Table of Contents
Overview of Fungi as a Kingdom
Fungi form one of the main kingdoms within the domain Eukarya. They are neither plants nor animals, although historically they were often grouped with plants. Today, fungi are recognized as a separate lineage with their own characteristic structures, life cycles, and ecological roles.
Fungi are especially important as decomposers, symbionts, and pathogens. They are also of great practical relevance: they provide food (e.g. mushrooms), beverages and bread (yeasts), medicines (e.g. penicillin), and industrial enzymes, but they can also cause crop failures and diseases in plants, animals, and humans.
Key features that distinguish fungi as a kingdom include:
- Eukaryotic cells with nuclei and membrane-bound organelles
- Cell walls containing chitin (not cellulose as in plants)
- Heterotrophic nutrition, usually by absorption
- Growth mainly as filamentous hyphae forming a mycelium, or as unicellular yeasts
- Reproduction by spores, sexually and/or asexually
- Life cycles often involving alternation of haploid, dikaryotic, and diploid stages
This chapter focuses on what is specific to fungi as a kingdom, not on general eukaryote features or on detailed molecular aspects that are treated elsewhere.
Fundamental Characteristics of Fungi
Nutrition and Lifestyle
Fungi are chemoheterotrophs: they obtain both energy and carbon from organic compounds, but unlike animals, they do not ingest food. Instead, they absorb small molecules from their surroundings.
Typical nutritional modes:
- Saprotrophs (decomposers)
- Use dead organic material: leaf litter, wood, dung, dead animals.
- Secrete extracellular enzymes (e.g. cellulases, ligninases, proteases) that break large polymers into absorbable units (sugars, amino acids).
- Essential for nutrient cycling in ecosystems, especially carbon and nitrogen.
- Parasites and pathogens
- Tap nutrients from living hosts, often causing disease.
- Plant pathogens: rusts, smuts, mildews, wilts.
- Animal and human pathogens: skin infections, systemic mycoses.
- Many have specialized structures, such as haustoria (penetrating feeding structures) in plant tissues.
- Mutualistic symbionts
- Both partners benefit.
- Mycorrhizae: fungi with plant roots (see below).
- Lichens: fungi with photosynthetic partners (algae or cyanobacteria).
- Endophytes: fungi living inside plant tissues without causing disease, often enhancing stress tolerance.
In all cases, fungi feed by absorptive nutrition: enzymes are secreted outward, digestion happens outside the fungal cells, and the resulting small molecules are taken up through the cell wall and membrane.
Cell Walls and Storage Substances
- Cell wall composition
- Main structural component: chitin, a nitrogen-containing polysaccharide also found in arthropod exoskeletons.
- Other components: glucans and proteins.
- This composition makes fungal walls rigid but flexible and differentiates them from plant and bacterial walls.
- Storage substances
- Fungi commonly store energy as glycogen, like animals, not as starch as in most plants.
- Lipid droplets can also be important energy reserves.
Body Forms: Hyphae and Mycelium, Yeast Form
Most fungi have a characteristic filamentous body.
- Hyphae
- Long, thin filaments of fungal cells.
- Grow at their tips (apical growth) by adding new cell wall and membrane.
- Can branch to explore and colonize substrates.
- Mycelium
- The mass of interconnected hyphae that makes up the main body (thallus) of the fungus.
- Often hidden in soil, wood, or other substrates.
- Large surface area relative to volume, ideal for absorption.
- Septate vs. coenocytic hyphae
- Septate hyphae: divided by cross walls (septa) into compartments; septa usually have pores for cytoplasmic continuity.
- Coenocytic (aseptate) hyphae: lack regular septa and contain many nuclei in a common cytoplasm.
- Yeasts
- Unicellular fungi that typically reproduce by budding or fission.
- Often adapted to liquid or moist environments (e.g. plant saps, animal surfaces, food).
- Many dimorphic fungi can switch between yeast and filamentous forms depending on environmental conditions.
Reproduction and Life Cycles
Fungal reproduction relies heavily on spores, which are usually microscopic, resistant units that can disperse via air, water, or animals.
