Organismal Biology

Learning Objectives

1. Place fungi on a phylogenetic tree
2. Identify and describe the key adaptations unique to fungi (cell walls made of chitin and external digestion), including morphological, life cycle, and
   metabolic traits
3. Describe the symbiotic relationship of fungi with plants and pathological relationships with other organisms
4. Explain ecosystem services of fungi and human nutrition applications
5. Explain why the colonization of land plants was facilitated by fungi

Many species of fungus produce the familiar mushroom (a) which is a reproductive structure. This (b) coral fungus displays brightly colored fruiting bodies. This electron micrograph shows (c) the spore-bearing structures of Aspergillus, a type of toxic fungi found mostly in soil and plants. (credit: mushroom: modification of work by Chris Wee; credit fungus: modification of work by Cory Zanker; credit Aspergillus: modification of work by Janice Haney Carr, Robert Simmons, CDC; scale-bar data from Matt Russell)

Fungi are eukaryotes with an enormous variety of body plans and, along with land plants and animals, are one of the major evolutionary lineages to occupy land. While scientists have identified about 100,000 species of fungi, this is only a fraction of the 1.5 million species of fungus likely present on Earth. Edible mushrooms, yeasts, black mold, and the producer of the antibiotic penicillin, Penicillium notatum, are all fungi. And currently the largest (and perhaps, oldest) living organism on Earth’s surface is a fungus!

So, how old are Fungi generally? Unfortunately, most fungal structures do not fossilize well, so it has been difficult to establish much of a fossil record for fungi. With the advent of new molecular techniques, the hypothesized origin of fungi has changed several times and is still hotly debated. The two prevailing hypotheses suggest fungi either evolved much earlier (1300 million years ago) or at the same time as the first land plants in the Pre-Cambrian. The key difference is whether fungi evolved first in water, and then became successful on land alongside plants, or whether their evolutionary origins coincided with the plant invasion of land. By either hypothesis, we have evidence that by 420 million years ago, plants and fungi were evolving at the same time on land, millions of years before the first vertebrates crawled out of the sea.

Characteristics of fungi

The information below was adapted from OpenStax Biology 24.1

Although humans have used yeasts and mushrooms since prehistoric times, until recently, the biology of fungi was poorly understood. Up until the mid-20th century, many scientists classified fungi as plants, largely due to sessile lifestyle and general morphology. However, molecular biology analysis of the fungal genome demonstrates that fungi are more closely related to animals than plants.

While fungi can be multicellular or unicellular, all fungi have two things in common:

1. cell walls made of a tough polysaccharide, called chitin, which provides structure
2. external digestion of food

In the next section, we will go review the typical characteristics of many fungi, but keep in mind, there are exceptions to the rule.

Cell structure and function

Fungi are eukaryotes, and as such, have a complex cellular organization. Being eukaryotes, a typical fungal cell contains a true nucleus, mitochondria, and a complex system of internal membranes, including the endoplasmic reticulum and Golgi apparatus.

Unlike plant cells, fungal cells do not have chloroplasts or chlorophyll. Many fungi display bright colors arising from other cellular pigments, ranging from red to green to black. Pigments in fungi are associated with the cell wall and play a protective role against ultraviolet radiation or predators.

Like plant cells, fungal cells have a thick cell wall, but in fungi, it is made of complex polysaccharides called chitin and glucans. Chitin, also found in the exoskeleton of insects, gives structural strength to the cell walls of fungi. The wall protects the cell from desiccation (‘drying out’) and predators.

Growth

Although dimorphic fungi can change from the unicellular to multicellular state (depending on environmental conditions), most fungi are actually multicellular organisms. They display two distinct morphological stages: the vegetative and reproductive. The vegetative stage consists of a tangle of slender thread-like structures called hyphae (singular, hypha), whereas the reproductive stage can be more conspicuous.

Fungal hyphae, although microscopic, allow for the rapid flow of nutrients and small molecules across the fungal body. Many fungi create networks of these hyphae into a mass called a mycelium. The mycelium can grow on a surface, in soil or decaying material, in a liquid, or even on living tissue. Although individual hyphae are tiny, the overall mycelium of a fungus can be very large, with some species truly being “the fungus humongous”. The giant Armillaria solidipes (honey mushroom) is considered the largest organism on Earth, spreading across more than 2,000 acres of underground soil in eastern Oregon; it is estimated to be at least 2,400 years old!

