Fungus

A fungus is any member of the group of eukaryotic organisms that includes yeasts, molds, as well as mushrooms. These organisms belong to the biological kingdom called Fungi. Unlike plants, fungi have chitin in their cell walls and, like animals, they are heterotrophs, meaning they get their nutrition by absorbing dissolved organic molecules. Fungi do not perform photosynthesis and instead grow to move, with some spores able to travel through air or water.

Fungi play an essential role in nature by breaking down organic matter and recycling nutrients in the environment. Over 90% of plants depend on a special partnership called Mycorrhizal symbiosis with fungi, which helps plants grow better and capture more carbon from the air, aiding in the fight against climate change. Fungi have been used for thousands of years as food, like mushrooms and truffles, and in making bread rise, as well as in fermenting foods such as wine, beer, and soy sauce.

Since the 1940s, fungi have also been used to produce antibiotics and various enzymes for industrial uses and detergents. Some fungi are used as natural biological pesticides to help control unwanted plants, diseases, and pests. However, fungi can also be harmful, causing diseases in plants, animals, and humans, and leading to losses in crops and food supplies. The kingdom of fungi is incredibly diverse, with estimates suggesting between 2.2 million to 3.8 million species exist, though only about 148,000 have been described by scientists.

Etymology

The word fungus comes from the Latin word for "mushroom." It was used by writers like Horace and Pliny. This Latin word itself comes from a Greek word meaning "sponge," which describes the shape and look of mushrooms and molds.

The study of fungi is called mycology. This word comes from Greek words for "mushroom" and "study." It started being used in English in the early 1800s. All the fungi in one area are called the mycobiota. Some people also use the word mycota to talk about fungi. Recently, there has been a suggestion to use the word funga along with words like fauna (animals) and flora (plants) when talking about nature.

Characteristics

Fungi used to be thought of as plants because they often grow in soil and look similar to plants like mosses. But scientists now know fungi are their own special group, very different from plants and animals. They have been separate from plants and animals for about one billion years.

Like animals, fungi do not have chloroplasts and need to get their food from other sources. Like plants, they have cell walls and can make spores to reproduce. One big difference is that fungi have something called chitin in their cell walls, which plants do not have. Some fungi grow in long, thread-like shapes called hyphae, while others grow as single cells called yeasts.

Diversity

Fungi can be found everywhere in the world, from deserts to places with lots of salt or ionizing radiation, and even in deep sea sediments. Some fungi can survive strong UV and cosmic radiation during space travel. Most fungi grow on land, but some live in water. For example, certain fungi called Batrachochytrium dendrobatidis and B. salamandrivorans are parasites that live in water and affect amphibian populations. These fungi can move through water using a special form called a zoospore to reach their hosts. Other water-loving fungi are found in very hot parts of the ocean called hydrothermal areas.

As of 2020, scientists have identified about 148,000 species of fungi, but we don’t know all the types that exist. One guess is that there could be between 2.2 and 3.8 million different species. Every year, more new fungi are discovered. In 2019, 1,882 new species were found, and in 2020, that number jumped to 2,905—the most ever recorded in one year. Scientists study fungi by looking at their size, shape, and how they react to different substances. They also use tools that look at the tiny building blocks inside cells to learn more about these amazing organisms.

Mycology

Main article: Mycology

Mycology is the study of fungi. It looks at their genes, chemicals, how they are grouped, and how they help or hurt us. Fungi are used for medicines, food, and some special substances for religious reasons, but they can also cause problems like poisoning or sickness. The study of plant diseases is closely related because many plant sicknesses are caused by fungi.

People have used fungi for a very long time. For example, a very old frozen person found in the Alps carried mushrooms that might have been used to start fires or for medicine. Ancient people also used fungi without knowing it when they made bread rise and drinks ferment. Some of the oldest writings talk about crops being destroyed, probably by harmful fungi.

Mycology became a proper science after microscopes were invented in the 1600s. Important early scientists observed fungal spores and showed they could grow into new fungi. Later scientists developed ways to classify fungi, and today we use advanced tools to learn more about them.

