The Wonderful World of Fungi, Part II
Characteristics of Fungi
By: H. Wayne Shew, Ph.D.
NAB Certified Pollen Counter
The first blog of this series on fungi dealt with describing which organisms are properly called fungi and how these organisms are currently grouped taxonomically. Before writing blogs on the individual groups of fungi (divisions), I wanted to write a little on some of the general attributes of fungi and why they are such a fascinating group of organisms.
All fungi are eukaryotic (meaning they have a true nucleus with chromosomes located inside a membrane bound organelle). Fungi have a body called a thallus. The thallus may be unicellular as in the Chytrids, but most fungi have a somatic structure that is composed of thread-like structures called hyphae that anastomose and form a mycelium (the Zygomycetes, Ascomycetes, and Basidiomycetes). The mycelium composed of these thread-like structures called hyphae is the vegetative body of the fungus.
Most fungi are nonmotile; the exception being the Chytrids which do have a motile phase. Those fungi that are nonmotile must obtain new food sources through hyphal growth, similar to the way plants reach additional sources of water and minerals through growth of new roots.
Fungi show many features that are plant-like in nature, (some are considered below), but nutritionally they are quite different from plants. Plants produce food using water, carbon dioxide, and light. Fungi act as saprobes and secrete exoenzymes into the environment that digest large molecules into their molecular subunits which are then ingested by absorption through the hyphal wall.
The role that fungi play nutritionally in the environment is crucial in the recycling of materials in nature. Fungi are the major organisms involved in cellulose decomposition and are some of the only organisms capable of metabolizing lignin, a component of secondary plant cell walls. Without the fungi (and bacteria) we would quickly be overrun with accumulating plant and animal products.
Fungal cell structure is more like that of plant cells than animal cells since they have a cell wall of varying compositions, but principally composed of the molecule called chitin. (Yes, this is the same molecule that makes up the exoskeleton of insects and crustaceans.) Fungi are also like plants in that they exhibit an alternation of generations—possessing a gametophyte and a sporophyte phase. However, the molecular composition of fungal cell membranes is closer to that seen in animals. Fungal cell membranes do have a unique sterol, ergosterol, which serves the role that cholesterol does in mammalian cell membranes. Fungi are also like animals in storing glycogen as a reserve material rather than starch as is typically seen in plants.
Reproduction in fungi typically involves the formation of spores. Spores are haploid structures that are easily dispersed in the environment and which can germinate and give rise to hyphae. The hypha emerging from a germinating spore grows and branches and eventually forms a mycelium. Fungal spores are important to many people because many of them are easily wind-disseminated and can trigger allergic responses in individuals who inhale them and are sensitive to them. Fungal spores can also trigger asthma attacks, and some fungal spores can get into the lungs and lead to development of systemic mycoses, defined as growth of a fungus in various human organs.
Fungal spores may be asexual spores (mitospores produced by mitosis) or sexual spores (meiospores produced by meiosis). Some fungi produce both types of spores while other fungi form only one type. More will be said about reproduction when discussing the individual groups of fungi in subsequent blogs. It should also be pointed out that some fungi reproduce vegetatively by fragmentation of the fungal body. If a piece of the mycelium is broken off the main thallus and reaches a suitable habitat, it can give rise to an entirely new thallus.