The phylum Ascomycota is divided into 6 classes, 18 sub-classes and 55 orders. The classes are based on the development of the ascomata, the structure of asci and the discharge of ascospores. The 6 classes are; Ascomycetes, Neolectomycetes, Pneumocystidomycetes, Saccharomycetes, Schizosaccharomycetes and Taphrinomycetes. We shall study only 2 classes – Saccharomycetes (which contains the genus Saccharomyces) and Ascomycetes (which contains the genera Uncinula Eurotium, Eupenicillium, Peziza, Morchella and Chaetomium).
Genus Saccaromyces (The Yeasts):
The term ‘yeast’ refers to ascomycetes that are unicellular and reproduce asexually by fission or budding and form naked asci not enclosed in ascoma. There are ten species in this genus, the most important being S. cerevisiae, the Brewers’ and Bakers’ yeast, also called as the ‘yeast of commerce’.
Saccharomyces Cerevisiae (Brewers’ and Bakers’ Yeast):
Man has known bread and wine since antiquity but not Saccharomyces cerevisiae. However, fermentation, which is the conversion of sugar into alcohol, attracted the attention of the best minds in the latter half of the 19th century. This naturally gave a place of pride to yeast.
Louis Pasteur, the great French scientist, contended that fermentation was brought about by the life activities of yeast, which grew and increased in dry weight in sugar solutions. “No fermentation without life”, claimed Pasteur. Leibig, a German chemist, challenged Pasteur’s findings and claimed that the ferment was an extraordinarily labile organic substance formed by interaction of air with something present in plant juices.
Two years after Pasteur’s death, Buchner (in 1897) produced cell-free yeast extract, which brought about the fermentation. The extract, though derived from yeast, was lifeless; something like cell-free extract of brain capable of thinking! Later, the name zymase was given to the chemical that caused fermentation. It was an enzyme, which literally means ‘in yeast’. This discovery of enzyme gave birth to the science of Biochemistry. Thus, Biochemistry owes as much to man’s thirst for alcohol as chemistry owes to his greed of making gold (alchemy).
Yeasts, because of their ability to ferment sugar, are named Saccharomyces (sugar fungi). In a well-aerated sugar solution, the yeast grows and undergoes normal aerobic respiration producing carbon dioxide and water. However, in the absence of oxygen, the aerobic respiration ceases and fermentation takes place resulting in the production of ethyl alcohol and carbon dioxide. In the brewing industry, alcohol is the desired product, whereas in bakeries, carbon dioxide is utilized to make the bread porous and light.
Yeast is the richest source of Vitamin-B Complex, and dried yeasts in the form of tablets, are used as a nutritional supplement in medical therapy.
Habitat – Yeasts occur abundantly on substrates rich in sugars, such as the surface of fruits and nectar of flowers, etc.
Thallus – Yeasts are unicellular organisms of all possible shapes – circular, oval, elongated, rectangular, dumb-bell shaped, and triangular. Individually, the cells are colourless but colonies appear white, cream- coloured, or light brown. The colony characteristics, along with physiological reactions, are used in the identification of species. The colonies of yeasts strongly resemble bacterial colonies.
The yeast cell has a cell wall which consists of mannan and glucan, in addition to chitin. Inside, there is a big vacuole which occupies a large portion of the cell. The nucleus lies on one side of the vacuole and very close to it. The cytoplasm, in addition to the different usual cell organelles, contains glycogen, oil, and refractile volutin granules (an inorganic metaphosphate polymer) as reserve materials.
The vacuole was previously considered to be a part of the nucleus but the electron microphotographs have conclusively demonstrated that the nucleus and the vacuole are separate entities, each surrounded by its own membrane.
S. cerevisiae shows an alternation of haploid and diploid generations which is a rare thing in fungi (another example is Allomyces, an aquatic Chytridiomycete). Each generation is perpetuated by budding and is equally extensive and important.
In budding, a small protuberance appears at one or more places on the cells, which grow into daughter cells. The nucleus divides by constriction; the nuclear envelope does not break. Chromosomes are not seen even in electron microphotographs.
