Fungi: A Comprehensive Overview

Fungi cause problems with crop production and storage.
Plants grow better in the presence of mycorrhizal fungi.
Fungi speed up the carbon cycle as they break down dead trees in terrestrial ecosystems.

Approximately 110,000 species of fungi have been described and named.
Hundreds more are discovered each year.
As many as 6 million species of fungi may be found worldwide.
In one study, over 650 fungal species were found in the guts of 27 beetle species, with 200 of these species never having been described.

Fungi have two growth forms:
- Single-celled fungi are called yeasts (5 μm).
- Multicellular fungi have weblike bodies called mycelia (20 mm).

Lichens are symbiotic associations between an Ascomycete and a Cyanobacterium or Green Alga.

Fungi can decompose even the hardest trees into soft soils over time.
The large surface area of a mycelium makes nutrient absorption exceptionally efficient.
Saprophytic fungi can grow toward the dead tissues that supply their food.

Fungi must digest their food before they can absorb it, performing extracellular digestion, which takes place outside the organism.
Simple compounds resulting from enzymatic action are absorbed by hyphae.
The two most abundant organic molecules on Earth are digested by fungi: lignin and cellulose.
- Lignin is found in plant secondary cell walls.
- Cellulose is found in plant primary and secondary cell walls.

Fungi break down lignin using an enzyme called lignin peroxidase.
This enzyme catalyzes an oxidation step that creates a free radical, leading to a series of uncontrolled reactions that split the polymer into smaller units.
Fungi cannot grow with lignin as their sole food source.
Six-carbon rings in lignin are difficult to metabolize and accumulate in soil.
Saprophytic fungi use lignin peroxidase to break down lignin and expose cellulose for growth and reproduction.

Fungi break down cellulose with enzymes called cellulases.
Fungi secrete these enzymes into the extracellular environment.
These cellulases together convert cellulose into glucose, which the fungus can absorb and use as a source of food.

Fungi have unusual life cycles involving both sexual and asexual reproduction.
Asexual Reproduction: Mitosis produces spores (n), which grow into mycelium (n), and spore-producing structures (n).
Sexual Reproduction: Plasmogamy (fusion of cytoplasm) creates a heterokaryotic mycelium (n+n). Karyogamy (fusion of nuclei) results in a diploid zygote (2n). Meiosis then produces haploid spores (n).

Known Species: 750
Absorptive Life Style: Many decompose plants by digesting cellulose. Mutualistic chytrids live in guts of cows, deer, and other mammals and help digest plant material. Parasitic chytrids infect many species of plants and animals.
Life Cycle: The only fungi that produce motile cells–both their spores and gametes swim via flagella. Most exhibit alternation of generations.
Relevance: Some are parasites of mosquitoes and are being studied as a biological control agent. Batrachochytrium dendrobatidis is largely responsible for declines in amphibian populations worldwide.

Known Species: 1050
Absorptive Life Style: Many are saprophytes and live on plant debris. Some parasitize other fungi, insects, or spiders.
Life Cycle: Asexual reproduction is very common; asexual sporangia produce spores. Sexual reproduction involves fusion of hyphae and formation of zygosporangium and sexual sporangia.
Relevance: Several species are often seen growing on bread and soft fruits. Some are used in the production of steroids, pigments, alcohols, and fermented foods.

Known Species: 32,000
Absorptive Life Style: Decompose wood by producing lignin peroxidase. Some form ectomycorrhizal associations with tree roots. Smut and rust fungi are plant parasites.
Life Cycle: Produce haploid spores in club-shaped basidia. Basidia of mushroom-forming species line gills found under the cap.
Relevance: Mushrooms are used as a source of food. Some mushrooms are toxic or hallucinogenic. Ectomycorrhizal basidiomycetes enhance growth of hardwoods and softwoods. Smuts and rusts cause serious economic damage to crop plants.

Known Species: 64,000
Absorptive Life Style: Many form ectomycorrhizal associations with tree roots. Some form mutualistic associations with photosynthetic algae or bacteria in lichens. Some are predatory on protists or nematodes.
Life Cycle: Produce haploid spores in sac-like asci. Asci of many species found on fleshy, cup-shaped structures. Asexual reproduction in many.
Relevance: Lichens are often sensitive to air pollution and are used as bioindicators.