Unit 3: Fungi & Lichens

1. Fungi: General characteristics

Definition: Fungi are a kingdom of eukaryotic, achylophyllous (lacking chlorophyll), and heterotrophic (saprophytic, parasitic, or symbiotic) organisms. Their bodies are typically filamentous (hyphae), and their cell walls are made of chitin. The study of fungi is called Mycology.

• Nutrition: Heterotrophic. They secrete digestive enzymes (exoenzymes) onto their food and then absorb the dissolved nutrients.
• Thallus: The body is a mycelium, which is a network of thread-like filaments called hyphae.
  • Aseptate (Coenocytic): Hyphae of "lower fungi" (like *Rhizopus*) that lack cross-walls (septa) and are multinucleate.
  • Septate: Hyphae of "higher fungi" (like *Penicillium*, *Puccinia*) that have cross-walls (septa).
• Cell Wall: Composed primarily of chitin and glucans.
• Reserve Food: Food is stored as glycogen and oil (not starch).
• Habitat: Fungi are ubiquitous, but most are terrestrial and prefer dark, moist environments with organic matter.

2. Fungi: Classification

A simplified outline of the major groups (based on reproductive structures):

*Note: Oomycota are now classified as protists (Stramenopiles), not true fungi, because their cell wall is cellulose and they have a diplontic life cycle. They are studied in mycology for historical reasons.

3. Fungi: Economic importance

Positive Importance (Beneficial)

• Decomposers: As primary saprophytes, they are essential for recycling nutrients (C, N, P) from dead organic matter.
• Food Industry:
  • Yeast (Saccharomyces): Used in baking (leavening bread with CO2) and brewing (fermenting sugar to ethanol).
  • Mushrooms: Many are edible (e.g., *Agaricus* - button mushroom, *Pleurotus* - oyster mushroom).
  • Cheese Production: *Penicillium* species are used to ripen and flavor blue cheese (e.g., *P. roqueforti*).
  • Fermented Foods: *Aspergillus* is used to make soy sauce and tempeh.
• Medicine:
  • Antibiotics: *Penicillium* produces penicillin.
  • Other Drugs: *Claviceps* produces ergot alkaloids (for migraines); statins (cholesterol-lowering) are derived from fungi.
• Symbiosis: Fungi form lichens and mycorrhiza, which are ecologically critical.

Negative Importance (Harmful)

• Plant Diseases: Fungi are the number one cause of plant disease, leading to massive crop losses.
  • Rusts: *Puccinia* (black stem rust of wheat).
  • Smuts: *Ustilago* (loose smut of wheat).
  • Blights: *Phytophthora* (late blight of potato).
• Human & Animal Diseases (Mycoses):
  • Cutaneous: Ringworm, Athlete's foot (caused by dermatophytes).
  • Opportunistic: *Candida* (thrush), *Aspergillus* (aspergillosis) in immunocompromised patients.
• Food Spoilage: Molds like *Rhizopus* (bread mold) and *Aspergillus* spoil stored food.
• Toxin Production: Some fungi produce deadly mycotoxins. E.g., *Aspergillus flavus* produces aflatoxins on peanuts, which are carcinogenic.

4. Fungi: Reproduction

Fungi reproduce by vegetative, asexual, and sexual methods.

Morphology and life cycle of *Rhizopus*

• Organism: *Rhizopus stolonifer* (black bread mold).
• Group: Zygomycota.
• Morphology:
  • The mycelium is aseptate (coenocytic) and haploid (n).
  • It consists of three types of hyphae:
    1. Stolons: Horizontal hyphae that spread over the substrate.
    2. Rhizoids: Root-like hyphae that anchor the fungus and absorb nutrients.
    3. Sporangiophores: Erect hyphae that bear the sporangia.
• Life Cycle (Haplontic):
  • Asexual: Sporangiophores grow upwards and develop a black, spherical sporangium at the tip. Inside, numerous haploid sporangiospores (n) are produced. The sporangium bursts, releasing the spores, which germinate into new mycelia.
  • Sexual: 1. Occurs when two compatible mating strains (+ and -) grow near each other. 2. The hyphae produce swellings (progametangia) that fuse. 3. The tips are cut off by septa to form gametangia. 4. The wall between them dissolves (plasmogamy), and the many haploid nuclei from both sides fuse (karyogamy) to form a multinucleate diploid (2n) structure. 5. This develops into a thick-walled, black, warty resting spore called a zygospore (2n). 6. After a dormant period, the zygospore undergoes meiosis and germinates to produce a sporangiophore with a sporangium, which releases new haploid (n) spores.

Morphology and life cycle of *Penicillium*

• Organism: *Penicillium* (blue-green mold). Source of penicillin.
• Group: Ascomycota.
• Morphology:
  • The mycelium is septate, highly branched, and haploid (n).
• Life Cycle:
  • Asexual (Anamorph): This is the dominant form. 1. Erect, septate hyphae called conidiophores grow from the mycelium. 2. The conidiophore is branched at the tip, forming a "brush-like" structure. 3. The final branches are bottle-shaped cells called phialides. 4. Phialides produce long chains of small, spherical, haploid (n) axsexual spores called conidia. 5. The conidia (which are blue-green) give the mold its color and are dispersed by air.
  • Sexual (Teleomorph): (Rare, not seen in all species) 1. Plasmogamy occurs between an antheridium and an ascogonium. 2. This leads to the formation of dikaryotic (n+n) hyphae, which form asci. 3. Inside each ascus, karyogamy (2n) and meiosis (n) occur, producing 8 haploid ascospores (n). 4. The asci are enclosed in a completely spherical, closed ascocarp called a cleistothecium.