Asexual Reproduction
Asexual processes do not involve nuclear fusion or meiosis and produce genetically identical offspring (clones).
Common asexual processes:
- Mitotic spores (conidia, sporangiospores, etc.)
- Produced by mitosis on specialized hyphae or in sporangia.
- Often numerous and well adapted for dispersal.
- Fragmentation
- Mycelium breaks into pieces; each fragment can regenerate a new mycelium under suitable conditions.
- Budding and fission in yeasts
- New cells form by budding off from a mother cell or by splitting.
Asexual reproduction allows fungi to exploit favorable conditions quickly and colonize new substrates efficiently.
Sexual Reproduction
Sexual reproduction in fungi is diverse but involves three fundamental steps:
- Plasmogamy: fusion of cytoplasm from two compatible fungal cells (often from different mating types).
- Karyogamy: fusion of nuclei to form a diploid nucleus.
- Meiosis: reduction division producing genetically diverse haploid spores.
A distinctive fungal feature is the possible existence of a dikaryotic phase:
- After plasmogamy, nuclei from each parent may remain separate for an extended period.
- Each cell contains two genetically distinct nuclei ($n + n$), not yet fused.
- Karyogamy occurs later, often in specialized reproductive structures; meiosis follows.
Fungal life cycles can be:
- Predominantly haploid (most of the life cycle is haploid, with a brief diploid stage around karyogamy and meiosis).
- With a prominent dikaryotic stage, especially in some major groups (e.g. many mushrooms).
Details of particular life cycles differ between major fungal lineages and are treated under those groups, not here.
Major Ecological Roles of Fungi
Decomposers (Saprotrophs)
Fungi are among the primary decomposers of complex plant materials.
- Capable of degrading lignin and cellulose, which most animals cannot digest.
- Break down wood, leaf litter, and other plant residues, returning carbon, nitrogen, and minerals to soil and atmosphere.
- Enable nutrient cycling and soil formation, especially in forests and grasslands.
- Some specialized decomposers target keratin (hair, nails), dung, or even paint and plastics.
Without fungal decomposers, ecosystems would accumulate undecomposed organic matter and nutrient cycles would stall.
Symbiotic Relationships
Mycorrhizae
Mycorrhizae are mutualistic associations between fungal hyphae and plant roots.
Basic principles:
- Fungal partner increases root surface area and penetrates soil pores inaccessible to roots.
- Fungus enhances uptake of water and mineral nutrients (especially phosphorus and some micronutrients).
- Plant supplies the fungus with carbohydrates from photosynthesis.
Main structural types:
- Ectomycorrhizae
- Fungal hyphae form a sheath around fine roots and penetrate between, but not into, root cells.
- Common in many trees (e.g. pines, oaks, beeches).
- Often associated with mushrooms that form conspicuous fruiting bodies.
- Endomycorrhizae (arbuscular mycorrhizae)
- Hyphae penetrate into root cortex cells and form highly branched structures (arbuscules) for nutrient exchange.
- Very widespread in herbaceous plants and many crops.
Ecological significance:
- Enhance plant growth and survival, particularly in nutrient-poor or dry soils.
- Influence plant community structure and productivity.
- Help stabilize soils and contribute to carbon storage.
Lichens
Lichens are symbiotic associations between a fungus (usually an ascomycete) and a photosynthetic partner (green alga and/or cyanobacterium).
Functional roles:
- Fungus provides structure, protection from desiccation and radiation, and supplies water and minerals.
- Photosynthetic partner supplies organic carbon (and sometimes nitrogen if cyanobacteria are present).
Characteristics:
- Able to colonize extreme environments: bare rock, arctic tundra, deserts, high mountains.
- Important pioneers in primary succession (e.g. on volcanic rock, glacial deposits).
- Contribute to rock weathering and soil formation.
- Sensitive to air pollution, often used as bioindicators of air quality.
Endophytes and Other Symbioses
- Endophytic fungi live within plant tissues (leaves, stems) without causing disease.
- May improve host resistance to herbivores, pathogens, heat, or drought.
- Some grasses rely on endophytes for resistance to grazing due to fungal toxins that deter herbivores.
