Characteristics and Structure of Fungi

MCB 102 · INTRODUCTORY MICROBIOLOGY: Characteristics and Structure of Fungi

This study summary covers the fundamental aspects of fungi, including their classification, structure, characteristics, reproduction, and ecological and economic significance.

Learning Objectives

• Define fungi and state their position in biological classification.
• Describe the general characteristics of fungi.
• Explain the structural features of fungal cells, hyphae, and mycelium.
• Distinguish between different types of fungi based on morphology.
• Describe cell wall composition and reproductive structures.
• Explain the ecological and economic significance of fungi.

What Are Fungi?

Key Facts

• Fungi are eukaryotic organisms that are either unicellular (yeasts) or multicellular (moulds, mushrooms).
• They lack chlorophyll and are heterotrophic, obtaining nutrients by absorption.
• Reproduction primarily occurs by spores.
• Multicellular fungi consist of masses of mycelia, which are composed of filamentous structures called hyphae.
• Fungi are vital for food production, medicine, and nutrient recycling from dead organisms.

Etymology & Historical Note

• Mycology: The study of fungi (Greek: mykes = mushroom).
• Mycologist: A scientist who studies fungi.
• Father of Mycology: Anton de Bary (1831-1888).
• Historically, fungi were classified as plants due to their sessile nature and cell walls. Modern molecular evidence places them in a distinct kingdom, sharing a more recent common ancestor with animals.

Fungi and Plants: Key Differences

• Cell Wall Material: Fungi have cell walls made of chitin; plants have cell walls made of cellulose.
• Energy Production (Photosynthesis): Fungi lack chlorophyll and chloroplasts, being heterotrophic and absorbing nutrients; plants contain chloroplasts and perform photosynthesis.
• Nutrient Storage: Fungi store excess carbohydrates as glycogen (like animals); plants store them as starch.
• Vacuoles: Fungal cells often have smaller, multiple vacuoles; plant cells typically have a single, large central vacuole.

Fungi and Animals: Key Differences

General Characteristics of Fungi

1. Eukaryotic: True membrane-bound nucleus, DNA organized into chromosomes, all cellular organelles present.
2. Heterotrophic Nutrition: Cannot photosynthesize; secrete exoenzymes into the substrate and absorb digested nutrients (absorptive heterotrophy).
3. Chitin Cell Wall: Cell walls composed of chitin, beta-glucans, and glycoproteins (unlike plant cellulose or bacterial peptidoglycan).
4. No Flagella (mostly): Most fungi are non-motile. Exception: Chytridiomycota produce flagellate zoospores.
5. Glycogen Storage: Store carbohydrates as glycogen (like animals), NOT starch (like plants). Enables rapid energy mobilization.
6. Spore Reproduction: Reproduce through sexual and asexual spores. Spores are lightweight, easily dispersed, and a single fruiting body can release billions.
7. Ergosterol in Membranes: Plasma membranes contain ergosterol (not cholesterol). Ergosterol is a target for antifungal drugs (e.g., amphotericin B, azoles).
8. Diverse Habitats: Mostly terrestrial; prefer warm, moist environments. Found in soil, decaying matter, on living hosts, and even in extremes.

Nutritional Types in Fungi

• Saprotrophic: Feed on dead/decaying organic matter (e.g., fallen leaves, dead wood, animal carcasses). They are primary decomposers.
  • E.g., Rhizopus (bread mold), mushrooms, Aspergillus.
• Parasitic: Absorb nutrients directly from living organisms (hosts), often causing harm, disease, or even death to the host.
  • E.g., Trichophyton, Puccinia.
• Mutualistic: Form mutually beneficial partnerships with other organisms. Both partners receive nutritional or environmental benefits.
  • E.g., mycorrhizae (fungi and plant roots), lichens (fungi and algae/cyanobacteria).
  • E.g., Glomus, Cladonia.
• Predatory: Trap and digest microscopic animals using specialized hyphae.
  • E.g., Arthrobotrys spp.

