Fungi

I.    Characteristics

A.    Nutrition
Fungi are heterotrophs.  They obtain their nutrients by absorption, thus they also are decomposers.
Fungi secrete hydrolytic enzymes which decompose complex molecules into smaller molecules that they can absorb.

Most fungi are saprophytic decomposers which absorb nutrients from nonliving organic material, such as fallen logs, dead plants or animals.  They attack anything organic including food, clothing, stored goods, books, and houses.

Parasitic fungi absorb nutrients from the cells of living hosts.  These fungi have special hyphae (see below) called haustoria which penetrate cells.  These are the type of fungi that cause athlete's foot.

Mutualistic fungi also absorb nutrients from host cells, but they provide something beneficial to the host in return.  Example:  Fungi that live in the roots of plants (called mycorrhizae) obtain nutrients for the plant but aborb minerals that the plant can use in return.

B.    Structure
Fungi are multicellular eukaryotes.
Bodies of fungi are composed of units called hyphae (hypha, singular). A mass of hyphae is called a mycelium.  There are actually two types of mycelia:  vegetative mycelia, involved in obtaining nutrients, and aerial mycelia, involved in reproduction.

Hyphae cells may or may  not have cross walls (septa).  The cell walls contain chitin.

Yeasts are the exception of fungi that have hyphae.  Yeast are primarily single-celled, however, the cells may stay together after division, forming pseudohyphae.
 

C.    Reproduction
Fungi reproduce by spore formation.  Spores are dispersed by wind (or the air currents created when you open your refrigerator door).

Spores are produced asexually or sexually.  Asexual reproduction is more common and occurs in all fungi.  For both asexual and sexual reproduction, specialized hyphae are involved.

Sexual reproduction involves the fusion of cells from two individual fungal organisms.  Fertilization, however, is delayed so these cells may remain dikaryotic (two nuclei) for some time. Remember that mature fungi are haploid.



II.    Diversity of Fungi
A.    Division Chytridiomycota - chytrids
This is a 'new' division of the fungal kingdom.  These organisms used to be part of Kingdom Protista, but now are included in Kingdom Fungi based on comparing protein and nucleic acid sequences.

Chytrids are mainly aquatic.  Some are saprobes, others parasitize protoists, plants, and aquatic invertebrates.

B.    Division Zygomycota - conjugation fungi
These fungi are mostly terrestrial and live in soil or on decaying plant and animal material.
Zygomycota are also called conjugating molds.
Asexual reproduction occurs by the formation of specialized hyphae called sporangia.  Asexual spores develop in these structures.
They reproduce sexually by fusing (conjugating) of hyphae from different mating strains, forming sexual spores called zygospores. Usually sexual reproduction occurs under stressful conditions such as when food is not available.

Examples:  Rhizopus stolonifer - black bread mold
 

C.    Division Ascomycota - sac fungi
These fungi can be found in marine, terrestrial, and freshwater habitats.
Asexual reproduction occurs by formation of specialized hyphae called conidiophores, the spores are called conidia.

This is a conidiophore from the fungus Aspergillus.

Sexual reproduction occurs in sac-like structures called asci (ascus- singular).  A group of asci are held in a structure called an ascocarp which is a fruiting body (mass of specialized hyphae involved in reproduction).  The sexual spores are called ascospores.

Types of sac fungi:

- morels, truffles
- powdery mildews
- blue and green molds (example:  Penicillium)

- Claviceps purpura
This is a parasite of rye and other grains.  It causes ergot, which is a plant disease.  In humans who eat infected rye, it causes ergotism (St. Anthony's Fire).  Ergotism causes a burning sensation in the hands and feet and causes hallucinations.
 

- yeasts
Yeasts are single-celled.  They are either round or oval and may form psuedohyphae.  Yeasts reproduce by budding.
Saccharomyces cerevisiae is the yeast used for baking and brewing.

Candida is a yeast that is part of the normal flora of humans (skin and vaginal lining).  Under certain circumstance (i.e. change in pH, destruction of normal bacterial flora by antibiotics, compromised immune system) Candida can become pathogenic.
 

Lichens represent a symbiotic relationship (mutualism) between a photosynthetic organisms (algae or cyanobacteria) and a fungus (usually a sac fungus). The fungus provdies the alga with a suitable physical environment for growth.  The fungus also absorbs minerals and water.  The algae provides food for the fungus.  The fungus usually gives the lichen its overall shape and structure.

Lichens are important pioneers on newly cleared rock and soil surfacesPhysical penetration of the outer crystal of rocks and chemical attack of rock by lichen acids help break down the rock and establish soil-trapping lichens.

Lichens are typed according to their physical appearance:
1.   crustose - "encrusting"
These lichens form dry, colorful colonies on rocky surfaces.
2.    foliose - "leafy"
These lichens grow on trees.  Some resemble Spanish moss.
3.    fructiose - "shrubby"
These lichens live on soil surfaces and form a tangled, netlike growth.  An example is reindeer moss.

D.    Division Basidiomycota - club fungi
Club fungi derive their name from the club shaped-structures that form to produce sexual spores.
Two groups of club fungi differ nutritionally and structurally:
1.    mushrooms, toadstools, and shelf fungi
These fungi are saprophytic and absorb nutrients from decaying organic matter.  The fleshy reproductive structure is called a basidiocarp (stalk and cap) and is above ground.  Sexual spores produced in club-like structures are called basidiospores.
 

     This fungus is a shelf fungus growing on the trunk of a tree.
From Campbell's Biology, 5th ed.
 

2.    wheat rusts and corn smuts
These fungi are parasitic.
 

E.    Division Deuteromycota - imperfect fungi
These fungi are called imperfect fungi because they have no known sexual reproductive stage.
A number of these fungi cause diseases in humans, i.e. Trichophyton which causes athlete's foot.
 

Mycorrhizae
These are a mutualistic relationship between a fungus and a plant.  The fungus lives in the roots of the plant - mycorrhizae means "fungal root."  The fungal hyphae increase the absorptive surface of the plant roots.  Teh fungus gains inroganic nutrients needed by both organisms and the plant provides organic nutrients for the fungus.  Over 95% of  all vascular plants have mycorrhizae.