Table of Contents
Overview of Fungi as “Lower Organisms”
Fungi form their own large kingdom of eukaryotic organisms. They are neither plants nor animals, even though they were long grouped with plants. They are especially important in ecosystems as decomposers and in human life as food, pathogens, and sources of medicines.
Key features in brief:
- Eukaryotic cells (with a true nucleus and organelles)
- Mostly non‑motile
- Cell wall usually made of chitin (not cellulose as in plants)
- Heterotrophic nutrition, usually by absorption
- Growth mainly at tips of filaments rather than by body movement
Basic Structure and Growth
Unicellular vs. Multicellular Fungi
- Yeasts
- Mostly unicellular
- Reproduce mainly by budding
- Individual oval cells; sometimes form short chains (
pseudohyphae) - Filamentous fungi (molds, mushrooms, many plant pathogens)
- Composed of long, threadlike filaments called hyphae
- A mass of hyphae is called a mycelium
- Mycelium is usually the main “body” of the fungus, often hidden in soil, wood, or other substrates
Hyphae and Mycelium
- Hyphae
- Tubular filaments surrounded by a cell wall
- Grow mainly at their tips by cell division and cell wall extension
- Two main types:
- Septate hyphae: divided by cross-walls (septa) with pores; common in many higher fungi
- Coenocytic hyphae: no cross-walls; a continuous cytoplasm with many nuclei
- Mycelium
- Network of interwoven hyphae
- Large surface area relative to volume, ideal for absorbing nutrients
- Can be:
- Vegetative mycelium: involved mainly in nutrient uptake and growth
- Reproductive mycelium: produces specialized structures for spore formation
Cell Wall and Internal Organization
- Cell wall composition
- Typically contains chitin, a tough polysaccharide also found in arthropod exoskeletons
- May include glucans and other polysaccharides
- Cytology
- Nuclei usually small and numerous in filamentous fungi
- Many fungi are dikaryotic in parts of their life cycle: two genetically distinct nuclei share a common cytoplasm before nuclear fusion (karyogamy)
Nutrition and Ecology
Heterotrophic by Absorption
Fungi do not photosynthesize. They obtain organic molecules from their environment by:
- Secreting digestive enzymes into their surroundings
- Breaking down complex organic substances outside their cells
- Absorbing the resulting small molecules (sugars, amino acids, etc.)
This mode of nutrition is called saprotrophic when they feed on dead organic matter.
Ecological Roles
- Saprotrophs (decomposers)
- Break down dead plant and animal material (leaf litter, wood, dung, carcasses)
- Crucial for nutrient cycling (carbon, nitrogen, and others) in ecosystems
- Some specialize in tough materials (e.g., lignin-degrading fungi in wood rot)
- Parasites and pathogens
- Live on or in living hosts, often causing disease
- Examples: rusts and smuts of crops; many plant wilts and rots; skin infections in animals
- Often form specialized penetration structures (e.g., haustoria) to withdraw nutrients from host cells
- Mutualists
- Live in close association where both partners benefit
- Key examples:
- Mycorrhizae: associations between fungal hyphae and plant roots, enhancing water and mineral uptake for plants; fungus receives sugars
- Lichens: symbiosis between a fungus and a photosynthetic partner (usually a green alga or cyanobacterium)
These different lifestyles deeply influence fungal morphology, reproduction, and life histories.
Reproduction in Fungi: General Features
Fungi can reproduce asexually and sexually, often switching depending on environmental conditions.
Asexual Reproduction
Common mechanisms:
- Fragmentation
- A piece of mycelium breaks off and grows into a new individual
- Effective in stable, resource-rich environments
- Budding (especially in yeasts)
- A small outgrowth (bud) forms on a parent cell, enlarges, and detaches
- Rapid propagation in nutrient-rich media
- Asexual spores
- Produced without fusion of gametes
- Types vary with groups; examples:
- Conidia: non-motile spores formed externally on specialized hyphae (conidiophores)
- Sporangiospores: produced inside a sporangium at the tip of a sporangiophore
- Often formed in large numbers; adapted for dispersal by air, water, or animals
Asexual reproduction is usually responsible for fast population increase and colonization of new substrates.
Sexual Reproduction
Sexual reproduction in fungi is highly diverse but generally involves these stages:
- Plasmogamy
- Fusion of cytoplasm from two compatible fungal cells or hyphae
- Nuclei from each parent usually remain separate initially
- Dikaryotic stage (in many higher fungi)
- Cells contain two genetically different nuclei (
n + n) - This can be a long-lived phase, forming extensive dikaryotic mycelium
- Karyogamy
- Fusion of the two nuclei to form a diploid nucleus (
2n) - Meiosis
- Diploid nucleus undergoes meiosis to produce haploid spores
- Restores the haploid condition typical of most fungal cells
Sexual spores are often produced in characteristic, group-specific structures and are crucial for:
- Generating genetic variation
- Surviving adverse conditions (often thick-walled or dormant)
- Long-distance dispersal
The exact details of sexual cycles differ among major fungal lineages and are treated in systematics-oriented chapters.
