Fungi (Eukaryote)

Fungi are eukaryotic organisms that produce long filaments called hyphae, which form a mass called mycellium. Chitin is a characteristic component of the cell wall of hyphae. In most fungi, the hyphae are septate and contain crosswalls that divide the filament into separate cells containing one nucleus each. In some others,the hyphae are nonseptate and contain several nuclei. They are called coenocytic hyphae.

Fungi are heterotrophic organisms that include both macroscopic and microscopic forms. They use organic compounds as a carbon source and energy and thus play an important role in nutrient recycling in aquatic and soil environments. Some fungi form traps that capture protozoa and nematodes. They grow well under acidic conditions (pH 5) in foods, water, or wastewater. Most fungi are aerobic, although some (e.g.,yeast) can grow under facultatively anaerobic conditions. Fungi are significant components of the soil microflora, and a great number of fungal species are pathogenic to plants, causing significant damage to agricultural crops. A limited number of species are pathogenic to humans and cause fungal diseases called mycoses. Airborne fungal spores are responsible for allergies in humans. Fungi are implicated in several industrial applications, such as fermentation processes and antibiotic production (e.g., penicillin).

Identification of fungi is mainly based on the type of reproductive structure. Most fungi produce spores (sexual or asexual spores) for reproduction, dispersal, and resistance to extreme environmental conditions. Asexual spores are formed from the mycelium and germinate, producing organisms identical to the parent. The nuclei of two mating strains fuse to give a diploid zygote, which gives haploid sexual spores following meiosis.

There are four major groups of fungi:

. Phycomycetes

These fungi are known as the water molds and occur on the surface of plants and animals in aquatic environments. They have nonseptate hyphae and reproduce by forming a sac called sporangium, which eventually ruptures to liberate zoospores, which settle and form a new organism. Some phycomycetes produce sexual spores. There are also terrestrial phycomycetes, such as the common bread mold (Rhizopus), which reproduces asexually as well as sexually.

. Ascomycetes

Ascomycetes have septate hyphae. Their reproduction is carried out by sexual spores (ascospores) contained in a sac called an ascus (eight or more ascopores in an ascus), or asexual spores called conidia, which are often pigmented. Neurospora crassa is a typical ascomycete. Most of the yeasts (e.g., baker’s yeast Saccharomyces cerevisiae) are classified as ascomycetes. They form relatively large cells that reproduce asexually via budding or fission, and sexually by conjugation and sporulation. Some of these organisms (e.g., Candida albicans) are pathogenic to humans. Yeasts, especially the genus Saccharomyces, are important industrial microorganisms involved in bread, wine, and beer making.

. Basidiomycetes

Basidiomycetes also have a septate mycelium. They produce sexual spores called basidiospores on the surface of a structure called a basidium. Four basidiospores are formed on the surface of each basidium. Certain basidiomycetes, the wood-rotting fungi play a significant role in the decomposition of cellulose and lignin. Common edible mushrooms (e.g., Agaricus) belong to the basidiomycete group. Unfortunately, some of them (e.g., Amanita) are quite poisonous.

. Fungi imperfectii

Fungi imperfectii have septate hyphae but no known sexual stage. Some of them (e.g., Penicillium) are used for the commercial production of important antibiotics. These fungi cause plant diseases and are responsible for mycoses in animals and humans (e.g., athlete’s foot).