General Characters.
The Phycomycetes (Gr. Phycos, seaweed; mycetes, fungus), or Algal Fungi, are the lowest class of Eumycetes or True Fungi and include about 254 genera and 1,300 species represented by such common fungi as bread mold (Rhizopus), water mold (Saprolegnia), white rust of mustard (Albugo), and downy mildews (Peronospora). The thallus is very simple and is composed of single-cell or filaments of cells forming hyphae.
Frequently, these hyphae are unseptate and coenocytic. Septa occur either rarely or at the time of the formation of the reproductive organs and in the old mycelium. Some of the higher forms, e.g., Entomophthora, however, show a septate mycelium. The lower forms, e.g., Synchytrium, are usually holocarpic, while the higher forms are eucarpic.
In the former, the entire thallus IS transformed into a single reproductive organ at maturity, while in the latter, only a part of the thallus is transformed into sex organs while the remaining continues to carry on its vegetative function.
Reproduction
Vegetative Reproduction:
It is effected by fragmentation.
Asexual Reproduction:
In the lower groups, asexual reproduction takes place by sporangia borne on sporangiophores. The spores are produced as a result of the cleavage of the contents of the sporangium into an indefinite or sometimes large number of protoplasts.
The protoplasts thus formed may develop one tinsel-type or hairy flagellum or two whiplash-type or non-hairy flagella and be termed zoospores or may secrete walls and form aplanospores.
A given species generally produce either zoospores or aplanospores, but under varying environmental conditions, certain exceptional species may produce either zoospores or aplanospores at different times.
Terrestrial species produce aplanospores, while zoospores are usually produced in aquatic species, and they are generally more primitive than those producing aplanospores. In biflagellate zoospores, the anterior flagellum is tinsel-type, while the posterior one is whiplash-type (Couch, 1941).
In certain advanced forms, e.g., Cystopus, there is a gradual transition; the sporangia themselves take on the function of conidiospores or conidia. They may be abstricted singly or in chains from the apices of the hyphae termed conidiophores. They are dispersed by wind and germinate directly.
Under wet conditions, the conidia, or more appropriately, the conidia-sporangia may, however, form one to several zoospores in a typical fashion. The zoospores come out by a rupture of the parent wall or through a terminal pore and may pass on to a special vesicle from where they escape later on.
They may exhibit diplanetism, which means that they show two periods of rest. Sometimes the hyphae become transversely septate, and their cells round are termed chlamydospores. They are comparable to the akinetes of green algae.
Sexual Reproduction:
It is very common in the Phycomycetes. It may be isogamous, i.e., by the fusion of two motile gametes (plasmodiogametes) which may be similar (Catenaria) or dissimilar (Allomyces, Olpidium) or non-motile (Aplanogametes), as in Mucor or oogamous (heterogamous), by the fusion of two dissimilar gametes, namely, the motile male gamete, the antherozoid produced in an antheridium, and a non-motile female gamete, the egg contained in an oogonium, e.g. Monoblepharis.
The male gametes are usually non-motile, e.g., Pythium, and are transferred to the oogonium through one or more fertilization tubes that penetrate the oogonial wall.
In the higher forms, the protoplasm of the antheridium and oogonium usually differentiates into an outer portion, the periplasm, and an inner portion called the gonoplasm and ooplasm, respectively. The gonoplasm passes through the fertilization tube to fuse with the ooplasm, which is usually uninucleate (Pythium) or multinucleate (Albugo bliti).
In some Phycomycetes, both gametes are uninucleate; in others, one is uninucleate and the other multinucleate; in still others, both gametes are multinucleate.
During gametic union, there is a complete fusion (karyogamy) of the nuclei in the zygote when the fusing gametes are uninucleate, while in the case of multinucleate gametes, the male and female nuclei first unite in pairs (plasmogamy), followed by multiple karyogamy, i.e. several pairs of nuclei fuse in pairs.
However, when a uninucleated male gamete unites with a multinucleate egg, its nucleus unites only with one of those in the egg.
The zygospore or oospore produced as a result of sexual union invests itself with a thick wall and undergoes a period of rest. On germination, it may produce a new mycelium directly or may first form one or more zoospores, which later on germinate to produce a new thallus.
Parthenogenesis or the formation of a zygospore is also found in certain species.
Homothallic species are of more widespread occurrence among the Phycomycetes than the heterothallic species. The Phycomycetes are predominantly aquatic and may live either as parasites or saprophytes.
By virtue of their aquatic habit and marked similarity to algae in structure, chlamydospores formation c.f. akinetes of algae and mode of reproduction, they were supposed to be derived from algae.
In fact, they were regarded as degenerate algae in which the chlorophyll had been lost. Accordingly, they were classified after their supposedly corresponding algal groups, i.e., Protococcales, Ulotrichales, Cladophorales, Siphonales, etc. The name Phycomycetes or “Algal Fungi” emphasizes this idea.
Classification:
The filamentous Phycomycetes were divided according to their mode of reproduction into two orders, namely, the Oomycetes and the Zygomycetes, the former showing oogamous and the latter isogamous reproduction (Martin, 1950).
The non-filamentous Phycomycetes having rounded or lobed mycelial thallus were placed in the order Archimycetes (Gaumann, 1926, 1949). But the inclusion of all oogamous forms in Oomycetes and all isogamous forms in Zygomycetes is considered to be rather artificial.