- Fungal symbioses with animals:
- Fungi in the guts of some herbivores help digest plant material.
- Leaf-cutter ants cultivate fungi as their primary food source.
- Termites with cultivated fungi contribute to wood decomposition.
Pathogens
Fungi can also negatively affect hosts.
- Plant pathogens
- Cause rusts, smuts, blights, wilts, mildews, and rots.
- Major contributors to crop losses (e.g. rust diseases in cereals, late blight of potato, smut in maize).
- Influence natural plant populations and community composition.
- Animal and human pathogens
- Skin infections (e.g. ringworm, athlete’s foot).
- Mucosal infections (e.g. candidiasis).
- Systemic mycoses, especially in immunocompromised individuals.
- Fungi can cause devastating declines in wild animals (e.g. chytrid fungal infections in amphibians).
- Mycotoxins
- Some fungi produce toxic secondary metabolites that contaminate food and feed (e.g. aflatoxins from Aspergillus species).
- These toxins can be carcinogenic, neurotoxic, or otherwise harmful even at low doses.
Diversity and Systematics of Fungi
Modern systematics recognizes several major lineages within the traditional kingdom of fungi. Molecular data have reshaped our understanding of their relationships, and some groups once treated as fungi are now placed elsewhere.
Key points for a beginner-level classification:
- Fungi within Eukarya are a monophyletic group (share a common ancestor distinct from plants, animals, and protists).
- Some organisms historically grouped with fungi (e.g. slime molds, water molds) are not true fungi and belong to other lineages.
Within the true fungi, several large groups (phyla) are recognized. Without going into detailed taxonomy, it is useful to know:
- Some groups are primarily aquatic and have flagellated spores.
- Others are primarily terrestrial and produce complex multicellular fruiting bodies (mushrooms, morels, truffles, etc.).
- The majority of known fungi belong to a few major phyla that include most of the familiar forms (yeasts, molds, mushrooms, lichens).
Exact classification and the names of these phyla, as well as their phylogenetic relationships, are handled in more specialized treatments of fungal systematics.
Economic and Practical Importance of Fungi
Beneficial Uses
- Food and beverages
- Edible mushrooms (cultivated and wild).
- Yeasts in baking (bread rising) and fermentation of beverages (beer, wine).
- Fungal fermentations in traditional foods (e.g. some cheeses, soy sauces, tempeh).
- Medicine
- Discovery of antibiotics (e.g. penicillin from Penicillium, many other antibiotics from diverse fungi).
- Production of immunosuppressive drugs, cholesterol-lowering agents, and other pharmaceuticals.
- Model organisms for biomedical research (e.g. yeasts in genetics and cell biology).
- Biotechnology and industry
- Enzymes for detergents, food processing, and textile treatments.
- Fungi used in bioremediation to degrade pollutants.
- Production of organic acids, vitamins, and other valuable metabolites.
Harmful Impacts
- Agricultural losses
- Plant disease epidemics leading to crop failure and famine.
- Post-harvest spoilage of stored grains, fruits, and vegetables.
- Economic costs in plant protection and yield reduction.
- Human and animal health
- Fungal infections (mycoses), especially problematic in immunocompromised patients.
- Allergies to fungal spores.
- Exposure to mycotoxins in contaminated food or feed.
- Material damage
- Decomposition of wood structures and timber.
- Damage to textiles, paper, and artworks under humid conditions.
Understanding fungal biology is therefore crucial in agriculture, medicine, environmental management, and industry.
Fungi in Ecosystems and Evolution
Fungi play central roles in ecosystems and have had major evolutionary impacts:
- They accelerated the breakdown of lignified plants when vascular plants first colonized land, influencing global carbon cycles.
- Mycorrhizal associations were likely critical for the early success of land plants.
- Lichen symbioses enabled life on bare rock and in harsh environments, shaping the development of soils and early terrestrial ecosystems.
- By driving plant disease and stress, fungi act as evolutionary pressures, shaping plant defenses and coevolutionary dynamics.
From an evolutionary and systematic perspective, fungi are thus a key group for understanding both the diversity of life and the functioning of the biosphere as a whole.