Morphology

Most fungi grow as tiny, thread-like structures called hyphae. These threads can be very small, just a few thousandths of a millimeter wide, but they can stretch for many centimeters. Hyphae grow by adding new tips, sometimes splitting to form more threads, and they can join together to create a network called a mycelium.

When you see fuzzy spots on old bread or mold on a wall, you are seeing fungal mycelia. In labs, these fuzzy patches grown on special plates are called colonies and can have different colors and shapes that help scientists identify the type of fungus. Some fungi can grow very large, like one that covers many acres and is thousands of years old. Certain fungi also produce cup-shaped structures used for making new spores, and some grow large mushroom-like shapes that we often eat.

Growth and physiology

Fungi grow in different ways depending on where they live. They can grow as tiny threads called hyphae on surfaces or as single cells in water. This helps them get food efficiently because they have a large surface area compared to their size. Some fungi can even push through plant surfaces using special structures that create strong pressure.

Fungi get their food by breaking down big molecules into smaller ones they can absorb. Most fungi need food from other organisms, but they can use many different kinds of substances to grow. A few fungi can even get energy from radiation, though this is not common and not well understood.

Reproduction

Fungi have many ways to reproduce, showing the variety in their lifestyles and genetics. Many fungi can reproduce in more than one way. For example, some fungi have two main stages in their life cycle: one for sexual reproduction and one for asexual reproduction. The environment can trigger these stages, leading to special structures that help spread spores or spore-containing parts.

Asexual reproduction

Fungi can reproduce asexually through special spores called conidia or by breaking into pieces. When a fungal piece breaks off, it can grow into a new fungus. This helps fungi spread quickly and stay adapted to their environment. Some fungi, known as Deuteromycota, do not have a known sexual stage and only reproduce this way.

Sexual reproduction

Sexual reproduction in fungi has been seen in all groups except one, though it is believed to happen there too. It works differently from animals or plants. Fungi can be identified by the shapes of their sexual structures and spores. They have special ways to mate, either only with opposite types or with any other individual.

Most fungi have stages where they are simple and more complex in their life cycles. When compatible fungi meet, they can join together. Some fungi have a special stage where the nuclei from both parents stay separate.

In one group, special structures form to ensure proper distribution of nuclei. An ascus is created where nuclear fusion happens, followed by meiosis and the production of ascospores. These spores can grow into new fungi after they are spread.

In another group, compatible simple hyphae join to create a special mycelium. A structure called a clamp connection helps control the transfer of nuclei. Basidia then create basidiospores after nuclear fusion and meiosis. Well-known forms of these are mushrooms.

In a former group, simple hyphae from two individuals join to form a gametangium, which becomes a thick-walled spore. When this spore germinates, it undergoes meiosis to create new simple hyphae, which can then form asexual spores for quick spreading.

Spore dispersal

Many fungi spread their spores through the wind. These spores are dry and do not absorb water, making them easy to scatter. Some fungi actively shoot their spores out of their structures to travel far.

Special mechanisms help spores shoot out. For example, in some fungi, the buildup of substances causes spores to be explosively discharged into the air. Other fungi use different methods, like relying on external forces or using insects to carry their spores.

Some fungi depend on animals to spread their spores. For example, truffles and other underground fungi produce smelly fruitbodies that attract small mammals, which eat them and spread the spores in their waste. Insects and birds can also help spread spores.

Homothallism

In a type of sexual reproduction called homothallism, two simple nuclei from the same individual join to form a zygote, which can then undergo meiosis. This type of reproduction might have been the earliest way eukaryotes reproduced.

Other sexual processes

Besides regular sexual reproduction, some fungi can exchange genetic material through special processes. This is known to play a role in hybridization and may have been important in fungal evolution.

Evolution

Fossil history

Unlike plants and animals, we don’t have many old fossils of fungi. This is because fungal bodies are soft and break down easily, and most fungal parts are too tiny to see clearly. Fungal fossils can be hard to tell apart from other tiny organisms, but they are easier to recognize when they look like fungi we know today. These fossils are often found inside preserved plants or animals and are studied using special microscopes.

