Fungi: Structure, Function, Diversity, and Ecological Roles

Fungi: Structure, Function, Diversity, and Ecological Roles

Introduction to Fungi

Fungi are a diverse kingdom of eukaryotic organisms that play essential roles in ecosystems as decomposers, mutualists, and pathogens. They are heterotrophs that feed by absorption, using hydrolytic enzymes to break down complex organic matter. Fungi exhibit a variety of body structures and reproductive strategies, and their evolutionary history reveals close relationships with animals.

Fungal Nutrition and Ecological Roles

Modes of Nutrition

• Heterotrophy: Fungi absorb nutrients from their environment by secreting enzymes that break down complex molecules into smaller, absorbable compounds.

• Decomposers: Break down nonliving organic material, recycling nutrients in ecosystems.

• Parasitic Fungi: Absorb nutrients from living hosts, often causing disease.

• Mutualistic Fungi: Form beneficial relationships with other organisms, exchanging nutrients for other resources or services.

Fungal Body Structure

Hyphae and Mycelium

The main body of most fungi consists of a network of filaments called hyphae. These hyphae form an interwoven mass known as the mycelium, which maximizes surface area for absorption. Fungi may also exist as single cells called yeasts, especially in moist environments rich in nutrients.

Cell Wall Composition

• Fungal cell walls are rich in chitin, a strong, flexible polysaccharide that prevents cell lysis during nutrient absorption.

Septa and Coenocytic Hyphae

Hyphae may be divided by cross-walls called septa, which have pores allowing movement of organelles. Some fungi lack septa, resulting in coenocytic hyphae—continuous cytoplasmic masses with many nuclei.

Specialized Hyphae

• Some fungi have specialized hyphae for feeding on living animals or for extracting nutrients from plants (e.g., haustoria).

Mycorrhizae and Arbuscules

Mutualistic associations between fungi and plant roots are called mycorrhizae. Specialized hyphae called arbuscules penetrate plant cell walls (but not membranes) to facilitate nutrient exchange.

Fungal Reproduction

Spore Production and Life Cycles

Fungi reproduce by producing vast numbers of spores, which can be dispersed by wind or water. Spores germinate in suitable environments to form new mycelia. Fungi may reproduce sexually, asexually, or both.

• Sexual Reproduction: Involves fusion of hyphae from different mating types, followed by plasmogamy (fusion of cytoplasm), a heterokaryotic stage (coexisting nuclei), karyogamy (fusion of nuclei), and meiosis to produce genetically diverse spores.

• Asexual Reproduction: Molds produce haploid spores by mitosis; yeasts reproduce by simple cell division or budding.

Evolution and Phylogeny of Fungi

Origins and Relationships

Fungi and animals are more closely related to each other than to plants. Both belong to the Opisthokonts clade, which evolved from a unicellular, flagellated ancestor. Multicellularity evolved independently in fungi and animals.

Major Fungal Lineages

Molecular analyses have clarified relationships among fungal groups. Major lineages include:

• Cryptomycetes

• Microsporidians

• Chytrids

• Zoopagomycetes

• Mucoromycetes

• Ascomycetes

• Basidiomycetes

Fungal Diversity: Key Groups

Cryptomycetes

• Unicellular, flagellated spores, found in diverse habitats, often parasites of protists and fungi.

• Can synthesize chitin-rich cell walls.

Microsporidians

• Unicellular parasites of protists and animals, with highly reduced mitochondria and genomes.

• Infect host cells using a harpoon-like organelle.

Chytrids

• Mostly aquatic, with flagellated spores (zoospores).

• Include decomposers, parasites, and mutualists.

Zoopagomycetes

• Parasitic or commensal on animals, fungi, or protists.

• Form filamentous hyphae and reproduce asexually via nonflagellated spores.

Mucoromycetes

• Includes molds such as Rhizopus stolonifer (bread mold).

• Reproduce sexually by forming zygosporangia, which are resistant to harsh conditions.

Ascomycetes

• Known as sac fungi; produce spores in saclike asci within fruiting bodies called ascocarps.

• Include yeasts, morels, truffles, and many plant pathogens.

• Reproduce asexually by conidia and sexually by ascospores.

Basidiomycetes

• Known as club fungi; include mushrooms, puffballs, and shelf fungi.

• Produce spores on club-shaped basidia within fruiting bodies called basidiocarps.

• Important decomposers of wood (lignin).

Fungi in Ecological Interactions

Decomposers

• Fungi efficiently decompose cellulose and lignin, recycling nutrients in ecosystems.

Mutualists

• Form mycorrhizal associations with plants, endophytes within plant tissues, and symbioses with animals (e.g., leaf-cutter ants).

Lichens

Lichens are symbiotic associations between fungi (usually ascomycetes) and photosynthetic organisms (algae or cyanobacteria). They are important pioneers on new surfaces and contribute to soil formation and nitrogen fixation.

Fungi as Pathogens

• About 30% of fungi are parasites, mainly of plants, causing significant agricultural losses.

• Some produce toxins (e.g., ergot on rye) that can affect humans and animals.

• Animal mycoses include ringworm and systemic infections.

Practical Uses of Fungi

• Fungi are used in food production (bread, cheese, alcoholic beverages).

• Source of antibiotics (e.g., penicillin from Penicillium).

• Medical applications (e.g., ergot derivatives for blood pressure control).

• Biotechnology: genetically engineered yeasts produce human proteins; fungi are being explored for biofuel production.

Summary Table: Major Fungal Groups

Group	Key Features	Reproduction	Ecological Role
Cryptomycetes	Unicellular, flagellated spores, chitin cell wall	Asexual, sexual (inferred)	Parasites of protists/fungi
Microsporidians	Unicellular, reduced mitochondria, harpoon organelle	Asexual	Parasites of animals/protists
Chytrids	Flagellated zoospores, aquatic	Asexual, sexual	Decomposers, parasites, mutualists
Zoopagomycetes	Filamentous hyphae, nonflagellated spores	Asexual, sexual (zygosporangium)	Parasites/commensals
Mucoromycetes	Coenocytic hyphae, zygosporangia	Asexual, sexual	Decomposers, pathogens, mutualists
Ascomycetes	Saclike asci, ascocarps, conidia	Asexual (conidia), sexual (ascospores)	Decomposers, pathogens, mutualists
Basidiomycetes	Club-shaped basidia, basidiocarps	Sexual (basidiospores)	Decomposers, mutualists, pathogens