The Morphology of Fungi

• Morphologically, fungi range from small unicellular yeasts and moulds to large, visible mushrooms, puffballs, and bracket fungi.
• The body or vegetative structure of a fungus is called a thallus (pl. thalli).
• Some are single cells (e.g., yeasts), but most exist as filaments known as hyphae (singular, hypha).
• Hyphae usually branch extensively, and the collective mass of hyphal filaments is called a mycelium.

The Hypha: Fundamental Structural Unit

A hypha (pl. hyphae) is a thin, thread-like, tubular filament – the basic building block of a multicellular fungus.

Types of Hyphae based on septation:

• Septate Hyphae:
  • Divided into compartments by cross walls called septa (internal cell walls formed at right angles to the cell wall).
  • Septa typically have pores allowing cytoplasm and organelles to flow between cells.
  • Found in: Ascomycota, Basidiomycota.
• Aseptate (Coenocytic) Hyphae:
  • A continuous tube with many nuclei in shared cytoplasm.
  • Also called coenocytic.
  • Found in: Zygomycota (e.g., Rhizopus, Mucor).

Types of Hyphae based on function/location:

• Vegetative hyphae: Penetrate the media and absorb food. They can grow on or within the substrate.
• Aerial hyphae: Directed above the surface of the media.
• Reproductive hyphae: Aerial hyphae that carry different spores.

Hyphal Growth

• Hyphae exhibit apical growth, meaning they elongate at their tips.
• In theory, they can grow indefinitely under favorable environmental conditions.
• Environmental conditions eventually limit their growth.
• Hyphae may initially develop from a germ-tube (a short, immature hypha) that emerges from a germinating spore.

The Mycelium: Fungal Vegetative Body

The mycelium is a mass of interwoven, branching hyphae forming the main body of a multicellular fungus – responsible for growth, digestion, and nutrient absorption.

• Vegetative Mycelium:
  • Grows into the substrate.
  • Functions like a root system.
  • Secretes enzymes to absorb nutrients.
  • Creates a huge surface area-to-volume ratio for efficient absorption.
• Aerial Mycelium:
  • Grows above the substrate.
  • Bears reproductive structures (spores).
  • Visible as fuzzy/cottony growth on food.
  • Also called sporophore in some fungi.

Fungal Cell: Internal Structure

• Cell Wall: Primarily composed of robust chitin and β-glucans; provides structural integrity.
• Plasma Membrane: Contains ergosterol; responsible for selective permeability; a major drug target for antifungals.
• Nucleus: True, membrane-bound; contains chromosomal DNA.
• Mitochondria: Sites of aerobic respiration and ATP production.
• Ribosomes (80S): Responsible for protein synthesis (similar to animal ribosomes).
• Vacuoles: Involved in the storage of metabolites and maintenance of turgor pressure.
• Spitzenkörper: A vesicle supply center at the hyphal tip; coordinates apical growth.
• Glycogen Granules: Serve as an energy reserve (not starch).
• Lomasome: Complex invaginations of the plasma membrane, function debated (possibly involved in cell wall synthesis or secretion).
• Endoplasmic Reticulum (ER): Network involved in protein and lipid synthesis and transport.
• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.
• Peroxisome: Involved in various metabolic processes, including fatty acid breakdown.

Yeasts and Dimorphic Fungi

Yeasts

• Unicellular growth form of fungi (mainly Ascomycota & Basidiomycota).
• Shape: Round/oval/cylindrical; 3-15 µm diameter.
• Reproduction: Primarily by budding (asexual); some by binary fission.
• Pseudohyphae: Elongated chains of cells formed under stress (resemble hyphae but differ structurally).
• Examples: Saccharomyces cerevisiae (baker's yeast), Candida albicans.

