" KINGDOM FUNGI: DECOMPOSERS and PARASITES "
TEACHER'S GUIDES
I. INTRODUCTION
" Kingdom Fungi: Decomposers and Parasites " is part of the exciting six part Basics of
Biodiversity series. The Basics of Biodiversity series in turn is part of the 30 part Basics of
Biology series which includes the following six part series; Basics of Ecology, Basics of Cell
Biology, Basics of Genetics, and Basics of Anatomy. The programs in the Basics of Biology
series are designed to cover most the topics brought up in high school biology courses.
" Kingdom Fungi: Decomposers and Parasites " begins by describing hyphae- the
filamentous, thread-like fungal cells that intertwine to form interwoven masses called mycelium
which are usually only visible when differentiated into sexual structures such as mushrooms,
puff-balls, and powdery molds. How fungi obtain nutrients either through parasitic or mutualistic
relationships or decomposing dead bodies is examined before looking at the economic,
ecological, and health impacts of various fungi. The major divisions of fungi are then examined
and some of the unique adaptations found in kingdom Fungi discussed.
" Kingdom Fungi: Decomposers and Parasites "- like all thirty programs in the Basics of
Biology series- contains a student study guide, a multiple-choice test and a crossword puzzle that
are stored as PDF files on the DVD. For information on other programs in the series or if you
have questions or comments call 1-800-325-1956 or e-mail us at info@greatpacificmedia.com.
II. STUDENT OBJECTIVES
Upon watching " Kingdom Fungi: Decomposers and Parasites "students will be able to:
*Explain why fungi are a critical part of the living world.
*Describe and identify fungal structures such as hyphae, mycelium, septa, and spores.
*Explain the various ways fungi obtain nutrients.
*Describe how the methods of sexual reproduction differ between different phyla of fungi.
*Describe the mycorrhizal associations that exist between many fungi and plants.
*Explain the relationship that exists between the fungi and algae that form lichen.
*Describe how certain fungi are actually active predators.
*Describe the differences between different phyla of fungi beyond their reproductive structures.
*Describe the unique ways Pilobolus and truffles disperse their spores.
*Give examples of how different fungi impact human health and the world economy.
III. VOCABULARY
Hyphae
Heterotroph
Chytrids
Mushroom
Pilobolus
Diploid
Mycelium
Chestnut Blight
Zygote Fungi
Penicillin
Decomposer
Saprobe
Septa
Lichen
Sac Fungi
Arthrobotry
Parasite
Chitin
Mycorrhizae
Club Fungi
Aflatoxins
Spores
Symbiotic
Pileus
Fairy Ring
Truffle
Haploid
IV. DISCUSSION QUESTIONS
1. Which do you think evolved first fungi, plants or animals? Why?
2. How many different types of fungus have you seen in your life?
3. Bread kept around long enough almost always has fungus grow on it. Why? How?
4. Why don't fungi have flower like structures to encourage insects to distribute their spores?
5. What do you think are the origins of the hallucinogens associated with certain mushrooms?
6. Why do you think it is that it is much harder to grow and raise truffles than oak trees?
7. Make a list of the fungi that would invade your body if your immune system failed.
8. In what biome would you expect to find the most species of fungi? Why?
9. Why do you think that the cells in the bodies of most fungi are haploid rather than diploid?
10. Would you be surprised to find out the worlds largest organism is a fungus? Why or why
not?
11.Describe sequence of events that led to the evolution of the truffle.
12. Why do think plants haven't evolved structures that take over the role of mycorrhizal fungi?
V. NARRATION FOR "KINGDOM FUNGI: DECOMPOSERS and PARASITES"
When most of us think of fungi, mushrooms and toadstools are what come to mind. But
mushrooms and toadstools are simply reproductive structures that represent just a small
percentage of the biomass of a typical fungus. The rest of the biomass of most fungi is largely
invisible because it is made up of thread-like strands which, wind and wander through dead logs,
dead animals and often deep into the soil.
