Fungi are a diverse kingdom of eukaryotic organisms that includes yeasts, molds, mushrooms, rusts, smuts, and many microscopic lineages (Webster & Weber, 2007). They are neither plants nor animals. Unlike plants, fungi do not photosynthesize. Instead, they release enzymes into their surroundings and absorb dissolved nutrients. This mode of nutrition allows them to act as decomposers, symbiotic partners, parasites, pathogens, and industrial organisms across nearly every ecosystem.
Older introductory textbooks often divided fungi into a small number of groups such as Chytridiomycota, Zygomycota, Ascomycota, Basidiomycota, and Deuteromycota. That scheme is now obsolete. Molecular phylogenetics and whole-genome analysis have shown that “Zygomycota” and “Deuteromycota” do not represent natural evolutionary groups. Modern fungal classification recognizes multiple early-diverging lineages and continues to change as environmental sequencing and genome-scale studies reveal previously unknown relationships (Spatafora et al., 2017; Naranjo-Ortiz & Gabaldón, 2019).
Structure and Classification
Most fungi have cell walls containing chitin and glucans and cell membranes containing ergosterol. These features distinguish them from plants and animals and also have clinical importance because several antifungal drugs target ergosterol synthesis or membrane function. Fungi generally store carbohydrates as glycogen rather than starch, another trait that reflects their closer evolutionary relationship to animals than to plants.
Filamentous fungi grow as microscopic threads called hyphae. A mass of interconnected hyphae forms a mycelium. Hyphae may be divided by cross walls called septa or may contain multiple nuclei in a continuous cytoplasm. Yeasts usually grow as individual cells and reproduce by budding or fission, although some species can form pseudohyphae or switch between yeast and filamentous growth depending on environmental conditions.
Modern classification relies increasingly on DNA and genome comparisons rather than one visible trait. Current research agrees that the old five-group system oversimplified fungal evolution, but the exact number and rank of recognized phyla can vary among taxonomic treatments. A widely cited phylogenomic framework recognizes lineages including Chytridiomycota, Blastocladiomycota, Neocallimastigomycota, Zoopagomycota, Mucoromycota, Glomeromycota, Ascomycota, and Basidiomycota, while some treatments also recognize additional early-diverging groups or use different rank assignments.
Chytrids are unusual among fungi because many produce flagellated zoospores and live in aquatic or moist environments. Some act as decomposers, while others are pathogens. Batrachochytrium dendrobatidis and related chytrids have caused severe amphibian population declines, demonstrating that microscopic fungal disease can alter entire ecosystems.
Organisms once grouped together as “zygomycetes” are now divided among lineages such as Mucoromycota and Zoopagomycota. Members of the order Mucorales include common fast-growing molds and medically important species that can cause mucormycosis, especially in immunocompromised or critically ill patients.
Glomeromycotan fungi form arbuscular mycorrhizal relationships with plant roots. These fungi grow into root cortical cells and form highly branched structures called arbuscules that increase the exchange surface between plant and fungus. The plant supplies carbon compounds, while the fungus helps the plant acquire phosphorus, water, and other soil resources.
Ascomycota is the largest described fungal group and includes yeasts, morels, truffles, many molds, lichen-forming fungi, plant pathogens, and human pathogens such as Candida and Aspergillus. Sexual spores develop in saclike structures called asci, although asexual conidia may dominate the visible life cycle.
Basidiomycota includes many familiar mushrooms as well as puffballs, shelf fungi, rusts, smuts, and yeasts such as Cryptococcus. Sexual spores are produced on basidia. Many basidiomycetes are important wood decomposers because they possess enzymes capable of breaking down lignin, while others form ectomycorrhizal partnerships with forest trees.
Reproduction and Growth
Fungi reproduce through asexual, sexual, or mixed strategies. Asexual reproduction can occur through budding, fragmentation, fission, sporangiospores, or conidia. This mode allows rapid colonization when environmental conditions are favorable. A clone may spread efficiently without requiring a compatible mating partner.
Sexual reproduction generally involves plasmogamy, in which cytoplasm from compatible cells fuses, followed by a stage in which genetically distinct nuclei may coexist. Karyogamy later fuses nuclei, and meiosis restores haploid nuclei while generating genetic variation. The sequence differs across lineages, so one universal “alternation of haploid and diploid generations” does not accurately describe all fungi.
In many ascomycetes and basidiomycetes, the diploid phase is brief. A dikaryotic phase, in which two genetically distinct nuclei share the same cell without fusing immediately, can be much more prominent. Some yeasts can exist for extended periods as either haploid or diploid cells. Fungal life cycles therefore need to be understood according to lineage rather than forced into a plant-style model.
Environmental conditions strongly affect fungal growth. Moisture, temperature, oxygen, substrate chemistry, pH, competition, and available nutrients all influence metabolism. Some fungi tolerate extreme cold or dryness, while thermophilic species grow at high temperatures. Dimorphic pathogens may shift between yeast-like and filamentous forms depending on temperature or host environment.
Fungal spores are adapted for dispersal by air, water, animals, or physical disturbance. A single fruiting body can release enormous numbers of spores, but successful colonization depends on whether they reach an appropriate substrate and environment. Spore production therefore increases opportunity rather than guaranteeing growth.
Ecological Roles
Fungi are central to decomposition and nutrient cycling (Watkinson et al., 2016). Their extracellular enzymes break down plant litter, wood, dung, and dead organisms, releasing carbon, nitrogen, phosphorus, and other nutrients that can be reused by plants and microorganisms. Without fungal decomposition, large amounts of organic material would accumulate and nutrient cycles would slow dramatically.
