspergillus species). They are also used to soften stone-washed jeans (Trichoderma species).
Research and Science: Fungi are important experimental organisms because they are easily cultured, occupy little space, multiply rapidly, and have short life cycles. As eukaryotes, they are more closely related to animals than bacteria, making their study highly applicable to human health problems. Fungi are used to study metabolic pathways, growth, development, differentiation, cell division mechanisms, and for microbial assays of vitamins and amino acids. They have also been crucial genetic tools; Neurospora was instrumental in the "one gene one enzyme" theory (Nobel Prize in 1958), and Saccharomyces cerevisiae (baker's and brewer's yeast) was the first eukaryotic genome to be sequenced, and also contributed to understanding mitosis genes (Nobel Prize in 2001).
Forensic Science: Fungal spores, due to their dispersal patterns, can be utilized in criminal investigations, though their range of dispersal is typically limited.
The study of fungi, known as mycology, encompasses many aspects of their biology, including systematics, taxonomy, classification, physiology, ecology, pathology, evolution, genetics, and molecular biology. Applied mycology fields include plant pathology, human pathology, fermentation, and mushroom cultivation.