While the occasional shelf fungus or mushroom seen on a walk through a forest would
appear, when compared to the trees and animal life, to represent a living kingdom of little
consequence to the biosphere we would be wrong. Fungi release over 8 billion tons of carbon
dioxide into the atmosphere each year and a symbiotic relationship with fungi is probably what
enabled the ancestors of modern plants to first establish themselves on the land. These symbiotic
relationships continue even today as over 85% of plants have a symbiotic relationship with at
least one species of fungi.
Further, fungi are not only essential to the growth of plants but also to recycling them
after they die. This is because a number of fungi have the relatively rare ability to breakdown the
cellulose and lignin found in plants to obtain energy and in the process they release at least some
of the nutrients contained in the plants back to the soil to support a new cycle of life.
However, other species of fungi are a constant threat to attack and kill other living
organisms including humans with compromised immune systems. Fortunately, we humans have
also found ways to make use of the biochemical activities of certain species of fungi to make a
variety of products including wine, bread, cheese and life saving antibiotics like penicillin.
Let's now take a closer look at the members of kingdom Fungi and the critical role they
play in the living world.
Fungal Structure- Hyphae Intertwine to Form Mycelium
Fungal bodies, with rare exception, consist of microscopically thin, threadlike filaments
called hyphae that grow into interwoven masses called mycelium. The hyphae of some fungi
consist of single elongated cells with numerous nuclei, while in others the hyphae are divided by
partitions called septa into many cells, each containing from one to many nuclei, depending on
the species. Pores in the septa allow cytoplasm to stream between cells and distribute nutrients.
In contrast to the nuclei found in animal cells, those found in fungal bodies are usually
haploid-that is they contain only a single set of chromosomes. Like plant cells, fungal cells are
surrounded by cell walls. However, the cell walls of fungus are largely made up of chitin, the
same substance found in the exoskeletons of arthropods, rather than cellulose like plant cells.
Fungal mycelium slowly work their way into aging bread or cheese, beneath the bark of
decaying logs, or into the soil. Periodically, hyphae aggregate and differentiate into reproductive
structures that usually project above the surface in which they live. These structures, including
mushrooms, puffballs, or the powdery molds on food, are often the only part of the fungus that is
easily seen. However, in a few species like truffles the sexual structures remain underground to
be dug up by animals, which then spread the spores produced by these sexual structures.
Fungal Nutrition- Fungi are Heterotrophic like Animals
Like animals, fungi are heterotrophic, living on nutrients stored in the bodies or wastes of
other organisms. Some fungi are saprobes; agents of decay that digest the bodies of dead
organisms. Other fungi are parasites that feed on living organisms. A third group of fungi such as
those found in lichens and mycorrhizae live in symbiotic relationships with other organisms.
The absorption of nutrients by fungi resembles that of bacteria; both secrete enzymes
outside their bodies that break down the complex macromolecules of their nutrient source into
smaller subunits that can be absorbed through pores in their cell walls. The fungal body,
composed of long filaments only one cell thick, provides an enormous surface area for nutrient
absorption. Rapidly growing filaments penetrate deeply into a source of nutrients, hastening
decomposition by digesting the material. Most small nutrient molecules diffuse through the
fungal cell walls and into the cells, but often some of the nutrients remain outside the fungus,
providing nourishment for other organisms.
Fungal Reproductive Structures -Mushrooms, Puffballs, Shelf Fungi, Etc.
Fungal reproduction is varied and often complex, involving both sexual and asexual
processes. During simple asexual reproduction, a mycelium breaks into pieces, each of which
grows into a new individual. Many fungi reproduce both asexually and sexually via different
types of spores formed on or within special structures that project above the mycelium, allowing
dispersal of the spores by wind or water. Haploid asexual spores are produced by mitotic
divisions of haploid fungal cells. In favorable habitats, the asexually formed spores begin mitotic
divisions that produce a new haploid mycelium. Sexual spore formation begins with the fusion of
two haploid nuclei of compatible mating types, resulting in a diploid zygote. This zygote then
undergoes meiosis to form haploid sexual spores. These are dispersed, germinate, and divide via
mitosis to form a new haploid mycelium. The reproductive capacity of some fungi is prodigious:
a single giant puffball may contain 5 trillion sexual spores.
