Lecture 03: What is a Fungus

Introduction

When I talk to people about fungi (sing.= fungus, from the Latin for "mushroom"), I hear many misconception as to what a fungus is. Many lay people think of fungi as being the same as bacteria or viruses, or that they are a subtype of these organisms. Both concepts are incorrect. The fungi are a group of organisms, with specific characteristics that make them unique from other organisms. An old video released by the BBC, in 1980, "The Rotten World About Us", that I hope you have had the opportunity to view, briefly summarizes the different types of fungi that are recognized and also shows us the both the misery and the value of these organisms. While the video is dated, it summarizes almost every aspect of fungi that will be covered this semester. Also, the time lapse photography, shows rarely seen footage of fungal spore germination, mycelial growth and mushroom development that are outstanding. Truly, a video worth seeing if you have an interest in fungi and one that is appropriate for this course.

From the introduction to fungi during our first lecture, as well as from the video, you now have some idea as to what a fungus is and that fungi are just about everywhere. They cause famines and economic losses due to plant diseases. Human diseases caused by fungi, while less common, are now of greater significance with larger number of people becoming afflicted with such diseases because their immune system have been severely damaged or at least compromised due to AIDS, immune suppressor compounds for organ transplants to prevent rejection of organs, or chemo and radiation therapy for various forms of cancers that usually damage immune systems. They are also the cause of rot and degradation of various commercial products. However, fungi have also been an asset as well. Many antibiotics, beverages and food products are of fungal origin and the same fungi that cause rot and degradation of commercial products are the same ones that recycle complex organic material to simple molecules that then serve as the "food" of plants. Thus, fungi have been both an asset and a detriment to society. The images below illustrate some of the benefits and harm that fungi have bestowed upon people: 

What is a Fungus?

However, what is a fungus? What combination of characteristics do we attribute to these organisms that make them unique from other organisms? Because of the diversity and variations that we see in fungi, they have been a difficult group of organisms to define. Our concepts of this unique group of organisms have changed drastically since I first took mycology, the branch of botany that studies fungi, back in the days, when I was an undergraduate, in 1969. At that time, fungi were classified in the Plant Kingdom and they were believed to be a type of plant that lost its chlorophyll and no longer able to make their own food. However, much has changed. Fungi are no longer classified as plants and now have their own kingdom: Fungi. The Kingdom Fungi, as presently defined, is much more restrictive than how fungi were defined back in 1969 and some organisms that were once classified as fungi have now been excluded and others that were previously excluded are now included. The following combination of six characteristics, is now used to define organisms classified in the Kingdom Fungi:

(1) Fungi reproduce by Spores

Spores are usually defined as unicellular, reproductive structures, but in fungi they can be multicellular as can be seen in the illustration, of various fungal spores in Fig 5.

When a fungus spore germinates, it will give rise to the fungus body. There are two types of fungus bodies.

(2) The Fungus Body

The most commonly recognized type of a fungus body is Hypha (pl.=hyphae). Hyphae are the filamentous growth of the fungus body. The germination of a fungus spore will initially give rise a filamentous germ tube, which will then begin to branch and continue to grow at its apices. Germination of a fungus spore can be viewed in the YouTube video below:

It is likely that most people have seen the resultant growth of mycelium. It is not uncommon for left over food to be long forgotten because they have been placed way at the back of your refrigerator. When discovered, we are shocked to see the filamentous growth on the forgotten food (Fig. 5-6).

When there is a lot of hyphal growth such as in the above figures, it is referred to as mycelium (pl.=mycelia). If we look under a microscope, we will also find that the mycelium growth is of two types. The Rhizopus mycelium of Fig. 5 will be seen to form a branched network of continuous filaments that are not divided into cells. This type of mycelium is said to be coenocytic (Fig. 7). The mycelium of the Penicillium, growing on the grapes in Fig. 6, however, will appear to be composed of rectangular cells, attached end to end. This mycelium is said to be septate (Fig. 8). The partitions that are perpendicular to the length of the filamentous growth are called septa (sing.= septum).