The daughter nucleus enters the bud along with the other organelles. Before the daughter cell is pinched off by the formation of a wall, it produces another cell and in this way a chain of cells is produced during active growth. The chains appear like a short hypha.
The haploid cells of (+) and (-) mating types grow independently and multiply by budding. After a period of growth two haploid ceils of opposite strains copulate to form a diploid cell. The diploid yeast cells are bigger and their colonies grow faster. The diploid cells also increase their number by budding, until the conditions for a change to haplophase arise. The diploid cells act as asci.
The diploid nucleus undergoes meiosis and four haploid nuclei, two of each strain, are produced. Ascospores are formed incorporating these nuclei. The ascospores are liberated by breaking of the ascus wall. Each ascospore resembles a yeast cell though much smaller. They grow into haploid cells and start budding to establish the haploid phase, thus completing the life cycle.
Genus Uncinula (Powdery Mildew of Grapes):
Uncinula nector causes much damage to grapes and frequently destroys the whole crop. In 1850, it caused unprecedented damage in France and attracted much attention. Millardet’s accidental discovery of Bordeaux mixture (a mixture of quick lime, copper sulphate and water in the ratio 4: 4: 50) was related to this disease. The disease occurs in India also. In addition to forming the powdery coating of conidia on leaves, stems, and fruits, it also causes stunting of plants, deformation and mummification of fruits.
The much-branched, septate hyphae consist of short uninucleate cells, which spread over the leaf surface. Globular haustoria enter the epidermal cells and draw nutrition from them.
Genus Eurotium (Aspergillus as the Conidial Stage):
Introduction and Importance:
Due to dominance of the conidial stage, the fungus continues to call as Aspergillus, though technically it should be referred as Eurotium. The genus, though widespread in distribution, is more prevalent in tropical countries. Conidia of Aspergillus are always present in the air and cause contamination in laboratory cultures of bacteria and fungi. A. niger is called ‘weed of the laboratory’.
The various species are of great importance because of their harmful as well as useful activities. When Aspergillus infects lungs (Pulmonary Aspergillosis) the symptoms resemble tuberculosis. In England, in the year 1960, about 100,000 Turkey poults died by eating groundnuts infected with Aspergillus flavus. A toxin called aflatoxin, produced by A. flavus, was responsible for the poultry deaths.
Aflatoxins are, toxic to human beings also. Strains of Aspergillus niger are used in the manufacture of citric acid, gluconic and itaconic acids. Species of Aspergillus are used (as for Mucor and Rhizopus) for hydrolysis of starch into glucose in alcoholic fermentation by yeast. Aspergillus nidulans is the historical fungus in which the parasexual phenomenon was discovered in 1952 by Pontecorvo and Roper.
Species of Aspergillus are found in any conceivable habitat because of the bizarre enzymes that they produce. They cause spoilage of stored products, such as grain, food stuffs – including jams and jellies, salted meat, fish and leather. They are the ‘green molds’ that cover the shoes or leather goods in humid weather.
Some species are weak parasites causing diseases of fruits during storage and transit.
Members of the Aspergillus glaucus, A. halophilicus and A. restrictus groups are the most xerophytic of all fungi and grow where all other fungi are excluded. The grains stored commercially below 13 per cent of moisture content (equivalent to RH below 70 per cent), are attacked by these three species of Aspergillus and thus called storage fungi. Other fungi cannot grow at this low relative humidity. As a result of their activities, the percentage germination of grains falls; they are rendered unfit for food and contain toxins which can cause health hazard.
The hyphae are hyaline, septate and branched. The cells are multinucleate.
Genus Eupenicillium (Penicillium as the Conidial Stage):
Introduction and Importance:
Though cosmopolitan in distribution, species of Penicillium are more prevalent in temperate countries. This is the opposite of Aspergillus. Because of its importance in industry, especially in antibiotic and cheese manufacture, extensive work has been done on various aspects of Penicillium.
The important activities of Penicillium are given below:
a. Antibiotic Production:
The antibiotic penicillin is manufactured from P.chryrsogenum whose several high antibiotic producing strains are available. P. notatum, from which Alexander Fleming had discovered penicillin, is however, not used in industrial production. The ‘wonder drug’ penicillin, though discovered by Fleming in 1927, was not put to commercial production until 1945. It is active (as a bacteriostatic agent) against Gram + ve bacteria and also against rickettsia and some of the larger viruses. Penicillin is now a generic term used for an entire group of antibiotics.