Ecological and Economic Importance of Fungi
The interesting array of mushrooms seen on forest floors hardly begins to indicate the
vast number of microscopic fungal filaments to be found penetrating the moist rich soil,
decaying vegetation and the wastes and dead bodies of animals. As decomposers, fungi make an
incalculable contribution to ecosystems. Their extra-cellular digestive activities liberate nutrients
that can be used by trees and other plants and the bodies of fungi provide food for small insects
and worms. If fungi and bacteria were to disappear suddenly, nutrients would remain locked in
the bodies of dead plants and animals, causing many nutrient cycles to grind to a halt, soil
fertility to decline rapidly, and ultimately to the collapse of the Earth’s ecosystems.
Parasitic fungi, in contrast, are responsible for the majority of plant diseases. American
chestnut and elm trees have been decimated by the fungi causing Chestnut blight and Dutch Elm
disease. Fungal parasites such as those causing corn smut also cause billions of dollars in crop
losses annually. It is estimated of that each year half the crops in storage in the third world are
destroyed by fungi and that toxins produced by the growth of Aspergillus flavus on groundnuts
and cereals kill thousands.
On the other hand several species of fungus are grown commercially for food and it has
been discovered that a number of fungal parasites can be used to control insect populations such
as citrus mites. Of course the importance of yeast fungus in the production of food staples such
as bread and cheese and beverages like wine and beer can hardly be overstated. Yeast is also, as
we will see later, a favorite in genetics labs.
However, different species of yeast and a number of other fungi are responsible for a
variety of human illnesses from the merely annoying like ring worm and athlete's foot to the
deadly like crytocococcosis and pneumocystic pneumonia.
Fungal Symbiotic Relationships-Lichens
Lichens are a unique symbiotic (literally "living together") association between fungi,
usually ascomycetes and unicellular green algae or cyanobacteria. Together, these organisms
form a living unit so tough and undemanding of nutrients that it is among the first to colonize
newly formed volcanic islands and be found on rocky surfaces uninhabitable by other organisms.
The algal partner provides food formed by photosynthesis, while the fungus provides support and
protection from dehydration. Lichens in a variety of bright colors can be found growing on bare
rock, and have invaded habitats ranging from deserts to the Arctic tundra. Based on their size and
slow rate of growth, some lichens in the Arctic are believed to be 4000 years old. About 20,000
lichens have been identified, each with a different combination of fungus and algae or blue-green
bacteria, and with a unique color and growth pattern.
Not only have fungi formed symbiotic relationships with diminutive photosynthetic
organisms like algae and cyanobacteria they have formed them with the largest photosynthetic
organisms as well.
Fungal Symbiotic Relationships-Mycorrhizae
Mycorrhizae are symbiotic associations between fungi and plant roots. Over 5000 species
of mycorrhizal fungi are found growing in intimate association with the roots of about 85% of all
vascular plants, including most trees in the great temperate and tropical forests of the world.
These associations benefit both partners. The hyphae of mycorrhizae surround the root and
frequently invade the root cells. The fungus digests and absorbs organic nutrients from the soil,
passing some of these nutrients directly into the plant root cells. The fungus also absorbs water
and passes it to the plant, an advantage in dry, sandy soils. Plants that participate in this unique
relationship tend to grow larger and more vigorously than those deprived of the fungus,
especially in poor soils. In return, the fungi receive food such as glucose and other organic
molecules produced by the plants.
The Classification of Fungi-An Overview
Although nearly 100,000 species of modern fungi have been described, biologists have
only begun to comprehend fungal diversity as at least 1,000 new species are described each year.
Some scientists estimate there might be as many as 1.5 million fungi, largely in tropical
rainforests, waiting to be described. Fungal taxonomy, like that of microorganisms, has been the
subject of considerable controversy in the past. But advances in molecular research is providing
a clearer picture of which organisms are and aren't fungi.