The second type of fungus body that is not seen as commonly is the yeast. Under the microscope, you can observe that this body type is composed of single cells that continually divides, by budding (Fig. 9) or fission (Fig. 10) to form lots and lots of cells. To the naked eye, this growth will appear to be a thick, waxy to syrupy growth (Fig. 11). Fission is the process where the yeast cell divides into two cells that are approximately equal in size, following the division of the nucleus. The process of budding is more complicated and is illustrated in the illustration below (right). A yeast cell that is about to bud will have a predetermined area of the cell that becomes blown out forming a new cell (the bud). The nucleus will divide, with one nucleus migrating into the new cell. When the new cell is approximately the size of the original cell, the cells will seal off the opening and separate, giving rise to two yeast cells: 

(3) Nutritional category: Fungi are Heterotrophs and Absorb their food

This means that they cannot produce their own food and derive their nutrition from complex organic compounds. That Fungi cannot produce their own food is emphasized here because they were formerly classified in the Plant Kingdom where most organisms were able to make their own food. Another reason for once classifying fungi in the plant kingdom is that they cannot move like animals. This places fungi at a significant disadvantage in obtaining food. However, fungi are able to cope with this disadvantage because the substrate that they grow on is their food". Thus, fungi literally grow in their "food". But, how do fungi "eat"? The process by which fungi eat is called absorption. This is the same mode of nutrition that is utilized by bacteria. Absorption occurs when the food that the fungus is growing in is transported through the cell wall of the fungus. If the food that they are growing in is a simple compound, such as simple sugars or amino acids, the food can be transported directly into their cells, through their cell walls, where they are metabolized. However, more often than not, the food is composed of complex chemical compounds, e.g. wood, that are too big to be transported through the "openings" of the cell walls of the fungus. These types of food must be broken down into smaller, simpler compounds that can be transported through their cell walls. The fungi do this by secreting digestive juices or enzymes that will break down the complex compounds into small compounds, which can then be "absorbed" through their cell walls. A summary of the absorption process is shown in Fig. 12.

A time lapse video, below, from YouTube demonstrates how fungi absorbs food and decomposes organic material.

Although this process may seem very different than our own means of obtaining food. It is not that different. The essential difference between fungi and animal digestive systems is that fungi digest their food first and then "eat" it, while animals eat their food before digesting it. The basic process of digestion is otherwise more or less the same. Our digestive system requires that our food is chewed by our teeth to reduce the surface area of the food, go through the esophagus, stomach, intestine and many associate organs. So there are a lot of things that can go wrong when we eat our food. The fungal digestive system is much more simplified and one which has been very successful for them.

Heterotrophs can further be divided categories:

• 1. Saprotroph: Heterotrophs that derive their food from non-living carbon sources (Saprobes and saprophytes are older, equivalent terms). This category of heterotrophs is of significance because they serve as nature's recyclers. They consume dead organic material and break down this material into simple inorganic compounds which can then be utilized by plants to produce more food. However, these are the same fungi that cause "rot" and "decay", in our food and products of material importance to people, such as furniture, art work, books, etc. However, the down side of fungal decomposition is that fungi do not distinguish between fallen branches and logs, from commercial wood and food products (Figs 10-11). Thus, to a wood rotting fungus, wood is food, regardless of its source.

  2. Parasite: Heterotroph that derives its food from the living cells of another organism, usually referred to as the host (Figs. 15 and 16a). Many fungi fit into this category, but not all, and not even most. This is just one of the many misconceptions/stereotypes that we have of fungi.

  3. Mutualistic Symbiont: Heterotroph that derives its food from another living organism, but the relationship is mutually beneficial to both organisms involved, e.g., lichens = fungus and alga. As you'll see later in the semester, mycorrhiza, the mutualistic relationship between a fungus and the roots of plants are very important, i.e. if they did not exist, the world as we know it also would not exist.

(4)Reproduction and the Different Types of Fungi

A spores is the structure that reproduces a fungus. The appearance of spores and the way that they are produced is the means by which we recognize the different types of fungi. The simplest means by which spores are borne is directly on modified mycelial structures. In Curvularia (Fig. 19), the mycelium gives rise to spores that are borne on modified hyphae:

Spores may also form on complex, specialized structures generally referred to as fruiting bodies. A mushroom is an example such a structure, but where do fruiting bodies or mushrooms come from? Recall from "The Rotten World About Us" video that there was a lawn area where there was a "fairy ring" of mushrooms. Where the mushrooms came from was explained by the narrator as mycelium that originated from mushroom spores that germinated in the center of the fairy ring, in the soil beneath the grass, and with time grew outwards, to form a circular growth of mycelium. It is this mycelium that will give rise to the mushrooms when conditions are right by, becoming tightly interwoven, much like someone knitting a sweater, that will form the mushroom where the spores will be borne. An analogy that can be used here is to compare mushrooms to a fruit, such as an apple. Apples are the fruits of the apple tree and are borne on the aerial branches. The roots in the soil are also important for their uptake of water and minerals and the leaves for utilizing the food and minerals to produce the energy needed to produce the fruits. In a similar matter the mushroom is the fruit of the fungus and the mycelium derives its food from the soil and when conditions are right will give rise to mushrooms. An example of a mushroom that produces fairy rings is Chlorophyllum molybdites (Figs. 20-21). The mycelium that gives rise to the mushroom is beneath the soil and is not visible in the pictures. 

