Fungi

Figure 31.1: Symbolizes the grandeur of the kingdom Fungi.
Fungi are eukaryotes, often overlooked except for mushrooms and athlete’s foot.
They are a significant component of the biosphere, with both single-celled and complex multicellular forms.
Found in diverse terrestrial and aquatic habitats.
Essential for ecosystem health by breaking down organic material and recycling nutrients.

Fungi feed by absorption, not ingestion.
They secrete hydrolytic enzymes to break down complex molecules into absorbable compounds.
Enzymes can penetrate cell walls to absorb nutrients.
Collectively, the different enzymes found in various fungal species can digest compounds from a wide range of sources, living or dead.
This diversity of food sources corresponds to the varied roles of fungi in ecological communities: decomposers, parasites, or mutualists.

Decomposers: Break down and absorb nutrients from nonliving organic material, such as fallen logs, animal corpses, and the wastes of organisms.
Parasitic Fungi: Absorb nutrients from the cells of living hosts. Some parasitic fungi are pathogenic, causing diseases in plants and animals.
Mutualistic Fungi: Absorb nutrients from a host but reciprocate with actions that benefit the host. For example, mutualistic fungi in termite guts break down wood.

Multicellular Filaments and Yeasts: Common fungal structures.
Hyphae: Tubular cell walls surrounding plasma membrane and cytoplasm.
Chitin: Strengthens cell walls, aiding in nutrient absorption.
Septa: Cross-walls dividing hyphae into cells, allowing organelle flow.
Coenocytic Fungi: Lack septa, forming a continuous cytoplasmic mass.
Mycelium: Interwoven mass of hyphae, maximizing surface-to-volume ratio for efficient feeding.

Haustoria: Modified hyphae extracting nutrients from plants.
Arbuscules: Specialized branching hyphae for nutrient exchange with plant hosts.
Mycorrhizae: Symbiotic relationships between fungi and plant roots, enhancing nutrient uptake.
Mycorrhizal fungi can improve delivery of phosphate ions and other minerals to plants because the vast mycelial networks of the fungi are more efficient than the plants’ roots at acquiring these minerals from the soil. In exchange, the plants supply the fungi with organic nutrients such as carbohydrates.
There are two main types of mycorrhizal fungi: ectomycorrhizal fungi and arbuscular mycorrhizal fungi.

Spores: Produced in vast numbers, dispersed by wind or water. If they land in a moist place where there is food, they germinate, producing a new mycelium.
Sexual Reproduction: Involves plasmogamy (cytoplasm fusion) and karyogamy (nuclear fusion), followed by meiosis. The nuclei of fungal hyphae and the spores of most fungi are haploid, although many species have transient diploid stages that form during sexual life cycles.
Asexual Reproduction: Includes spore production by mitosis, budding in yeasts, and formation of conidia in ascomycetes. Many fungi reproduce asexually by growing as filamentous fungi that produce (haploid) spores by mitosis; such species are informally referred to as molds if they form visible mycelia.

Fungi evolved from a flagellated ancestor, closely related to animals.
Opisthokonts: Clade including fungi, animals, and their protistan relatives.
Nucleariids: Unicellular protists closely related to fungi.
Fungi likely colonized land before plants, forming early symbiotic relationships.
Phylogenetic analyses suggest that fungi evolved from a flagellated ancestor. While the majority of fungi lack flagella, two basal lineages of fungi (the cryptomycetes and the chytrids) do have flagella.

Cryptomycetes and Microsporidians: Basal fungal lineages, often parasitic. Cryptomycetes are unicellular and have flagellated spores. Microsporidians are unicellular parasites of protists and animals, including humans.
Chytrids: Ubiquitous in aquatic environments, some decomposers, others parasites. Nearly all chytrids have flagellated spores, called zoospores.
Zoopagomycetes: Parasites or commensal symbionts of animals. They form filamentous hyphae and reproduce asexually by producing nonflagellated spores.
Mucoromycetes: Includes fast-growing molds and mycorrhizal fungi. The life cycle of Rhizopus stolonifer (black bread mold) is typical of mucoromycete species.
Ascomycetes: Sac fungi, producing spores in asci, important decomposers and pathogens. Ascomycetes reproduce asexually by producing enormous numbers of asexual spores called conidia.
Basidiomycetes: Club fungi, including mushrooms, puffballs, and shelf fungi, important decomposers. The life cycle of a basidiomycete usually includes a long-lived dikaryotic mycelium.

Decomposers: Essential for nutrient cycling. Fungi are well adapted as decomposers of organic material, including the cellulose and lignin of plant cell walls.
Mutualists: Form symbiotic relationships with plants, algae, cyanobacteria, and animals. Mutualistic fungi absorb nutrients from a host organism but reciprocate with actions that benefit the host.
Parasites: Cause diseases in plants and animals. About 30% of the 145,000 known species of fungi make a living as parasites or pathogens, mostly of plants.

Food: Mushrooms, truffles, and yeasts for baking and brewing. Fungi are used to ripen Roquefort and other blue cheeses.
Medicine: Antibiotics like penicillin, cholesterol-lowering drugs, and immunosuppressants. Some fungi produce antibiotics that are effective in treating bacterial infections.
Research: Model organisms for studying genetics and biotechnology applications. The yeast Saccharomyces cerevisiae is used to study the molecular genetics of eukaryotes because its cells are easy to culture and manipulate.