Where’s the ‘Fun’ in Fungi? An Overview of Fungi and Their Role
Fungi are eukaryotic [have a nucleus], heterotrophic [utilize organic materials for their nutrition] organisms. In the past they have been considered to be a part of the plant kingdom but now are considered to comprise their own Kingdom.
Their form varies from single spheroidal cells to networks of microscopic tube-like structures covering vast expanses to macroscopic fruiting structures easily recognized as mushrooms. Some fungi survive extremes and of particular significance is some species’ ability to grow in dry and acidic conditions. Most fungi have a means of sexual reproduction but many important species appear to be asexual.
The group can be usefully divided into the yeasts, moulds, water moulds and higher fungi. Yeasts are typically unicellular though some, especially some medically significant species, form a filamentous habit in certain conditions. Common genera are Saccharomyces [in which bakers and brewers yeast fall] and Candida, the genus which causes thrush.
Water moulds are very numerous but rarely noticed except when they cause disease in fish and amphibians or plants. They have the animal characteristic of producing cells with flagella at some stage of their life-cycle. Saprolegnia, Achlya and Phytophthora are three of the more important genera of this group. Saprolegnia has become a very important pathogen in fish farming operations while Phytophthora causes potato blight and fatal wilt diseases in many species of plant. Chytrids are another sub-group here which have motile stages but eventually attach to some substrate [which can be aquatic or marine organisms] as vase-shaped unicells.
The Moulds are largely nondescript, often ‘hairy’ accretions to the naked eye. Microscopically they are networks of tube-like structures called hyphae that collectively form what is called a mycelium in which there is differentiation into specialized tissues and spores. Included amongst these are fruiting bodies, equivalent to mushrooms but on a microscopic or barely visible scale. Because of the large surface area, these organisms are incredibly effective in penetrating substrates and degrading them so form an essential element in regenerating soil nutrients. The important genera are too numerous to list.
From the left, spores of Alternaria; young head of Aspergillus; Cladosporium spores and hyphae
What one would recognize as mushrooms are mostly in the group called Basidiomycetes although some Ascomycetes also have mushroom-like fruiting bodies. The fruiting bodies are what we notice but they belie the business end of the organism which, like the moulds, is the mycelium forming a network in whatever the substrate the mushroom appears on. Rusts and smuts which look like moulds are also Basidiomycetes. The more obvious ‘mushroom’ part is merely a structure for producing and disseminating spores.
Perhaps because fungi go largely unnoticed, their ecological significance is often overlooked. Fungi are especially important in decay and recycling cycles. In an agricultural context their potent ability to produce enzymes that can degrade biological polymers [cellulose, lignins, starch etc.] causes huge losses of food before and after harvest. But they also form symbiotic relationships with trees, for example, in which many species can grow without producing any disease symptoms. On the other hand, some species are host-specific or general plant pathogens. Lichens are another form of fungal symbiosis in which a fungus and an alga collaborate to form a new organism. One lichen, Cladonia, provides the main source of food for reindeer. Other species, especially in the genus Usnia, produce fragrant volatile compounds essential to the perfume industry.
Asymptomatic fungi from coffee; sporing heads and sclerotia in Aspergillus
Although relatively simple in structure, fungi as a group produce a vast array of biologically active chemicals, sometimes referred to as ‘secondary metabolites’. Some are beneficial, antibiotics for example, while others are harmful, referred to as ‘mycotoxins’ often detected in foodstuffs. Several common species are also implicated in ‘sick building syndrome’. Species from the genera Aspergillus, Penicillium, Fusarium and Claviceps are both amongst the most common fungi and most significant in these contexts.
Fungi have many important industrial roles. They are utilized in food fermentations [wine / beer, bread, cocoa, tempe, tofu, cheese, to name a few examples] and certain organic compounds such as citric acid. Interestingly, many of the same genera and sometimes even the same species that produce important food-borne mycotoxins are used in food fermentations.
Remarkably few fungi are human pathogens but ‘athletes foot’, ‘thrush’ and ‘ring worm’ are well known. Others can be very serious and difficult to treat but fortunately rare. In recent times, however, fungi have taken on a greater medical significance with the increase of immune deficiency conditions, whether from HIV , cancer or auto-immune treatments, that depress the immune system. Numerous new infections caused by common moulds have been recorded in patients with immune deficiency.
The basidiomycete Pleurotus; foliose and fruiticose lichens in the Agulhas Negras mountains in Brasil