The oldest fossils that look like fungi are from about 2,400 million years ago. Other studies suggest fungi might have appeared between 760 and 1,060 million years ago. One special fossil of a fungus named Ourasphaira giraldae was found in the Canadian Arctic and may have lived on land over a billion years ago—before plants existed on land! Fungi were around during many different times in Earth’s history, changing and adapting to new environments.

External phylogeny

Fungi are more closely related to animals than to plants. Scientists use special tools to study how fungi evolved, and these tools often show that older classifications of fungi need to be updated. Because of this, the way we group fungi changes a lot, and they can have different names depending on how they reproduce.

Internal phylogeny

In 2007, scientists worked together to create a new way to group fungi into seven main groups. Two big groups, Ascomycota and Basidiomycota, include many familiar fungi like mushrooms and yeasts. These groups make up most of the fungi we know.

Taxonomic groups

Fungi are grouped based on how they reproduce. As of 2019, there are nine main groups of fungi. These include groups like Ascomycota (sac fungi) and Basidiomycota (club fungi). Some fungi, like the Microsporidia, were once thought to be simple organisms but are now known to be highly specialized fungi.

The Chytridiomycota, or chytrids, are found all over the world. They can move with tiny tail-like structures called flagella. The Blastocladiomycota are similar but have different ways of reproducing. The Neocallimastigomycota live in the stomachs of animals that eat plants and help break down tough materials.

The Glomeromycota form important partnerships with plants, helping them get nutrients from the soil. The Ascomycota include many types of fungi, such as yeasts and molds, and they produce special spores called ascospores. The Basidiomycota include most mushrooms and some fungi that cause diseases in plants and animals.

Fungus-like organisms

Some organisms look like fungi but are not actually fungi. For example, slime molds and water molds were once thought to be fungi but are now known to belong to different groups of organisms. These organisms differ from true fungi in how they get their food and in the materials that make up their cell walls.

Ecology

Fungi are found everywhere on Earth and play important roles in many ecosystems. Along with bacteria, they help break down dead plants and animals, which is important for keeping the environment healthy. They also help cycle nutrients, moving important substances like nitrogen and phosphorus through the ecosystem.

Many fungi have special relationships with other living things. For example, some fungi live in partnership with plants, helping them grow by bringing nutrients from the soil. These fungi can also help plants share food and nutrients with each other through networks underground. Fungi also team up with tiny plants called algae to form lichens, which can survive in harsh places like deserts and cold regions. Some insects, like ants and certain beetles, also grow fungi for food and to help build their homes.

Mycotoxins

Many fungi make special compounds that can be harmful to animals or plants, called mycotoxins. Some of these come from molds that spoil food and from poisonous mushrooms. For example, some mushrooms in the Amanita group have very strong toxins. There are also harmful compounds from a fungus called Claviceps purpurea, which can grow on rye and other grains.

Other important mycotoxins include ones made by Aspergillus species, which can grow on grains and nuts. These compounds can hurt the liver and may cause health problems over time. Mycotoxins are special chemicals that fungi make, and they can help the fungi survive by protecting them or helping them compete with other tiny living things. Some of these fungal chemicals are even used to help people in medicine.

Pathogenic mechanisms

Some fungi can make plants and animals sick. For example, Ustilago maydis is a fungus that causes disease in crops like maize and teosinte. Plants have strong defenses against these harmful fungi, including creating substances that can harm the invader. U. maydis has special ways to protect itself from these plant defenses, which helps it cause disease.

Another fungus, Cryptococcus neoformans, can infect both plants and animals, often affecting the lungs. It has unique survival strategies that let it live inside the body’s defense cells. These strategies help the fungus repair its own genetic material when damaged, allowing it to continue causing illness.

Human use

See also: Human interactions with fungi

Humans have used fungi for a very long time, mainly for food and medicine. Growing and collecting mushrooms is a big business in many places. Fungi can also make useful substances, like medicines that fight bacteria, lower cholesterol, or help with cancer. Scientists can even change yeast to make medicines more easily.

Therapeutic uses

Modern chemotherapeutics

See also: Medicinal fungi

Many medicines come from fungi. One important group is called antibiotics, which help fight infections. These include penicillins, made from a type of mold. Other medicines from fungi help with organ transplants and certain kinds of bacteria.

