Phycomycetes: Algal Fungi, Structure and Reproduction

Phycomycetes are the simplest true fungi, often called algal fungi because their aquatic lifestyle resembles that of algae. Members are found in water, on decaying wood in moist and damp places, or as obligate parasites living on plants. The familiar black bread mould, Rhizopus, and the white rust of mustard, Albugo, both belong to this group.

Phycomycetes matter because they introduce the basic fungal body plan: a network of tube-like threads that digest food externally. They spoil stored food, cause plant diseases like white rust, and serve as textbook examples of how fungi reproduce with motile spores, a feature lost in most higher fungi.

Key Concepts

• Coenocytic mycelium: the fungal body (mycelium) is made of hyphae with no cross-walls (aseptate), so many nuclei share one continuous cytoplasm.
• Zoospores: motile asexual spores with flagella, produced in aquatic species.
• Aplanospores: non-motile asexual spores, produced by species like Rhizopus inside a sac called a sporangium.
• Zygospore: the thick-walled sexual spore formed by the fusion of two gametes; isogamous if the gametes look alike, anisogamous or oogamous if they differ.
• Obligate parasite: an organism like Albugo that can only grow on a living host plant.
• Examples: Rhizopus (bread mould), Albugo (white rust parasite of mustard).

How It Works

Rhizopus grows on bread as a cottony mass of coenocytic hyphae, with root-like rhizoids anchoring it and runner-like stolons spreading across the surface. For asexual reproduction it raises upright stalks called sporangiophores, each tipped with a round sporangium packed with dark aplanospores; when the sporangium bursts, spores scatter and germinate on fresh bread. For sexual reproduction, two compatible mycelia produce short branches that fuse tip to tip, and their nuclei merge into a rough, thick-walled zygospore that can survive winter and germinate when conditions improve.

Worked Example

Question: A slice of bread carries 25 mature sporangia of Rhizopus. If each sporangium releases about 400 aplanospores, how many spores land on the bread in total?

Step 1: Spores per sporangium = 400 (approximate, for illustration).

Step 2: Total spores = 25 x 400.

Step 3: 25 x 400 = 10,000.

Answer: About 10,000 spores, which explains why mould spreads across bread so quickly.

Real-World Applications

• Food spoilage control: understanding Rhizopus helps design better bread packaging and storage.
• Crop protection: recognising Albugo white rust on mustard guides fungicide use and resistant-variety breeding.
• Fermentation industry: Rhizopus species are used to ferment soybeans into tempeh, a protein-rich food.
• Teaching: the simple coenocytic body and clear sporangia make phycomycetes ideal for learning fungal structure.

Phycomycetes are simple fungi with aseptate, coenocytic mycelium, found in water, on damp decaying matter and as plant parasites. They reproduce asexually by zoospores or aplanospores and sexually through zygospores, with Rhizopus and Albugo as key examples in food spoilage, fermentation and crop disease.