Morphology and life cycle of *Puccinia*

• Organism: *Puccinia graminis* (causes black stem rust of wheat).
• Group: Basidiomycota.
• Morphology:
  • An obligate parasite (cannot be grown on lab media).
  • The mycelium is dikaryotic (n+n) and septate, growing between the host cells.
• Life Cycle: Extremely complex. It is heteroecious (requires two different hosts) and macrocyclic (produces 5 different spore types).
  • Host 1: Wheat (Primary Host) 1. Urediniospore Stage (n+n): Dikaryotic aeciospores from the barberry infect wheat. They produce red-brown, rust-colored pustules (uredia) containing urediniospores (n+n). This is the "repeating stage" that re-infects wheat all summer. 2. Teliospore Stage (n+n → 2n): Late in the season, the fungus produces black pustules (telia) containing thick-walled, two-celled teliospores (n+n). These are the resting spores. Karyogamy (n+n → 2n) occurs within each cell, and the spores overwinter.
  • Host 2: Barberry (Alternate Host) 3. Basidiospore Stage (n): In spring, the diploid (2n) teliospore germinates, undergoes meiosis, and forms a basidium, which produces 4 haploid basidiospores (n). These spores are wind-blown and can *only* infect the barberry leaf. 4. Pycniospore/Spermatial Stage (n): The basidiospores form flask-shaped pycnia on the upper leaf surface. These produce pycniospores (spermatia) (n) (male gametes) and receptive hyphae (female). Cross-fertilization (plasmogamy) occurs between different mating types. 5. Aeciospore Stage (n+n): The resulting dikaryotic (n+n) mycelium grows to the lower leaf surface and forms cup-shaped aecia, which produce chains of aeciospores (n+n). These spores are released and infect the wheat, completing the cycle.

5. Lichens

General account

Definition: A lichen is a stable, self-supporting, mutualistic (symbiotic) association between a fungus (the mycobiont) and a photosynthetic partner (the phycobiont, which is either a green alga or a cyanobacterium).

• The Symbiosis:
  • The Fungus (Mycobiont): Provides the body, structure, protection from UV light and drought, and absorbs water and minerals.
  • The Alga (Phycobiont): Performs photosynthesis and provides the fungus with sugars (food).
• Thallus Forms:
  1. Crustose: A flat, crust-like thallus, tightly embedded in the substrate (rock/bark).
  2. Foliose: A leaf-like, lobed thallus, loosely attached by rhizines (e.g., *Parmelia*).
  3. Fruticose: A shrub-like, branched, and erect or hanging thallus (e.g., *Usnea*).
• Reproduction:
  • Asexual (Vegetative): This is the main method, as it disperses *both* partners together.
    • Soredia: Powdery "packages" of algal cells wrapped in fungal hyphae.
    • Isidia: Small, coral-like outgrowths that break off.
  • Sexual: This is reproduction of the *fungus only*. The fungus produces ascospores, which must find a new algal partner by chance to form a new lichen.

Economic importance

• Ecological Pioneers: Lichens are the first organisms to colonize bare rock. They secrete acids that break down the rock, beginning the process of soil formation.
• Bioindicators: Lichens are extremely sensitive to air pollution, especially sulfur dioxide (SO2). Their absence in an area is a strong indicator of poor air quality.
• Dyes: Historically used to produce dyes. Litmus, the classic pH indicator, is extracted from *Roccella*.
• Fodder: "Reindeer moss" (*Cladonia*) is a major food source for reindeer and caribou in arctic regions.
• Medicine: *Usnea* produces usnic acid, which has antibiotic properties.

6. Mycorrhiza

General account and significance

Definition: Mycorrhiza ("fungus-root") is a mutualistic (symbiotic) association between a fungus and the roots of a higher plant.

• The Symbiosis:
  • The Fungus gets: Sugars (carbon) from the plant's photosynthesis.
  • The Plant gets: A vastly increased absorptive surface area. The fungal hyphae are much finer than root hairs and can explore a larger volume of soil. This gives the plant enhanced uptake of:
    • Water (increasing drought resistance).
    • Minerals, especially Phosphorus (P), which is often immobile in soil.
• Types of Mycorrhiza:
  1. Ectomycorrhiza (ECM):
     • The fungus forms a dense sheath (mantle) *around* the outside of the root.
     • Hyphae grow *between* the root cells to form a "Hartig net" for nutrient exchange.
     • They do not penetrate the cells.
     • Common in trees like pines, oaks, and birches.
  2. Endomycorrhiza (AM):
     • The fungus does *not* form an outer sheath.
     • Hyphae grow *into* the root cortex and penetrate the cell walls (but not the plasma membrane).
     • Inside the cell, they form highly branched, tree-like structures called arbuscules, which are the site of nutrient exchange.
     • This type, also called Vesicular-Arbuscular Mycorrhiza (VAM), is the most common, found in >80% of all land plants, including most crops.

Significance:

Mycorrhiza are fundamental to plant life. Most plants on Earth depend on this symbiosis for survival. In agriculture and forestry, inoculating soil with mycorrhizal fungi can significantly improve plant growth, reduce the need for phosphate fertilizers, and increase plant hardiness.