For example, slime molds, and many water molds that were once considered fungi
largely on the basis of appearance, are now after a review of their molecular structure classified
as protists with a very different evolutionary history from fungi.
But one tradition in the classification fungi from the pre-molecular era remains- fungi
continue to be classified on the basis of their sex structures. The four major fungi phyla are
Chytridiomycota, Zygomycota, Ascomycota, and Basidiomycota. The 25,000 or so species of
fungi in which the sex structures have not been identified, or have been lost through evolution
are thrown into a group referred to as Fungi Imperfecti.
The Classification of Fungi-Phylum Chytridiomycota
Members of phylum Chytridiomycota or the Chytrids are believed to be the modern fungi
most closely related to the first fungi. Chytrids are largely aquatic indicating that fungi like
plants and animals evolved in the water. Chytrids differ from the water molds of kingdom
Protista in that their cell walls contain chitin like all other fungus rather than the cellulose found
in cell walls of protist water molds.
Some chytrids are unicellular while others form branching chains of cells. Chytrids,
which reproduce both sexually and asexually are the only fungi to produce motile, flagellated
gametes. Haploid male gametes search out haploid females gametes via pheromones released by
the females. Once these two flagellated gametes fuse they produce a diploid zygote that under
goes numerous cell divisions to form a small diploid organism. This new organism produces
flagellated diploid zoospores that scatter about and create more new diploid chytrids. These
diploid chytrids give rise to sporangia that produce flagellated haploid gametes that start a new
reproductive cycle.
Though most chytrids live in fresh water, some live in estuaries or moist soil. A few
anaerobic species of chytrids live in the guts of chud chewing herbivores. While the chytrids
living in the guts of herbivores have a mutualistic relationship with their hosts, most chytrids
living in association with other living organisms act as parasites. Common victims of chytrid
parasitism are nematodes, aquatic insect larvae, plants and other fungi including other chytrids
and fungus-like oomycetes. The remaining species of chytrids are saprobes, which break down
dead organic matter to obtain energy and nutrients.
The Classification of Fungi-Phylum Zygomycota: The Zygote Fungi
The approximately 1,000 known species of zygomycetes or zygote fungi are
named for the ability of their haploid hyphae to "mate" and produce zygotes, which develop into
zygospores. The hyphae of most zygomycetes have no cross walls or septa, allowing cytoplasm
to stream easily throughout the thread-like hyphae. When hyphae of opposite mating types grow
side by side they each send out branches that are attracted to one another by pheromones released
by each. When they finally touch, the ends of each swell up and coalesce together to form a
zygote. One or more haploid nuclei from each of the hyphae flow into the zygote and fuse in
pairs, producing diploid nuclei. Afterward the zygote develops into a thick walled zygospore.
Eventually when conditions are right the diploid nuclei in the zygospore undergo meiosis and
create haploid spores. These newly created haploid spores are released by sporangium that sprout
from the zygospore.
These tough, resilient haploid spores are dispersed through the air and remain dormant
until conditions favorable for growth are encountered at which point they grow into new hyphae.
These new hyphae may reproduce either asexually, by producing haploid spores that are released
from sporangia growing directly from the hyphae, or sexually, by forming a zygospore with
another hyphae.
Zygomycetes include the dung fungus Pilobolus, which we'll look at in more detail
shortly and more familiar and annoying zygomycetes of the genus Rhizopus, which cause soft
fruit rot and black bread mold. But probably the most important zygomycetes are those of that
form symbiotic mychorrizal relationships with nearly 80% of vascular plants. These relationships
are particularly important in tropical rainforests where zygomycetes provide the trees and other
plants with critical phosphorus they would otherwise be unable to obtain because it bonds so
tightly to tropical soils.
The Classification of Fungi-Phylum Ascomycota: The Sac Fungi
The 32,000 or so species of ascomycetes, or sac fungi, are named after the sac-like case,
or ascus, in which spores form during sexual reproduction. While ascomycetes unlike
zygomycetes have septa dividing their hyphae, large pores in the septa nonetheless allow for the
flow of cytoplasm, nuclei, and other organelles between cells.