Mycelium growing beneath the soil can become quite extensive. On August 1, 2000, a report by the BBC that the mycelium of Armillaria solidipes was discovered in the Blue Mountains/ Malheur National Forest in Eastern Oregon that covered 900 hectares (2,200 acres) and is estimated to be 2400 years old. It is known to be the largest and oldest organism in the world. Similar, but smaller records have been found. Smith, Bruhn & Anderson (1992) discovered that the mycelium of Armillaria bulbosa (now A. gallactica) from an area near Crystal Falls, Michigan, near the Wisconsin border that was occupying at least 15 hectares (=37 acres) of soil, estimated as weighing approximately 10,000 Kg (»22,000lbs or about 100 tons) and to be about 1500 years old. That same year another larger fungus was discovered south of Mt. Adams in southwestern Washington. This fungus was also A. solidipes and it covered 600 hectares (»1500 acres). These record size fungi were all dubbed "Humongous Fungus" as each discovery was larger than the previous.. 

We can divide spores into two broad categories based on whether they are sexually or asexually produced:

Sexual spores when produced by an individual fungus are genetically variable and will give rise to genetically diverse individuals. Using a human analogy can probably help you understand better than using a fungus example. You are all aware that this type of reproduction must involve two parents, and that the children from two parents will inherit characteristics from each parent. For example, all of you have features that can be recognized as being maternally or paternally inherited. This will also be true for any siblings that you may have. However, you and your siblings are genetically unique in appearance and personality because the process of sexual reproduction is such that no two individuals will be exactly alike unless they are identical twins. This is advantageous for survival of a species because if environmentally unfavorable conditions should come about, genetic variations will ensure that some individuals will survive.

Asexual spores requires only a single parents and the "children" produced would be genetically identical to the parent. Genetically, identical individuals are said to be clones. Asexual reproduction is currently more easily understood due to the extensive coverage of cloning in the news media of Dolly, the cloned sheep and the mice that have been cloned right here on the University of Hawai‘i, Manoa campus. Although some animals, naturally, have this type of reproduction, there are far more examples of asexual reproduction in fungi and plants. Asexual reproduction occurs when a part of an individual regenerates itself into another individual. Since this new individual was originally part of the parent, the two are genetically identical.  Many agricultural plants are reproduced asexually because if you have a plant with all the qualities that you want, growing clones of this individual will ensure that everything you are growing will also have these qualities. Examples of such plants are illustrated in Figs 22-24.

The "eyes" of the potato can be used to produce a new potato plant, the leafy tops of the pineapple and carrot can be grown to produce more plants. While asexual spores/reproduction will quickly give you the quality plant, animal that you desire, the individuals are genetically identical. If unfavorable environment conditions or disease should come about, all individuals will die from these unfavorable conditions. We will go examples where this has been detrimental to the farmers.

(5) Other Characteristics

Cell Wall

The mycelium or yeast cell of fungi is surrounded by a rigid cell wall that is typically composed of chitin, the same material that makes up an insect's exoskeleton. However, one group of fungi that we will be studying has cell wall composed of cellulose, which is  is the same material that is found in plant cells. The presence of a cell wall was once used as evidence for fungi being closely related to plants, but presently the two groups are not thought to be closely related.

Eukaryotes

Fungi and bacteria share the characteristic that they are both decomposers/recyclers. However, fungi are eukaryotes. The term literally means "true nucleus", referring to the organelle in the cell that contains DNA, the genetic material that is organized into chromosomes. This is in contrast to bacteria that are prokaryotes and do not have nuclei. 

Summary of Characteristics of Fungi

Organisms that classified as fungi are eukaryotes that have mycelium or yeast bodies with cell walls that are composed of chitin and derive nourishment through the process of absorption. They reproduce by sexual or asexual spores that are either derived from pre-existing yeast cells or directly from mycelium. Mycelium may also become interwoven to give rise to fruiting bodies in or on which spores are borne.

We have just defined Fungi in its modern concept. In this concept of Fungi, we now have excluded two groups of organisms that were formerly classified as Fungi. However, we still study these organisms in mycology, even though we recognize that they are now related to the Fungi. When we refer to organisms in these two groups, we refer to them as "fungi", note lower case "f". We will briefly describe these two groups below.

