Ch 29: Fungi

Introduction to Fungi

Fungi are one of the three major lineages of large, multicellular eukaryotes, alongside plants and animals. Unlike plants, which produce their own food, and animals, which ingest food, fungi absorb nutrients from other organisms through extracellular digestion. They are crucial decomposers in terrestrial ecosystems, capable of digesting lignin and cellulose, the two most abundant organic molecules on Earth.

• Extracellular digestion: Fungi secrete digestive enzymes into their environment to break down complex molecules, then absorb the resulting small organic compounds.

• Decomposers: Fungi are more important than prokaryotes in decomposing dead vegetation due to their ability to digest lignin and cellulose.

• Parasites: Some fungi absorb nutrients from living hosts, causing disease (parasitism).

• Mutualists: Many fungi form beneficial relationships with other organisms, especially plants (mutualism).

Key Terms:

• Extracellular digestion – Digestion that occurs outside the organism's cells.

• Decomposer – An organism that breaks down dead organic material.

• Parasite – An organism that lives on or in a host, causing harm.

• Mutualist – An organism engaged in a mutually beneficial relationship.

Example: Mycorrhizal fungi form mutualistic associations with plant roots, aiding in nutrient absorption.

Why Do Biologists Study Fungi?

Economic and Ecological Importance

The study of fungi is called mycology. Fungi have significant impacts on human health, agriculture, and ecosystems.

• Human parasites: About 200 species infect humans, causing diseases such as athlete’s foot, vaginitis, diaper rash, ringworm, fungal pneumonia, and oral thrush.

• Antibiotics: Fungi are sources of important antibiotics (e.g., penicillin).

• Food spoilage: Fungi cause major losses in grain crops (rusts, smuts) and spoil fruits and vegetables.

• Ecological disasters: Fungal blights have devastated tree populations (e.g., chestnut and elm trees in the U.S.).

• Benefits: Fungi are used in food production (mushrooms, yeast for bread, beer, wine, cheese, soy sauce, and chocolate fermentation).

Fungi and Plant Nutrition

• Mycorrhizae: Fungi that form mutualistic relationships with plant roots, enhancing nutrient and water uptake.

• Saprophytic fungi: Decompose dead organic matter, recycling nutrients in forests, croplands, and rangelands.

Fungi and the Carbon Cycle

• Saprophytic fungi: Digest dead vegetation, releasing carbon as CO2 through cellular respiration.

• Carbon fixation: Plants and some protists remove CO2 from the atmosphere, incorporating it into organic molecules.

• Fungi's role: Accelerate the carbon cycle by breaking down lignin and cellulose, releasing stored carbon.

Example: In forests, fungi decompose wood, returning carbon to the atmosphere and making nutrients available for new growth.

Additional info: Human activities such as deforestation and burning fossil fuels disrupt the carbon cycle, increasing atmospheric CO2 and contributing to global warming.

How Do Biologists Study Fungi?

Morphological Traits

• Growth forms:

  • Yeasts: Single-celled fungi.

  • Mycelia: Multicellular, filamentous forms composed of hyphae.

• Hyphae: Tiny filaments (<10 µm diameter) that make up the mycelium and reproductive structures. Most have haploid nuclei and chitin cell walls.

• Types of hyphae:

  • Septate: Hyphae divided by cross walls (septa) into cell-like compartments.

  • Coenocytic: Hyphae lacking septa, multinucleate.

• Surface area: Mycelia have the highest surface area to volume ratio among multicellular organisms, ideal for nutrient absorption but prone to desiccation.

• Reproductive structures: Produce haploid spores for dispersal. Spores may be asexual (genetically identical to parent) or sexual (resulting from meiosis).

Molecular Phylogenies

• Fungi and animals: DNA evidence and shared traits (chitin, glycogen storage, flagella structure) indicate a close evolutionary relationship.

• Medical relevance: Similarities make fungal diseases difficult to treat without harming animals.