Approximately half of all ascomycetes enter into symbiotic relationships with algae or
cyanobacteria to form a wide variety of lichen. Many of the colorful molds that "decorate"
decaying food or attack and destroy fruit crops are ascomycetes. Both the fungus causing Dutch
elm disease, which has destroyed nearly all the elm trees in the United States, and the one
causing chestnut blight, are ascomycetes.
Another ascomycete Claviceps purpurea, which attacks rye plants, produces a toxin
called ergot. Ergot contains lysergic acid amide a precursor of lysergic acid diethylamide or
LSD. If infected rye is made into flour and consumed, as often happened in the Middle Ages the
victim comes down with a condition called ergotism or St. Anthony's fire. The symptoms of St.
Anthony's fire include a burning sensation in the hands, convulsions, hallucinations, and often
death. Interestingly, the active ingredient in ergot is now extracted, and used in minute doses to
treat migraine headaches, induce uterine contractions and control hemorrhaging after childbirth.
Some ascomycetes live in decaying forest vegetation and form beautiful cup-shaped
reproductive structures or corrugated, mushroom-like fruiting bodies called morels. Another
gastronomic delicacy, the truffle, which we will look at in more detail later, is also a member of
this diverse phylum.
Among the ascomycetes we also find yeast, one of the few unicellular fungi. Certain
species of yeast metabolize glucose and in the process produce ethanol and carbon dioxide. In
baking the ethanol bakes away but the carbon dioxide gas given off creates air pockets that give
bread its light texture. In beer fortunately, both the carbon dioxide and the ethanol remain even
after the yeast are gone. The yeast used by bakers and brewers is also popular with genetists.
These strains of yeast are among the most genetically studied eukaryotic cells, because of their
ease of growth in the lab, rapid reproduction rate, relatively small genome size, and no doubt
because of their commercial value.
Other species of yeast cause a variety of diseases in humans particularly those with
compromised immune systems. Thus, both crytocococcosis which often causes severe meningitis
and pneumoycytis pneumonia which is caused by another strain of yeast are opportunistic
diseases that are particularly likely to attack AIDS patients and others with lowered resistance.
Thrush a disease that attacks the mucus membranes of the mouth and the related yeast infections
of women are caused by yet another species of yeast. This species of yeast also attacks up to
70% of AIDS patients, but usually much more aggressively-going to the esophagus and often
causing bleeding ulcerations, perforations, pain, and nausea.
The Classification of Fungi- Phylum Basidiomycota: The Club Fungi
This phylum is referred to as the club fungi because its members produce club-shaped
reproductive structures. Phylum Basidiomycota consists of about 22,000 species, including the
familiar mushrooms, puffballs, and shelf fungi. It also includes some devastating plant pests
descriptively called rusts and smuts, which cause billions of dollars worth of damage to grain
crops yearly. Although several mushroom species are considered delicacies, others can be
deadly. The good-tasting members of the genus Amanita contain a number of potent and deadly
toxins.
Basidiomycetes typically reproduce sexually. Mushrooms and puffballs are actually
reproductive structures: dense aggregations of hyphae that emerge under proper conditions from
a massive underground mycelium. Below the cap or pileus of mushrooms are leaf-like gills that
produce specialized club-shaped diploid cells called basidia. Basidia form haploid reproductive
basidiospores by meiosis. These are released by the billions from the gills of mushrooms or the
inner surface of puffballs to be dispersed by wind and water.
Falling on fertile ground, a mushroom spore may germinate and produce an underground
network of mycelia. These grow outward from the original spore in a roughly circular pattern as
the older mycelia in the center die. The subterranean body periodically sends up numerous
mushrooms, which emerge in a ring-like pattern called a fairy ring. The diameter of a fairy ring
can reveal the approximate age of a fungus. The fairy rings of some fungus indicate an age of
nearly 700 years.