The antibiotic griseofulvin is produced from the fungus P. griseofulvum. It is used in the treatment of dermatophytic diseases (diseases of skin, nails, hair and feathers) like ringworms, athlete’s foot and epidermophytics. It is fungistatic and not fungicidal, i.e., does not kill the fungus. It is active only against fungi having chitin walls and thus, ineffective on Oomycetes, yeasts and bacteria.
b. Cheese Industry:
P. roqueforti and P. camemberti are used in the production of cheese having special flavours, called Roquefort and Camembert cheese.
c. Plant Parasites:
Blue mold of citrus fruits (P. italicum), green mold of citrus fruits (P. digitatum) and rot of apples (P. expansum) are some of the important diseases caused by Penicillium. Several species of Penicillium are associated with spoilage of food, leather and clothes.
The mycelium consists of highly branched and septate hyphae consisting of uninucleate cells.
Peziza, which has 80 species, grows saprobically on rotten wood, manure heaps and soils rich in organic matter.
The branched septate, mycelium consists of uninucleate cells which lies buried in the soil. Its presence is indicated by the cup-shaped apothecia, which are the conspicuous and visible part of the fungus. The feeding mycelium is perennial, while the fruiting body is short-lived and degenerates after production and liberation of the ascospores.
The apothecium of Peziza is cup-shaped, one to several centimeters in diameter and pale (P. vesiculosus) to brilliantly orange-coloured (P. aurantia). The cup of P. aurantia looks like a piece of orange peel.
Genus Morchella (The Morels and Sponge Mushrooms):
The morels are extremely delicious to eat and are popular in India by the name ‘gucchi’. The various true morels are the different species of Morchella, e.g., M. conica (conical morel), M. delicosa (delicious morel), M. hybrida (hybrid morel), and M. esculenta (the common morel). All are edible.
Unfortunately, they have not been cultivated commercially. Their fruiting is uncertain and we don’t know the conditions that induce fruiting in the vegetative thallus. Even in the laboratory, we grow them only in the mycelial stage; fruiting never occurs in laboratory pure cultures. This is the reason that morels continue to be the food of the rich.
Like Peziza, species of Morchella grow on manure heaps, humus-rich soils, old logs, and stumps in forests.
The mycelium grows inside the substratum and derives nutrition from the organic material of the substrate. The fruiting bodies, the visible edible part, are formed especially after rains.
It is a stalked apothecium, 10-15 cm in height. The thick stalk, stipe, is hollow and bears a sponge-like or ‘honey comb’ like pitted pileus. Both stalk and the pileus are hollow. The pileus is whitish grey to dark brown, depending on the species and the age. The pileus has convoluted ridges and furrows. The furrows are lined by hymenium consisting of asci and paraphyses. The asci have curved tips and contain 8 ascospores. The ridges lack the hymenium. The ascospores are violently discharged.
The ascospores give rise to haploid monokaryotic mycelia. Somatogamy between cells of opposite strains results in the formation of a dikaryotic cell. Ascogenous hyphae originate from this dikaryotic cell which later forms the asci. The fruiting body is formed by the somatic hyphae and, ultimately the stalked apothecium is formed.
It is a cellulolytic fungus which damages paper and fabrics. C. globosum rapidly degrades cotton cellulose and also causes soft rot of wood. It is also coprophilous, i.e., grows on herbivore dung. C. thermophile is thermophilic, cellulolytic and coprophilous, growing on composting manure.
Asexual reproduction is rare. Only five out of over 85 species are reported to produce conidia. The perithecium, a product of sexual reproduction, is flask-shaped having an elongated beak which has a distinct ostiole. The perithecium is covered by long hairs. These are of two types.
Those at the neck are curly, while those at the base are straight. The ascospores lie free in the perithecial cavity in a gelatinous mass, formed by dissolution of the ascus walls. These are released passively in the gelatinous mass. The ascospores are always unicellular and dark. They germinate to form the mycelium which later produces the perithecia.