Dimorphic Fungi

• Can exist in two forms depending on temperature:
  • 25°C (room temp) → Mould (filamentous)
  • 37°C (body temp) → Yeast (unicellular)
• Mnemonic: "Mould in the cold, yeast in the heat."
• Examples: Histoplasma capsulatum, Blastomyces dermatitidis, Coccidioides immitis (medically significant human pathogens).

Reproduction in Fungi

Fungi reproduce both sexually and asexually. Both often involve the production of spores.

Asexual Reproduction (Mitotic Spores & Vegetative Methods)

Asexual spores are formed from aerial hyphae of one organism, producing genetically identical offspring.

1. Sporulation (Spore Formation): Most common type. The process is called SPORULATION. A spore is a small, microscopic, unicellular reproductive unit and the primary agent of fungal dispersal.
2. Sporangiospores:
   • Produced within a sac-like structure called a sporangium, which develops on a sporangiophore.
   • Characteristic of Zygomycota (e.g., Rhizopus, Mucor).
3. Zoospores:
   • Motile sporangiospores found in lower fungi.
   • Possess flagella for movement.
   • E.g., Allomyces (Chytridiomycota).
4. Conidiospores (Conidia):
   • Not enclosed in a sac but produced at the tips or sides of a hypha (called conidiophores).
   • Characteristic of Ascomycota (e.g., Aspergillus, Penicillium, Alternaria).
5. Chlamydospores:
   • Thick-walled, large resting spores containing stored food.
   • Rise directly from hyphal cells.
   • Life stage that survives unfavorable conditions (e.g., winter).
   • Can occur singly or in chains.
   • E.g., Candida albicans.
6. Fragmentation:
   • Involves breaking the fungal mycelium into several fragments.
   • Each fragment develops into a new fungus. Also known as disarticulation.
7. Fission:
   • Simple splitting of a cell into daughter cells by a constriction.
   • The nucleus divides mitotically, and a new cell wall forms.
8. Budding:
   • Parent cell produces one or more projections called buds.
   • Buds develop necessary structures and detach to grow into new individuals.
   • Common in yeasts.

Sexual Reproduction (Meiotic Spores)

Sexual reproduction involves the union of two compatible nuclei with subsequent meiotic division, yielding recombinant progeny. Fungi typically do not have distinct male and female individuals, but rather different mating types.

Three stages in fungal sexual reproduction:

1. Plasmogamy: The cytoplasm of two compatible hyphae fuses, bringing their protoplasts together. The nuclei coexist as a pair in a single cell, resulting in a unique dikaryotic (or heterokaryotic) stage, denoted as (N + N).
2. Karyogamy: The two compatible haploid nuclei of a cell fuse, forming a single, diploid nucleus (2N).
3. Meiosis: The diploid zygote undergoes a reductional cell division to form haploid nuclei.

Methods of Sexual Reproduction:

1. Planogametic Copulation: Fusion of motile gametes called planogametes (zoospores).
   • Isogamy: Both gametes are motile and morphologically similar. E.g., Synchytrium.
   • Anisogamy: Both gametes are motile but differ in size. E.g., Allomyces.
   • Heterogamy / Oogamy: One gamete (male) is smaller and motile, and the other (female) gamete is larger and non-motile.
2. Gametangial Contact: Gamete-bearing structures (gametangia) come close, and a fertilization tube allows the male gamete to migrate into the female gametangium. E.g., Phytophthora, Albugo.
3. Gametangial Copulation: Gametangia fuse directly at the point of contact and develop into a zygospore. E.g., Mucor, Rhizopus.
4. Spermatization: Formation of small spores or seed-like structures called spermatia, which act as male gametes and are carried by wind or insects to receptive hyphae or female gametangia.
5. Somatogamy: Vegetative hyphae of opposite mating types fuse, acting as male and female gametes to bring about sexual reproduction. E.g., Smut Fungi.