Antibiotics

Other drugs from fungi include treatments for fungal infections and cholesterol. Some fungi also produce substances being studied to help with cancer and viruses.

Traditional medicine

The fungi Ganoderma lucidum (left) and Ophiocordyceps sinensis (right) are used in traditional medicine practices

Some mushrooms have been used in old healing ways, like in traditional Chinese medicine. Examples include Agaricus subrufescens, Ganoderma lucidum, and Ophiocordyceps sinensis.

Cultured foods

Baker's yeast or Saccharomyces cerevisiae, a unicellular fungus, is used to make bread and other wheat-based products, such as pizza dough and dumplings. Yeast species of the genus Saccharomyces are also used to produce alcoholic beverages through fermentation. Shoyu koji mold (Aspergillus oryzae) is an essential ingredient in brewing shoyu (soy sauce) and sake, and the preparation of miso, while Rhizopus species are used for making tempeh. Several of these fungi are domesticated species that were bred or selected according to their capacity to ferment food without producing harmful mycotoxins (see below), which are produced by very closely related Aspergilli. Quorn, a meat alternative, is made from Fusarium venenatum.

In food

Edible mushrooms include commercially raised and wild-harvested fungi. Agaricus bisporus, sold as button mushrooms when small or Portobello mushrooms when larger, is the most widely cultivated species in the West, used in salads, soups, and many other dishes. Many Asian fungi are commercially grown and have increased in popularity in the West. They are often available fresh in grocery stores and markets, including straw mushrooms (Volvariella volvacea), oyster mushrooms (Pleurotus spp.), shiitakes (Lentinula edodes), and enokitake (Flammulina spp.).

Many other mushroom species are harvested from the wild for personal consumption or commercial sale. Milkcap mushrooms, morels, chanterelles, truffles, black trumpets, and porcini mushrooms (Boletus edulis) (also known as king boletes) demand a high price on the market. They are often used in gourmet dishes.

Certain types of cheeses require inoculation of milk curds with fungal species that impart a unique flavor and texture to the cheese. Examples include the blue color in cheeses such as Stilton or Roquefort, which are made by inoculation with Penicillium roqueforti. Molds used in cheese production are non-toxic and are thus safe for human consumption; however, mycotoxins (e.g., aflatoxins, roquefortine C, patulin, or others) may accumulate because of growth of other fungi during cheese ripening or storage.

Poisonous fungi

Many mushroom species can be harmful to humans and cause problems like stomach upset, allergic reactions, hallucinations, or even serious health issues. Some of the most dangerous mushrooms include types of [Amanita], like the death cap. Because it is hard to tell which mushrooms are safe, it is best not to eat wild mushrooms unless you are sure they are safe.

Pest control

Main article: Biological pest control § Fungi

Fungi can also help control pests in farming. Some fungi can stop harmful insects, mites, weeds, and other plant diseases. These fungi are used as natural ways to protect crops without harmful chemicals.

Bioremediation

See also: Mycoremediation

Some fungi can break down harmful chemicals like insecticides and pollution, turning them into harmless substances. This can help clean up polluted areas.

Model organisms

Several pivotal discoveries in biology were made by researchers using fungi as model organisms, that is, fungi that grow and sexually reproduce rapidly in the laboratory. For example, the one gene-one enzyme hypothesis was formulated by scientists using the bread mold Neurospora crassa to test their biochemical theories. Other important model fungi are Aspergillus nidulans and the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe, each of which with a long history of use to investigate issues in eukaryotic cell biology and genetics, such as cell cycle regulation, chromatin structure, and gene regulation. Other fungal models have emerged that address specific biological questions relevant to medicine, plant pathology, and industrial uses; examples include Candida albicans, a dimorphic, opportunistic human pathogen, Magnaporthe grisea, a plant pathogen, and Pichia pastoris, a yeast widely used for eukaryotic protein production.

Others

Fungi are used extensively to produce industrial chemicals like citric, gluconic, lactic, and malic acids, and industrial enzymes, such as lipases used in biological detergents, cellulases used in making cellulosic ethanol and stonewashed jeans, and amylases, invertases, proteases, and xylanases.