Example: The large mycelium in Oregon covers 2100 acres, weighs hundreds of tons, and is thousands of years old.

Themes in Fungal Diversification

Symbioses

• Mycorrhizae: Mutualistic associations with plant roots; fungi provide nitrogen, phosphorus, and water, while plants supply sugars.

• Lichens: Symbiosis between a fungus and cyanobacteria or green algae.

  • Fungus provides protection and water retention.

  • Photosynthetic partner supplies sugars.

  • Ecological roles: winter food for Arctic mammals, pioneer species in soil formation, bioindicators of air quality.

  • Growth forms:

    • Crustose: Very flat.

    • Foliose: Leafy.

    • Fruticose: Highly branched.

Decomposition

• Some saprophytic fungi and a few bacteria are the only organisms capable of completely digesting wood.

• Fungi break down lignin to access cellulose, then secrete cellulase to convert cellulose into glucose for absorption.

Example: Lichens as pioneer species help form soil on bare rock, and their presence indicates clean air.

Key Lineages of Fungi

Phylum Chytridiomycota

• Only fungi with flagellated spores and gametes.

• Ecological roles:

  • Decomposers in freshwater and moist soil (produce cellulase).

  • Parasites of plants and animals; some species have caused catastrophic declines in amphibian populations.

Phylum Zygomycota

• Primarily soil-dwelling saprobes (decomposers).

• Coenocytic hyphae (multinucleate, no septa).

• Asexual reproduction: Haploid sporangiospores in sporangia.

• Sexual reproduction:

  • Fusion of opposite mating strains forms a zygosporangium with haploid nuclei from each parent.

  • Fertilization produces diploid zygotes, which undergo meiosis to form haploid zygospores.

  • Example: Rhizopus (black bread mold).

Phylum Basidiomycota (Club Fungi)

• Saprophytes: Mushrooms, bracket fungi, puffballs.

• Septate hyphae.

• Parasites: Corn smut, rusts.

• Asexual spores vary.

• Sexual reproduction: Four basidiospores form on a basidium (club-shaped structure).

• Life cycle:

  • Fusion of opposite haploid mating strains forms dikaryotic (n + n) mycelium.

  • Aboveground reproductive structure (mushroom) forms.

  • Fertilization in basidia produces diploid zygote, which undergoes meiosis to yield four haploid basidiospores.

• Parts of a mushroom: Pileus (cap), lamellae (gills), stipe (stalk), annulus (ring), base.

Phylum Ascomycota (Sac Fungi)

• Comprises about 75% of all fungi.

• Includes yeasts, morels, truffles, Penicillium, ergot, Tinea, zombie ant fungus, fungal component of lichens, nematode-capturing fungi.

• Septate hyphae.

• Asexual spores: Conidiospores on conidiophores.

• Sexual spores: Ascospores in an ascus (sac-like structure).

• Life cycle:

  • Fusion of opposite haploid mating strains forms dikaryotic (n + n) mycelium.

  • Aboveground reproductive structure forms.

  • Fertilization in ascus produces a zygote, which undergoes meiosis to yield four haploid nuclei, followed by mitosis to produce eight haploid ascospores.

Comparison of Major Fungal Phyla

Key Processes and Terms

• Sexual reproduction: Begins with fusion of haploid hyphae from different mating strains.

• Dikaryotic stage: Cells contain two genetically distinct haploid nuclei (n + n).

• Diploid zygote: The only diploid cell in the fungal life cycle, produced by fertilization, immediately undergoes meiosis.

• Spore formation: Haploid nuclei are partitioned into dispersal cells called spores.

Summary Table: Fungal Life Cycles

Key Equations and Concepts

• Cellular respiration (carbon release):

• Photosynthesis (carbon fixation):

Additional info: Fungi are essential for nutrient cycling, ecosystem productivity, and have profound impacts on human society through both beneficial and harmful interactions.