Saprolegnia, Mucor, Rhizopus and Schizosaccharomyces

Saprolegnia (Class Oomycetes)

Species of Saprolegnia are common in soil and fresh water; hence they are commonly called water molds. They are saprophytic on plant and animal remains, but a number of species such as S. Ferax and S. Parasitica have been implicated in disease among fish and their eggs S. Parasitica causes severe Epidemics of disease among fish in the natural environment.

The somatic portion of the organisms consists of two types of hyphae; the rhizoidal hyphae which enter the substratum and serve to anchor the organism and to absorb nutrients, and the branched hyphae on which the reproductive organs are formed. Elongated, tapering sporangia are formed at the tips of somatic hyphae. The nuclei present differentiate into zoospores. Am opening develops at the tip of the sporangium and the pear-shaped primary zoospores escape into the surrounding aqueous environment. They swim for some time (from a minute to over an hour), then withdraw their flagella and encyst. This cyst, after a period of rest (2 to 3 h, depending on the species), germinates to release a further bean-shaped secondary zoospore. The secondary zoospore may swim vigorously for several hours before encysting again. This encysted spore now germinates by sending a germ tube that develops into a hypha, forming a new colony. When conditions are favorable for sexual reproduction, the somatic hyphae give rise to oogonia and antheridia.

Mucor (Class Zygomycetes)

Members of this genus occur abundantly in soil and manure and on fruits, vegetables, and starchy foods. Some are responsible for food spoilage, but others are used in the manufacture of some cheeses and other foods. Common species are M. Racemosus and M. Rouxii. Morphologically, their mycelia are usually white usually white or gray and red nonseptate. Sporangiophores may be branched. The columellae (sterile structures in Sporangium) are round, cylindrical, or oval. Spores are black or brown and are smooth in appearance. Zygospores are produced when plus and minus strains (so called because there is no morphological differentiation between the male and female strains even though there is physiological heterothallism) of the organism are both present. Zygospores are also formed in growth on artificial media in the laboratory. No stolons or rhizoids are produced. It is interesting that in the absence of fermentable carbon source in a medium consisting of yeast extract and peptone supplemented with potassium acetate, mucor may grow in a yeast like instead of filamentous form.

Rhizopus (Class Zygomycetes)

These are common bread molds, which cause much food spoilage. They grow on bread, vegetables, fruits, and other food products. The most commonly encountered species is R. stolonifera. Morphologically, they have non-septate, cottony mycelia with sporangiophores arising at the nodes where the rhizoids form. Their sporangia are usually quite large and black; their columellae are hemispherical. The base of the sporangium, or apophysis, is cup-shaped. These molds produce clusters of root-like holdfasts called rhizoids, as well as stolons or ‘runners’ (like those of strawberry plants) capable of taking “root” where they may give rise to new organisms. The life cycle of R. stolonifera reproduction requires two thalli of different mating types. Since the thalli are indistinguishable morphologically, they are designated plus and minus instead of male and female. (Generally, with gametes that differ morphologically, the larger gamete of the pair is regarded as female, the smaller as male. Also, a gamete that leaves the structure where it was formed and later fuses with a relatively stationary gamete is regarded as male.) When both mating thalli are present, the hyphal tips differentiate into progametangia which comes into contact and develops into gametangia by the formation of septa. The walls between two gametangia then dissolve and their protoplasts coalesce. Nuclei of both mating types fuse in pairs, producing many zygote nuclei. The structure that contains them is then called a coenozygote. The wall of the coenozygote soon thickens, turns black, and becomes rough; it develops into the zygospore. The resting stage of the zygospore lasts from 1 to 3 months, and sometimes even longer. Upon germination, meiosis takes place, the zygospore breaks open, and a single sporangiophore bearing a germ sporangium at its tips emerges. The germ sporangium is similar to an asexually produced sporangium. Some germ sporangia contain spores all of one mating type (either plus or minus), but others contain spores of both mating types in about equal numbers.

Schizosaccharomyces (Class Ascomycetes)

All fission yeasts belong to this genus. They reproduce by the transverse division as well as by ascospores for the life cycle of Schizosaccharomyces. The best-known species is S. octosporus, which has been isolated from currants and honey. Its cell is globose to cylindrical, uninucleate and haploid. S. pombe is the fermenting yeast of some kinds of beer (such as African millet beer), and it has been isolated from sugar molasses and from grape juice. S. versatile, isolated from grape juice, grows like a yeast, but it can also form a true mycelium.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.