Wood decay illustrates their special ecological role. Many fungi can degrade cellulose and hemicellulose, while some basidiomycetes are particularly effective at breaking down lignin, one of the most resistant components of plant cell walls. This capability gives fungi a major role in forest carbon cycling.
Mycorrhizal symbiosis is equally important. Fungal hyphae extend beyond the immediate root zone and increase access to nutrients and water. In exchange, plants transfer photosynthetically fixed carbon to the fungus. Arbuscular and ectomycorrhizal relationships influence forest productivity, seedling establishment, soil structure, drought response, and competition among plants.
Fungi also form lichens with photosynthetic algae or cyanobacteria. Lichens can colonize rock, bark, deserts, polar regions, and other harsh environments. They contribute to weathering and soil formation and can serve as indicators of air quality. Endophytic fungi live within healthy plant tissues and may alter growth, chemical defense, drought tolerance, or disease resistance.
Not all fungal interactions are beneficial. Plant-pathogenic fungi cause rusts, smuts, blights, wilts, rots, and mildews that reduce crop yield and reshape natural communities. Animal pathogens affect insects, amphibians, bats, livestock, and humans. Fungi therefore regulate populations while also creating conservation and food-security problems.
Climate and land-use change can alter fungal distribution. Temperature, drought, flooding, wildfire, agriculture, and movement of plants or soil can change where fungi occur and how spores disperse. These effects are complex and species-specific, but environmental surveillance is becoming increasingly important as fungal pathogens expand into new regions.
Human Health
Humans use fungi extensively. Yeasts produce bread, beer, and wine. Filamentous fungi contribute to cheese, soy-based fermentation, enzymes, organic acids, vitamins, antibiotics, and other industrial products. Fungal enzymes are used in food processing, textiles, paper production, and biotechnology. Mushrooms are cultivated as food, and fungi such as Saccharomyces cerevisiae are major model organisms in genetics and cell biology.
Fungi can also cause disease. Superficial infections affect skin, hair, nails, and mucosal surfaces. Endemic dimorphic fungi can cause serious respiratory or systemic disease following environmental exposure, while opportunistic invasive infections are especially dangerous in people with neutropenia, cancer treatment, organ transplantation, advanced HIV, critical illness, or other immunosuppressive conditions.
The World Health Organization’s fungal priority pathogens initiative highlights the increasing public-health importance of invasive fungal disease and antifungal resistance. WHO’s 2025 diagnostic landscape report estimates that invasive fungal infections affect millions of people globally each year and notes major gaps in rapid, affordable, and accessible diagnostics, particularly in low- and middle-income countries. The 2022 priority list includes critical pathogens such as Cryptococcus neoformans, Candida auris, Aspergillus fumigatus, and Candida albicans (World Health Organization, 2022, 2025).
Diagnosis may involve microscopy, culture, histopathology, antigen tests, antibody tests, molecular assays, or mass spectrometry. A positive result must be interpreted in clinical context because some fungi can colonize skin or mucosa without causing invasive disease. Conversely, culture can be negative even when infection is severe.
Antifungal treatment is challenging because fungi and humans are both eukaryotes, making it harder to find drug targets that damage the pathogen without harming the patient. Existing drug classes include azoles, echinocandins, polyenes, and others, but resistance is increasing and treatment options remain limited for some infections. WHO’s 2025 pipeline analysis emphasized that the development of new antifungal medicines and diagnostics remains insufficient relative to global need.
Agricultural fungicide use also has One Health implications. Related chemical compounds may select resistance mechanisms in environmental fungi that later affect human medicine. This is particularly important for azole resistance in Aspergillus fumigatus. Human, agricultural, animal, and environmental health therefore cannot be managed independently.
Scientific Importance
Fungi demonstrate why biological classification changes as evidence improves. Visible morphology once placed unrelated organisms together, while molecular and genomic methods revealed deeper evolutionary relationships. The abandonment of Deuteromycota and the breakup of the old Zygomycota concept are clear examples of taxonomy being revised rather than simply expanded.
Fungal biology also connects cell structure with ecological function. Hyphae allow fungi to penetrate substrates, explore soil, and connect distant nutrient patches. Extracellular digestion allows them to decompose resistant material. Symbiotic interfaces such as arbuscules allow nutrient exchange with plants. Dimorphic growth allows some pathogens to adapt to different environments.
The kingdom’s importance therefore extends far beyond mushrooms and visible molds. Fungi recycle nutrients, support forests and crops, shape animal and plant populations, produce food and medicines, and cause diseases that are increasingly recognized as major public-health threats. Understanding fungi requires integrating classification, cellular biology, reproduction, ecology, evolution, and medicine rather than treating them as separate topics.
References
Naranjo-Ortiz, M. A., & Gabaldón, T. (2019). Fungal evolution: Diversity, taxonomy and phylogeny of the Fungi. Biological Reviews, 94(6), 2101–2137.
Spatafora, J. W., Chang, Y., Benny, G. L., et al. (2017). A phylum-level phylogenetic classification of zygomycete fungi based on genome-scale data. Mycologia, 108(5), 1028–1046.
Watkinson, S. C., Boddy, L., & Money, N. P. (Eds.). (2016). The Fungi (3rd ed.). Academic Press.
Webster, J., & Weber, R. (2007). Introduction to Fungi (3rd ed.). Cambridge University Press.
World Health Organization. (2022). WHO Fungal Priority Pathogens List to Guide Research, Development and Public Health Action.
World Health Organization. (2025). Landscape Analysis of Commercially Available and Pipeline In Vitro Diagnostics for Fungal Priority Pathogens.
World Health Organization. (2025). Antifungal Agents in Clinical and Preclinical Development: Overview and Analysis.
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