Fungi thrive in environments that are moist and slightly acidic, and can grow with or without light and oxygen. Most fungi are obligate aerobes, requiring oxygen to survive, however some species, such as the Chytridiomycota that reside in the rumen of cattle, are obligate anaerobes; for these species, anaerobic respiration is used because oxygen will disrupt their metabolism or kill them. Yeasts, like those used in wine or beer-making, are intermediates: facultative anaerobes. They grow best in the presence of oxygen using aerobic respiration, but can survive using anaerobic respiration when oxygen is not available.

Reproductive life cycle

Fungi can reproduce sexually and/or asexually. ‘Perfect’ fungi reproduce both sexually and asexually, while the so-called ‘imperfect’ fungi reproduce only asexually (by mitosis). Because of the variety of reproductive methods, the specific structures produced by a fungal for reproduction help to classify it among fungal phyla (subgroups), such as Basidiomycota, Ascomycota, Glomeromycota, and Chytridiomycota. We will cover these groups more in-depth in class.

Fungi may have both asexual and sexual stages of reproduction. Credit: OpenStax

In both sexual and asexual reproduction as shown above, fungi produce many small, light-weight spores that disperse from the parent organism by either floating on the wind or hitching a ride on an animal. The huge number of spores released increases the likelihood of landing in an environment that will support growth.

The (a) giant puff ball mushroom releases (b) a cloud of trillions 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)

Asexual Reproduction

Fungi can reproduce asexually by fragmentation, budding, or producing spores. Fragments of hyphae can grow new colonies, whereas, during budding, a bulge forms on the side of the cell, the nucleus divides mitotically, and the bud ultimately detaches itself from the mother cell.

The dark cells in this bright field light micrograph show budding in the pathogenic yeast Histoplasma capsulatum, seen against a backdrop of light blue tissue. Histoplasma primarily infects lungs but can spread to other tissues, causing histoplasmosis, a potentially fatal disease. (credit: modification of work by Dr. Libero Ajello, CDC; scale-bar data from Matt Russell)

The most common mode of asexual reproduction is through the formation of asexual spores, which are produced by one parent only (through mitosis) and are genetically identical to that parent. Spores allow fungi to expand their distribution and colonize new environments. They may be released either outside the body or within a special reproductive sac called a sporangium.

This bright field light micrograph shows the release of spores from a sporangium at the end of a hypha called a sporangiophore. The organism is a Mucor sp. fungus, a mold often found indoors. (credit: modification of work by Dr. Lucille Georg, CDC; scale-bar data from Matt Russell)

Sexual Reproduction

Sexual reproduction introduces genetic variation into a population of fungi. In fungi, sexual reproduction occurs in a variety of ways and often in response to adverse environmental conditions. During sexual reproduction, two mating types (rather than distinct ‘sexes’, e.g. male and female) are produced; we will provide a general outline of this process, but note the details vary greatly by fungal species.

Although there are many variations in fungal sexual reproduction, all include the following three stages. First, during plasmogamy (literally, marriage or union of cytoplasm’), two haploid cells fuse, leading to a dikaryotic stage where two haploid nuclei coexist in a single cell. During karyogamy (‘nuclear marriage’), the haploid nuclei fuse to form a diploid zygote nucleus. Finally, meiosis takes place in the gametangia (singular, gametangium) organs, in which gametes of different mating types are generated. At this stage, spores are disseminated into the environment, and the cycle can start again.

Metabolism and nutrition

Like animals, fungi are heterotrophs: they use complex organic compounds as a source of carbon, rather than fix carbon dioxide from the atmosphere as do some bacteria and most plants. In addition, like animals, fungi do not fix nitrogen from the atmosphere and must obtain it from their environment.

However, unlike most animals, which ingest food and then digest it internally in specialized organs, fungi perform these steps in the reverse order: digestion precedes ingestion. Thus, digestion occurs outside of the body. In multicellular fungi, first, exoenzymes are transported out of the hyphae, where they process nutrients in the environment. Then, the smaller molecules produced by this external digestion are absorbed through the large surface area of the mycelium. As with animal cells, the polysaccharide of storage is glycogen, rather than starch, as found in plants.

