SACCHAROMYCES - CLASSIFICATION, VEGETATIVE STRUCTURE, REPRODUCTION, ECONOMIC IMPORTANCE

            A. CLASSIFICATION:

Division – Mycota

    Sub-division – Eumycotina

         Class – Ascomycetes

               Sub-class – Hemiascomycetidae

                     Order – Endomycetales

                           Family – Saccharomycetaceae

                                Genus – Saccharomyces

            Species of Saccharomyces are commonly called Yeasts in English. The species of Saccharomyces are predominantly unicellular; reproduce vegetatively by budding, fission or both; produce ascospores in a naked ascus developing from a zygote or parthenogenetically from a single vegetative cell and carry out alcoholic fermentation producing alcohol and carbondioxide, when placed in a sugar solution.

            Species of Saccharomyces are cosmopolitan in distribution. They are abundantly found to occur saprophytically in substrata which contain sugars such as sugar solution, on the surface of ripe fruits, in the nectar of flowers, etc. They are also found in soil, in milk, in animal excreta and on decaying vegetables. Some are found to occur as parasites in plants and animals including man.

            The genus Saccharomyces contains about 40 species, out of which the best known is Saccharomyces cerevisiae.

       B. STRUCTURE OF THE VEGETATIVE BODY:- Vegetative body of Saccharomyces is very simple and consists of a single cell. The vegetative cells sometime remain in chains forming a pseudomycelium.

          Each individual cell is rounded, spherical or elliptical. The size of the cells usually ranges between 2-8µ in diameter and 3-15µ in length. The cell appears hyaline in colour. It consists of a tiny mass of protoplasts surrounded by a cell wall. The cell wall of Saccharomyces is thick and composed of three walls – the outer layer consists of mannan protein and some chitin, middle layer consists of glucan and the inner layer consists of glucan.

            Beneath the cell wall lies the cytoplasmic membrane (plasmalemma), which surrounds the cytoplasm. Embedded in the cytoplasm are the cell organelles such as golgi apparatus, ribosomes, endoplasmic reticulum, mitocondria, sphaerosomes and nucleus. A large well develop, centrally located vacuole is present in the mature cell of Saccharomyces.

                 C. REPRODUCTION:- The species of Saccharomyces reproduces both by vegetative and sexual methods of reproduction.

            1. Vegetative Reproduction:- Vegetative reproduction takes place by two common methods – budding and fission. Accordingly the species of Saccharomyces are called budding yeast and fission yeasts.

            Budding – Under favourable condition Saccharomyces exclusively reproduces by this method. At the time of budding, a small outgrowth known as bud develops. The bud develops and becomes constricted at the base and on maturity gets detached from the mother cell. More often the bud produces new buds, before separation from the mother cell. The process is repeated. In this way a large number of buds are formed without being detached from one another which results in the formation of chains of cells constituting the pseudomycelium.

            Fission – Vegetetative reproduction by fission has been reported to occur in some yeast which is known as fission yeasts for this reason. In this method, the division is transverse. The mother cell elongates. The nucleus divides into two, which move apart. Meanwhile, ring-like ingrowths appear at the wall of the yeast cell in the middle which later extends towards the centre dividing the two daughter nuclei by a septum. The septum thickens and finally separates resulting in the formation of two daughter cells.

        2. Asexual Reproduction:- When grown in a nutrient rich medium, the species of Saccharomyces (S. cerevisiae) reproduce by asexual method. In this case, the yeast cell itself develops into ascus, in which usually 4 ascospores (endospores) are developed endogenously. Out of the 4 haploid ascospores, two are of one mating type and the other two of different mating type. The spores are finally liberated by breaking up of the ascus wall. These haploid cells multiply by budding.

            3. Sexual Reproduction:- It occurs during unfavourable condition. Two somatic cells of opposite mating type behave as gametangia and undergo gametangial conjugation. It is followed by plasmogamy (fusion of protoplasm) and karyogamy (fusion of nuclei), resulting in the formation of diploid zygote.

            The detailed process of sexual reproduction in Saccharomyces can be discussed under the following heads –

            *Gametangial Conjugation – During the conjugation process the + and – strain secretes a sex hormone, elongate to a pear shaped form and approach close to each other. The two haploid cells of opposite mating types bend towards each other and fuse to form a conjugation bridge.

            Plasmogamy – Fusion between the protoplasts of the + and – strain takes place through the conjugation bridge (karyogamy). A large fusion cell with two nuclei (+ and –) lying side by side to form a dikaryon.

            Karyogamy – The two nuclei of the dikaryon finally fuse (karyogamy). Karyogamy is equivalent to fertilization. As a result a diploid zygote or synkaryon is formed. The cell containing the diploid zygote is larger in size and employed in industrial purposes. Such cells undergo multiplication by the process of budding.

            Ascus formation – During unfacourable condition, the diploid cell functions as ascus. Its nucleus divides by meiotic division to form 4 haploid nuclei. Out of the 4 nuclei, two are of one mating type while the other two of different mating type. All these haploid nuclei after accumulating cytoplasm around them change into ascospores.

            On return of the favourable condition, the ascospores bulge up, rupture the wall of ascus and get liberated, which behaves as a fresh haploid Saccharomyces cell.

        D. ECONOMIC IMPORTANCE:- Different strains of Saccharomyces are among the economically most important fungi. Some of the economic importance is as follows –

            1. Yeast has the ability to ferment carbohydrates. During respiration the yeast cell oxidizes sugar to form a simple organic acid and release energy. When the supply of oxygen is restricted in the process, the organic acid is splited into carbon-dioxide and alcohol. Carbon-dioxide and alcohol are industrially important products.

            2. In baking industries CO₂ is an important product and alcohol is a waste. The former is responsible for giving odour and giving spongy texture to the bread.

            3. The alcohol produced during the baking process id driven off as a bi-product. In brewing industries, on the other hand, it is an important product. It is used in the manufacture of commercial ethyl alcohol.

            4. Yeasts are sold in the marked in the form of dried yeast cakes or cakes of commerce, which is generally used in the laboratory and alcohol industry.

            5. Yeast can also be found to synthesize proteins from molasses and ammonia – this activity of yeast is very important for the production of protein foods. As yeasts are great source of vitamins (vitamin B and C) and proteins, they are as good as valuable food.

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