Major Functional Groups and Morphological Types
Without going into full classification, some recurring types are important to understand:
Molds
- Fast-growing, filamentous fungi often found on food, soil, and organic waste
- Reproduce mainly by abundant asexual spores (e.g., conidia)
- Rapid colonizers, important decomposers, and often first examples seen in everyday life
Yeasts
- Mostly unicellular, often adapted to liquid or moist environments rich in sugars (nectar, fruit, plant juices, animal-associated habitats)
- Often facultatively anaerobic, capable of fermentation (important in baking, brewing, and winemaking)
- Some can switch between yeast-like and filamentous forms depending on conditions (dimorphic fungi)
Macrofungi (Mushroom-Formers)
- Produce conspicuous fruiting bodies (e.g., mushrooms, brackets, morels)
- The fruiting body is only a reproductive structure; the main fungus is the extensive mycelium in the substrate
- Fruiting bodies:
- Elevate spore-producing tissue into the air for effective wind dispersal
- Show diverse forms (gills, pores, teeth, coral-like branches) that increase surface area for spore production
Not all fungi form large fruiting bodies; many remain microscopic.
Fungi and Other Organisms: Close Associations
Fungi and Plants: Mycorrhizae
- Mycorrhizae link plant roots with fungal hyphae
- Benefits:
- Plants: improved uptake of water, phosphate, and micronutrients
- Fungi: access to plant-produced carbohydrates
- Types:
- Ectomycorrhizae: fungal hyphae surround roots and form a sheath; common in many forest trees
- Endomycorrhizae (arbuscular mycorrhizae): hyphae penetrate root cortex cells, forming tree-like arbuscules inside
Mycorrhizal relationships are widespread and foundational for many ecosystems, especially forests and nutrient-poor soils.
Fungi and Photosynthetic Partners: Lichens
- Lichen = fungus (usually the main structural partner) + alga or cyanobacterium
- Fungus:
- Provides structure, protection, and water retention
- Photosynthetic partner:
- Provides carbohydrates via photosynthesis
- Lichens:
- Colonize harsh environments (rocks, tree bark, bare soil, tundra)
- Important pioneers in ecological succession
- Sensitive to air pollution, often used as bioindicators
Fungal Pathogens
- Plant pathogens
- Cause blights, rusts, smuts, wilts, mildews
- Can devastate crops and natural plant communities
- Animal and human pathogens
- Often infect skin, nails, mucous membranes, or lungs
- Some produce toxins; health aspects are detailed in disease-related chapters
Parasites often show specialized structures: appressoria for adhesion and penetration, haustoria for nutrient uptake, and complex lifecycles involving multiple hosts.
Fungi and Humans
Beneficial Uses
- Food and food production
- Edible mushrooms as direct food sources
- Yeasts in bread, beer, wine, and other fermented products
- Fungi in the ripening of cheeses and fermented soy products
- Industrial and biotechnological applications
- Production of organic acids, enzymes, vitamins, and other metabolites
- Use of fungal enzymes in detergents, food processing, and textiles
- Medicines
- Antibiotics (e.g., penicillins, cephalosporins) originally discovered in fungi
- Immunosuppressants and cholesterol-lowering drugs from fungal metabolites
- Ongoing role of fungal diversity in drug discovery
Harmful Aspects
- Food spoilage
- Molds on bread, fruit, and stored grains
- Production of mycotoxins (e.g., aflatoxins), which can be harmful or carcinogenic
- Plant disease and economic impact
- Major crop losses through fungal infections
- Need for plant protection strategies and resistant varieties
- Allergens and opportunistic infections
- Fungal spores as respiratory allergens
- Infections especially in immunocompromised individuals (outlined in disease chapters)
Fungi therefore play a double role for humans: indispensable partners in food and medicine, yet also potential threats in agriculture and health.
Life Strategies and Adaptations
Reproductive Strategies
- Massive spore production compensates for low individual survival probability
- Dormant spores survive:
- Drought
- Nutrient shortage
- Temperature extremes
- Many fungi coordinate fruiting body formation with favorable environmental cues (moisture, temperature, day length)
Environmental Tolerance
- Fungi occupy a wide range of habitats:
- Soil, freshwater, marine environments
- Dead wood, dung, leaf litter
- Extreme environments: cold, dry, nutrient-poor sites (especially lichens)
- Some tolerate:
- Low pH (acidic environments)
- High osmotic pressure (salty or sugary substrates, e.g., on jams, syrups)
Specialized Lifestyles
- Endophytes
- Fungi living inside plant tissues without obvious disease symptoms
- May increase host stress tolerance (e.g., against drought or herbivores)
- Insect-associated fungi
- Fungi that help leaf-cutter ants or termites digest plant material
- Entomopathogenic fungi that infect and sometimes manipulate insect behavior
These strategies highlight fungal flexibility and their integration into many ecological networks.
Summary
Fungi are a diverse kingdom of eukaryotic organisms characterized by:
- Filamentous or unicellular growth forms
- Chitinous cell walls
- Heterotrophic, absorptive nutrition
- Complex asexual and sexual reproduction with characteristic spores
As decomposers, pathogens, mutualists, and partners of humans in food and medicine, fungi are central players in ecosystems and in everyday life. Their reproductive strategies and symbioses underpin many of the processes discussed elsewhere in reproduction, growth, development, ecology, and health.