Groups of Organism No Longer Classified as fungi

Plasmodial Slime Molds

This group of organisms is quite interesting because of its unusual life cycle. Members are spore heterotrophic  producers, which is why they were once classified as fungi. However, mycelium is not produced nor is there cell walls around their cells. Also, the when the germination of their spores occur, an amoeba is borne (Fig. 40-41), which feeds via phagocytosis just as a regular amoeba does. They reproduce asexually by fission and when a critical number has been reached, the amoeba stage also serves as gametes and fuse to form a zygote that gives rise to the plasmodium (Fig. 42) stage that will continue to feed like a giant amoeba. The plasmodium eventually gives rise to numerous sporangia (Fig. 43) that contain spores. Because of the animal-like amoeba and plasmodium stage, slime molds have been referred to as the fungus that walks. 

Phylum: Oomycota

This phylum is commonly referred to as the water molds. It is very fungal in appearance with most species producing extensive, coenocytic mycelium. Cells of this phylum have cell walls, but are composed of cellulose, not chitin, as in the Fungi. Swimming zoospores also occur. The phylum has many members that are of economic importance as plant pathogens. During sexual reproduction, large spherical structures called oogonia (sing. = oogonium) are produced that contain eggs (Fig. 44) . this is how this phylum gets its name. The phylum name literally means egg fungus. Asexual zoospores are borne in zoosporangia (Fig. 45-47).

Important Terms and Concepts

• 1. Mycology: The part of biology that studies fungi. 

  2. Spore: Usually a single celled reproductive structure. However, in fungi spores can be made up of more than one cell.

  3. Chitin: The material that makes up the cell wall of fungi.

  4. Hyphae: Branched filamentous growth that makes up the fungus body.

  5. Mycelium: All of the hyphal growth of the fungus body, i.e. lots and lots of hyphae.

  6. Fruitbody: A reproductive structure composed of interwoven mycelium that functions in producing spores, e.g. mushroom is an example of a fruitbody.

  7. Yeast: Unicellular fungus body that produces asexually by budding or fission.

  8. Heterotroph: Organisms that derive food from consuming the cells or bodies of live or dead organisms.

  9. Absorption: The process by which fungi obtain nourishment.

  10. Saprotroph: Organism that derives nourishment from non-living carbon sources, i.e. dead organisms waste products of organisms.

  11. Parasite: Organism deriving nutrition from the living protoplasm of host cell.

  12. Mutualistic symbiosis: Two organisms of different species that interact with each other, such that both species will benefit from the relationship.

  13. Eukaryotes: Organisms that have their cellular internal structures surrounded by a membrane, i.e. DNA surrounded by a membrane is the nucleus in eukaryotes. Thus, often we simply define eukaryotes as those organisms that have a nucleus.

  14. Prokaryotes: Organisms that do not have their DNA surrounded by a membrane. Thus, they are often defined as organisms that do not have a nucleus.

  15. Plasmodial Slime Molds: Group of organisms no longer classified as fungi because of absence of cell walls and hyphae, and food obtained by phagocytosis.

  16. Water Molds: Group of organisms no longer classified as fungi because of absence of chitin in their cell wall. Cell wall is composed of cellulose in this group of organisms. However, it does have other fungal characteristics, such as mycelium and obtaining nourishment by absorption.

Questions to Think About

1. What is the study of mycology?

2. What are the combination of characteristics that define what a fungus is? 

3. Fungi are eukaryotes. What does this mean?

4. What is a spore, and in fungi, what does a spore give rise to?

5. Reproduction may be sexual or asexual. Define the two terms. How does sexual and asexual reproduction differ?

6. What is the branched filamentous structure of the fungus body called? What reproductive structures do they give rise to?

7. Not all fungi produce filamentous growth. Some fungi, instead are unicellular. What are these fungi called and how do they reproduce?

8. Fungi, like most organisms cannot produce their own food, like plants. What are these types of organisms called? What is the mode of nutrition called by which fungi obtain their food?

9. How does this process differ from how animals obtain food?

10. What is a heterotroph? What are the three categories of heterotrophs?

11. Fungi have cell walls that is composed of a unique compound. What is the composition of their cell walls?

12. Based on the the characteristics that we have just defined for fungi, what two group of organisms are now no longer classified as fungi?

13. What group of prokaryotic organisms are now no longer classified as fungi, and that we no longer study in mycology?

Literature cited

Dodge, Sherri Richardson.2001. An Even More Humongous Fungus, http://www.fs.fed.us/pnw/news/fungus.htm

Smith, M.L. J.N. Bruhn and J. B. Anderson 1992. The fungus Armillaria bulbosa is among the largest and oldest living organisms. Nature 356: 428-431.