The Evolutionary Ingenuity in Fungi- Nematode Nemesis
Several species of fungi including Arthrobotrys have evolved deadly snares to trap
nematodes. Natural selection has resulted in a variety of ingenious modifications of nematode
nabbing hyphae. Some produce sticky pods that adhere to passing nematodes. These will
germinate into hyphae that penetrate the nematode body, digesting it from within. Others ensnare
roundworms in sticky tangled hyphae. Another species shoots a harpoon-like projectile 1/10,000
of a centimeter in diameter into passing nematodes. The projectile develops into a new fungus
inside the worm, using it as food. The fungal strangler, Arthrobotrys, produces nooses formed
from three hyphal cells. When a nematode blunders into the noose, the three noose cells take in
water and swell. The resulting constriction of the noose takes only one tenth of a second and
anchors the worm firmly. Other hyphae then penetrate and feast on the hapless prey.
Natural selection, operating over millennia on diverse forms of fungi, has also produced
some interesting adaptations by which fungi disperse their spores. Let's now look at two of these
adaptations for spore dispersal.
The Evolutionary Ingenuity of Fungi -Truffles
Although many fungi are prized as food, none are as avidly sought after as truffles. A
single specimen resembling a small, shriveled, blackened apple may sell for $100. Truffles are
the reproductive structures of an ascomycete that forms a mycorrhizal association with the roots
of oak trees. Formed underground, the truffle is faced with the problem of dispersing its spores.
Its solution is to entice animals to dig it up. Although human beings cannot smell underground
truffles, their odor serves as an irresistible attractant to certain mammals, especially pigs. But
why? It was recently discovered that truffles release a chemical that closely resembles the sexual
attractant produced by pigs. This causes aroused pigs to dig up and devour truffles and in the
process scatter millions of spores via the winds and other means.
Since amorous pigs are very hard to handle most human truffle hunters now use trained
dogs to hunt their quarry. Because truffles are so expensive there is a continuing effort on the
part of entrepreneurs to plant oak groves, sow them with truffle spores, and harvest the truffles
commercially. But such commercial cultivation has proven difficult, as truffles require very
specific soil and climate conditions in order to grow and fruit. So truffles remain so rare and
costly that few of us will ever taste one.
The Evolutionary Ingenuity in Fungi- Shotgun Spore Dispersal
Only by closely scrutinizing piles of horse manure are you likely to observe the
diminutive beauty of Pilobus. Pilobus hyphae penetrating into the dung send up clear bulbs
capped with sticky black spore cases. As they mature, the sugar concentration in the bulbs
increases, which draws in water by osmosis. Meanwhile, the bulb begins to weaken just below its
cap. Suddenly, like an over inflated balloon, it bursts, blowing its spore-carrying top up to two
meters away. Pilobolus is known to bend toward the light. Likely to increase the probability that
its spores will land on open pasture. There to adhere to grass blades until consumed by a grazing
herbivore, such as a horse or cow. Later and some distance away the spores are deposited
unharmed in a pile of their favorite food: manure. Growing hyphae once again penetrate this rich
source of nutrients and send up new projectiles, continuing the cycle of life.
Summary
Although only 100,000 species of modern fungi have been described, some scientists
estimate there might be as many as 1.5 million. These various species of fungi are incredibly
important members of the biosphere. It was probably a symbiotic relationship between fungi that
enabled the ancestor of modern plants to first survive on land and these symbiotic relationships
continue even today as over 85% of plants have a symbiotic relationship with at least one species
of fungi. Further, lichen are almost always the first photosynthetic organisms to colonize a rocky
terrestrial environment and begin the process that eventually leads to establishment of a new
living community.
In established ecosystems fungi are critical to recycling plants after they die since many
have the relatively rare ability to breakdown the cellulose and lignin found in plants and in the
process release some of the nutrients contained in them back to the soil to support a new cycle of
life. However, other species of fungi are a constant threat to attack and kill other living
organisms including humans. Fortunately, humans have found ways to make use of various
species of fungi to make a variety of products including bread, beer, wine, cheese and life saving
antibiotics such as penicillin.
Although they are often easy to overlook, fungi nonetheless play a critical role in the
survival of the biosphere in their roles as parasites, decomposers, and symbiotic partners with
other members of the living world.