Fungi vs. Plants vs. Animals: Key Comparisons

Ecological Importance of Fungi

• Decomposers: Primary decomposers of dead organic matter. Break down lignin, cellulose, & tough polymers, cycling nutrients back into ecosystems.
• Mycorrhizae: Form symbiotic associations with over 90% of plant species. Fungi provide phosphorus and nitrogen; plants provide fixed carbon. Critical for forest ecosystems.
• Lichen Formation: Symbiotic partnership between a fungus and an alga/cyanobacterium. Pioneer organisms on bare rock, contributing to soil formation (pedogenesis).
• Food Web: Important food source for insects, slugs, squirrels, and humans. Support entire trophic levels in soil ecosystems.

Economic Importance: Beneficial Applications

• Food: Edible mushrooms (Agaricus bisporus, Pleurotus), truffles, morels – commercially cultivated worldwide.
• Fermentation: Saccharomyces cerevisiae for bread, beer, wine, bioethanol. Aspergillus oryzae for soy sauce, miso.
• Antibiotics: Penicillium notatum → Penicillin (Fleming, 1928), a landmark medical discovery.
• Enzymes & Acids: Aspergillus niger → citric acid, amylases, proteases, lipases. Used in food, beverage, and industrial processes.
• Biocontrol: Beauveria bassiana against aphids & whiteflies. Entomopathogenic fungi as safe biopesticides.
• Pharmaceuticals: Cyclosporin A (organ transplant immunosuppression), Lovastatin (cholesterol reduction), Ergotamine (migraines).

Economic Importance: Harmful Effects

• Plant Diseases: Wheat rust (Puccinia graminis), Rice blast (Magnaporthe oryzae), Corn smut (Ustilago maydis). Cause billions in annual crop losses.
• Human Mycoses (Fungal Infections):
  • Superficial: Ringworm, athlete's foot (e.g., Trichophyton).
  • Systemic: Candidiasis, Aspergillosis, Cryptococcosis, Histoplasmosis (more severe, internal infections).
• Mycotoxins: Toxic secondary metabolites produced by fungi.
  • Aflatoxins (Aspergillus flavus): Contaminate groundnuts/maize; potent carcinogens.
  • Also: Ochratoxin, Fumonisin, Zearalenone.
• Material Decay: Wood-rotting fungi (e.g., Serpula lacrymans - dry rot). Cause billions of dollars in structural timber damage annually.

Review Questions (Self-Assessment)

1. Define fungi and state THREE features that distinguish them from plants.
2. Distinguish between septate and aseptate hyphae with examples.
3. Why is ergosterol clinically important? Name one drug that targets it.
4. What is a dimorphic fungus? Give TWO examples and state their medical significance.
5. Compare asexual and sexual spore types in fungi with examples of each.
6. Outline the ecological roles of fungi in terrestrial ecosystems.
7. What are mycotoxins? Name TWO and state their health effects.

Lecture Summary

• Fungi: Eukaryotic, heterotrophic; Kingdom Fungi; phylogenetically closer to animals than plants.
• Cell wall: CHITIN (not cellulose); membrane sterol: ERGOSTEROL (not cholesterol).
• Food reserve: GLYCOGEN (not starch); ribosomes: 80S (same as animals).
• Basic unit: hypha; mass of hyphae = mycelium; unicellular form = yeast.
• Hyphae: Septate (Ascomycota, Basidiomycota) vs aseptate (Zygomycota).
• Asexual spores: conidia, sporangiospores, zoospores, chlamydospores.
• Sexual spores: ascospores, basidiospores, zygospores.
• Dimorphic fungi: mould at 25°C, yeast at 37°C – medically significant.
• Ecological roles: decomposers, mycorrhizae, lichens.
• Economic importance: antibiotics, fermentation, food, biocontrol, pharmaceuticals (beneficial); plant diseases, human mycoses, mycotoxins, material decay (harmful).
• "Without fungi, life on land as we know it would not exist."