In this lesson, we will examine the group of organisms known as the fungi. This group includes the familiar organisms mushrooms, yeasts and molds. We will look at several major characteristics of fungi, as well as a few disease-causing fungi.
Age of the Fungi
Ancient Earth had its fair share of dominant organisms. For most of the history of life, Earth has been dominated by the microscopic bacteria and archaea, floating, swimming and thriving in the ancient oceans that covered much of the globe. Through time, organisms got larger and more complex. There was a time when organisms resembling insects were dominant, then fish and reptiles. We've all heard of the dinosaurs ruling Earth, followed by mammals evolving. Now, humans are clearly the dominant organism. But, there was a time in history you have probably never heard of: the age of the fungus!
Around 250 million years ago, the earth was populated by ancient plants and animals. The Permian extinction changed all that. Over 90% of life on Earth vanished. The animals were decimated and so were the plants. This included many millions to billions of trees, leaving massive fields of dead wood. And, what thrives on dead wood and dead animal tissue? That's right, fungi. With few animals to tromp on them or eat them, few plants to crowd them and a massive dead wood buffet, the fungi after the Permian extinction were able to dominate the landscape. The little known, and even less heralded, golden age of the fungus had begun!
Fungi, singular, fungus, is a group of eukaryotic, non-phototrophic organisms with rigid cell walls, that includes mushrooms, molds and yeasts. This definition has some words in it that probably need definitions of their own. Eukaryotic simply means that fungal cells have a nucleus, like plant and animal cells, which distinguishes them from the Bacteria and Archaea. Non-phototrophic means that they can't use light for energy because they lack chlorophyll, distinguishing them from plants. The cell walls of fungi are unique in that they contain large amounts of chitin, a structural component only found in the cell walls of fungi. The chitin makes the cells walls rigid.
So, if fungi can't perform photosynthesis, and I've never seen a mushroom hunting game on the savannah, what do they do for nutrients? Many fungi are saprophytes. A saprophyte is an organism that acquires nutrients from dead organic matter. This can be as common as seeing mushrooms growing on dead wood or mold surviving in your refrigerator on last week's takeout food. These fungi are actually very important to the health of the ecosystem, rapidly breaking down plant and animal material and returning it to a more usable form. Other fungi are parasites, obtaining nutrients from a host species. Many common plant diseases are a result of a fungus siphoning nutrition from the plant. There are even some human diseases caused by fungi, but we'll talk more in-depth about those later.
Many fungi are common contaminants of foods. Most fungi reproduce by releasing spores, which are the resistant, reproductive, resting stage of a fungus. These spores are able to survive the high temperatures and pH extremes often used for food preservation. Spores are also not killed by freezing and some can even germinate, resulting in actively growing fungi in your freezer! There are three major types of fungus: mushrooms, molds and yeasts. Let's take a quick look at each.
To the average person, mushrooms are probably the best known fungi. Usually found springing up on dead wood after cool, wet weather, the mushroom is a common sight. Mushrooms generally consist of a stalk with a large cap on top. This cap produces the spores that the mushroom releases in order to reproduce and colonize new environments. Many species of mushrooms are edible and delicious, but others can be very toxic. Since knowing the difference can take a very experienced eye, eating wild mushrooms is usually not a fantastic idea for the average person.
Like mushrooms, molds are very recognizable. Forget about that brick of cheddar cheese for too long and suddenly it's covered with green fuzz. That fuzz is why molds are often called filamentous fungi, and the filaments give it that fuzzy or fluffy appearance. Hyphae is a technical term for the long, branching mold filaments. Specialized hyphae, called conidia, are often pigmented and reach up above the surface of the food source. These conidia are responsible for producing and releasing the mold spores for reproduction and dispersal. The common and ubiquitous bread mold is in the genus Rhizopus.
To a microbiologist, yeasts are the most important fungi, so I'll spend a little more time here. Yeast is the common term for any unicellular fungus. The mushrooms and molds are all multicellular organisms, but yeasts usually exist as only a single cell. The cell is larger than a bacterial cell and contains a nucleus and other organelles. Yeasts reproduce by budding. The new offspring cell begins as a small outgrowth on the membrane of the parent cell, which gets larger before eventually pinching off. This process is asexual, resulting in two clones. Some yeast can reproduce sexually by simply fusing and creating a genetically recombined zygote cell. The zygote contains DNA from both parents.
Yeasts are unicellular, yet eukaryotic, making them an important group of organisms for scientists. Humans are also eukaryotic, so performing experiments on yeast, like investigating cellular processes or testing new drugs, can provide data that can be applied to humans more accurately than work done on the more distantly related bacteria.
Saccharomyces cerevisiae is the most common yeast used in research as a model organism for higher eukaryotes. In 1997, it gained the distinction of being the first eukaryote to have its genome sequenced. Saccharomyces cerevisiae might sound familiar to you for another reason. Bakers and brewers have been using this yeast for many years to make bread and alcohol. S. cerevisiae thrives by fermenting sugars, releasing carbon dioxide or ethanol in the process. The bakers and brewers began experimenting on Saccharomyces in an attempt to optimize growth and production, which expanded the knowledge base and may have led the way to its use as a model organism in research.
We briefly mentioned that some fungi can cause disease in humans. Mycosis is the term given to any illness caused by the growth of a fungus either on or inside the body. There are several important human diseases caused by fungi, but they usually infect the body surfaces or respiratory tract and are rarely serious for healthy individuals. It's a good thing, too, because fungal infections can be difficult to eliminate. As a fellow eukaryote, most of the medications that will kill a fungus are also toxic to human cells, often producing nasty side effects.
Ringworm is a common fungal disease passed between infected individuals or picked up from contaminated surfaces. Trichophyton and Microsporum are the two most common genera of fungi that cause the characteristic ring-shaped scaly lesions. Candida albicans is another pathogen that causes several diseases in humans, depending on where it is located. Candida in the vaginal canal can cause yeast infections, characterized by itchiness and discharge. Candida in the mouth is called thrush, characterized by white spots that grow together forming a mass of crumbly white milk-like curds.
Let's review. The fungi are a group of eukaryotic, non-phototrophic organisms with rigid cell walls. This includes mushrooms, molds and yeasts. Many fungi are saprophytes, getting nutrients by absorbing them from dead, decaying plant material. Still, other fungi are parasitic, stealing resources from a host organism. Many fungi reproduce by releasing spores. These resistant, reproductive structures are often food contaminants due to their ability to survive most common food preservation methods.
Mushrooms are multicellular fungi with stalks and caps that are most often found growing on decaying plant material. Molds are multicellular, filamentous fungi that are commonly found on foods. Yeasts are any unicellular fungus. Being single celled and eukaryotic, yeasts make excellent research organisms for studying processes that can be applied to humans as well. Saccharomyces cerevisiae is a research model organism that is also used to make bread rise and beverages alcoholic.
A mycosis is a disease caused by the growth of a fungus on or inside a human body. Most mycotic diseases infect the body surfaces or respiratory tract and are not considered serious diseases. Treating fungal infections can be difficult as most drugs that target fungi will also harm human cells.
Following this lesson, you will have the ability to:
- Describe the general characteristics of fungi
- Summarize the structure and function of mushrooms, molds and yeasts
- Explain why yeasts are particularly important to scientists
- Define mycosis and explain why fungal diseases in humans are difficult to treat