Fungi are mostly decomposers which derive nutrients from dead or decaying organic matter (usually plants). Fungal exoenzymes are able to break down insoluble polysaccharides, such as the cellulose and lignin of dead wood, into readily absorbable glucose molecules. Other fungi form special roles, such as mutualisms with plants, where fungi trade water and key nutrients with plants in exchange for plant sugars. Still other fungi are parasitic, infecting either plants or animals; for example, smut and Dutch elm disease affect plants, whereas athlete’s foot and candidiasis (thrush) are medically important fungal infections in humans. In environments poor in nitrogen, some fungi even resort to predation by trapping other small organisms, like nematodes, via constricting rings within their hyphae. Fungi really do it all!

Fungal symbioses

The information below was adapted from OpenStax Biology 24.3

Symbiosis is the ecological interaction between two organisms that live together, however, the definition does not describe the quality of the interaction. When both members of the association benefit, the symbiotic relationship is called mutualistic. Fungi form mutualistic associations with many types of organisms, including cyanobacteria, algae, plants, and animals.

Among the examples of fungal-plant mutualism are the endophytes: fungi that live inside tissue without damaging the host plant. Endophytes release toxins that repel herbivores, or confer resistance to environmental stress factors, such as infection by microorganisms, drought, or heavy metals in soil.

For the most common example, most terrestrial plants form symbiotic relationships with fungi via their roots. The roots of the plant connect with the underground parts of the fungus forming mycorrhizae (from the Greek words myco meaning fungus and rhizo meaning root). In a mycorrhizal association, the fungal mycelia use their extensive network of hyphae and large surface area in contact with the soil to channel water and minerals from the soil into the plant. In exchange, the plant supplies the products of photosynthesis to fuel the metabolism of the fungus. Even some plants, such as orchids, have developed so strong an association with fungi that their seeds generally cannot germinate and grow without a fungal mycorrhiza partner!

Think: If symbiotic fungi are suddenly absent from the soil, what impact do you think this would have on plant growth?

Fungal ecosystem services

With their versatile metabolism, fungi can break down organic matter which would not otherwise be recycled in the ecosystem. Some elements, such as nitrogen and phosphorus, are required in large quantities by biological systems, and yet are not abundant in the environment unless this breakdown takes place. Even trace elements present in low amounts in many habitats are essential for growth would remain tied up in rotting organic matter if fungi and bacteria did not return them to the environment via their metabolic activity. Thus, fungi make it possible for other living things to be supplied with the nutrients they need to live.

Because of their varied metabolic pathways, fungi can fulfill many important roles. Not only do they help to stabilize ecosystems and supply us with food, but they are also directly used in the production of beer, cheese, and bread, as well as various medicines. Some fungi are also extremely sensitive to air pollution, especially to abnormal levels of nitrogen and sulfur. The U.S. Forest Service and National Park Service can monitor air quality by measuring their relative abundance and health in an area (read more on this here). Currently, fungi are being investigated as potential tools in bioremediation; For example, some species of fungi can be used to break down diesel oil, polycyclic aromatic hydrocarbons (PAHs), and even heavy metals, such as cadmium and lead.

How fungi helped plants onto land

The colonization of land by fungi is much entangled with plants. At the very least, it is clear that plants could not have colonized land some 420 million years ago without the help of fungi.

The first association between fungi and photosynthetic organisms on land involved moss-like plants and endophytes, before the evolution of plant roots. These plants could not survive in permanently dry areas, so fungi helped to provide needed moisture. True roots appeared in later in vascular plants, where a system of thin extensions from the rhizoids (rootlike structures found in mosses) are thought to have had a selective advantage: Because they had a greater surface area of contact with fungal partners than their root-less ancestors, these plants could access more nutrients in the ground. Slowly, the benefits of this interaction led to present-day mycorrhizae; up to about 90 percent of today’s vascular plants have associations with fungi in their rhizosphere. A well-accepted theory proposes that fungi were instrumental in the evolution of the root system in plants and contributed to the success of Angiosperms (flowering plants).

For an overview of what you’ve just read, and some new cool tidbits, check out this cool video:

Note that the video is a bit outdated (e.g., uses terminology like “Kingdom”), but is otherwise quite on point!