Gaumann (1952) divided the class into five orders, based mainly on the structure of the thallus, the type of flagella, and sexual reproduction. These are Chytridiales, Blastocladiales, Monoblepharidales, Oomycetes, and Zygomycetes.
According to recent classification, the class Phycomycetes is divided into seven orders: Chytridiales, Monoblepharidales, Plasmodiophorales, Saprolegniales, Peronosporales, Mucorales, and Entomophthorales.
We shall, however, consider examples from the following orders only:
- Chytridiales, e.g., Synchytrium, Rhizophidium, Olpidium.
- Monoblepharidales, e.g., Monoblepharis.
- Plasmodiophorales, e.g., Plasmodiophora.
- Saprolegniales, e.g., Saprolegnia.
- Peronosporales, e.g., Pythium, Cystopus, Peronospora.
- Mucorales, e.g., Mucor, Rhizopus, Pilobolus.
Classification in detail:
Phycomycetes are classified into three subclasses, each with distinct characteristics and orders.
Subclass I. Archimycetes:
Archimycetes are characterized by their rounded or lobed non-mycelial thallus, which is formed by the enlargement of zoospores. In some higher forms, a rudimentary mycelium may develop. This subclass includes three orders:
Order 1. Chytridiales: Chytridiales have a variable thallus form, mostly holocarpic, and produce one or more sporangia or gametangia. The zoospores and zoogametes are uniflagellate. Common genera in this order are Synchritium and Olpidium.
Order 2. Ancylistales.
Order 3. Protomycetales.
Subclass II. Oomycetes:
Oomycetes are characterized by a well-developed coenocytic mycelium, which often becomes septate. They undergo sexual reproduction through oogamy, where the zygote is always an oospore. Most oomycetes produce motile zoospores as accessory spores. This subclass includes five orders:
Order 1. Blastocladiales.
Order 2. Monoblepharidales: Monoblepharidales are aquatic saprophytes with foamy cytoplasm. Their zoospores are uniflagellate, and resting spores bear warts. Sexual reproduction involves the union of a motile male gamete with an oosphere contained in an oogonium.
Order 3. Leptomitales.
Order 4. Saprolegniales: Saprolegniales are mostly aquatic, sometimes terrestrial, and parasitic on animals and plants. Their mycelium is aseptate and branched, with net constrictions at intervals. The sporangia are located at the tips of hyphae. Their zoospores are biflagellate and typically exhibit diplanatism.
Sexual reproduction in Saprolegniales is oogamous, and the oogonium contains one to several oospheres without periplasm. Common genera in this order include Saprolegnia and Achlya.
Order 5. Peronosporales: Peronosporales are mostly parasitic on land plants, although some are saprophytic. They liberate biflagellate zoospores from zoosporangia, which can function as conidia under specific external conditions.
Sexual reproduction in Peronosporales is oogamous, and the oogonium contains a solitary oosphere with a well-defined periplasm. Common genera in this order are Peronospora, Pythium, Phytophthora, and Albugo.
Subclass III. Zygomycetes:
Zygomycetes are characterized by a well-developed mycelium, which often becomes septate. Their characteristic mode of sexual reproduction is through the conjugation of two gametangia, resulting in the formation of a zygote called a zygospore.
They also produce non-motile aplanospores, known as sporangiospores, within a unicellular sac called a sporangium. This subclass includes two orders:
Order 1. Mucorales: Mucorales are mostly saprophytes that thrive on various substrata, with rare cases of parasitism. Their mycelium is coenocytic, initially aseptate and regular but becoming irregular and septate with age. Sporangiospores are formed inside globose or ovoid sporangia.
Order 2. Entomophthorales.
Importance to Man.
The purely aquatic Phycomycetes are not of much economic importance, except for some parasitic forms such as Saprolegnia. which may parasitize fish and fish eggs and cause considerable damage to fish hatcheries. Some may attack aquatic plants of economic importance as well.
Many of the terrestrial Phycomycetes are very destructive to crop plants, causing serious diseases like mildews and blights, which may sometimes reach epiphytotic proportions, severely affecting extensive areas of crops. Examples Of such diseases are the late blight of potatoes and the downy mildew of grapes and onions.
Soil-inhabiting Phycomycetes cause serious root troubles and collar rots of economic plants as well as damping-off of seedlings. Still, some Phycomycetes such as Rhizopus, whose spores are afloat in the air, cause food spoilage in the home, and fruit and vegetable rots in transit and storage.
Some attack insects, nematodes, etc., while others cause certain human diseases; & few are, however, utilized in industry nonliving fermentation.
FAQs
While Phycomycetes can cause plant diseases, they are generally not harmful to humans. However, some species may cause opportunistic infections in individuals with weakened immune systems.
While many Phycomycetes are adapted to aquatic habitats, some species can also be found in soil and other terrestrial environments.
No, not all Phycomycetes are pathogenic. Some species play beneficial roles, such as decomposers and biological control agents.
Yes, many Phycomycetes can be cultured in the laboratory under controlled conditions for research and industrial purposes.
Phycomycetes belong to the Kingdom Fungi but represent an early diverging group within the fungal kingdom, distinct from the true fungi (Ascomycetes, Basidiomycetes, etc.).