RHIZOPUS (Rhizopus stolonifer) – Common bred mold

Biological classification

Kingdom fungi

Reasons:

• They lack chlorophyll.
• Presence of hyphae which form mycelium
• They possess no true roots, stems and leaves
• Presence of cell wall made of chitin
• They are eukaryotic.

Phylum Zygomycota

Reasons:

• Possess gametangia which form zygospores.
• Presence of sporangium on sporangiophore.
• They are non-septate (their hyphae have no cross wall)

Habitat of Rhizopus stolonifer

• Moist bread
• Fermented kenkey.
• Rotten fruits and vegetables.

Structure of Rhizopus.

The basic unit of Rhizopus is a microscopic thread-like structure called hypha.

The hyphae form an interwoven mass called mycelium (plural – mycelia).

Mycelium is the vegetative part of a fungus.

There are three main parts of the mycelium.

These are stolon, rhizoid and sporangiophore.

Stolon: This is a hypha that grows horizontally on the substrate

Rhizoid: This is a root-like hypha which is more branched than the other two types and grows from the stolon into the substrate.

• Rhizoid supports the fungus onto the substrate
• Secretes digestive enzymes
• Also, absorbs nutrients from the substrate.

Sporangiophore – This is an upright and unbranched hypha.

• It bears a swollen end called sporangium (plural =sporangia).
• Sporangia contain spores which are used in asexual reproduction.

Adaptive features of Rhizopus.

• Presence of sporangia which produce spores and protect them.
• Possesses sporangiophore which holds sporangia in position for dispersal of spores.
• Mycelium for vegetative growth.
• Also, spores are present for asexual reproduction.
• Presence of rhizoid for absorption of nutrients, anchorage and secretion of digestive enzymes.
• Presence of stolon for vegetative growth.

Mode of reproduction.

• Sexual reproduction by gametangia, which is a means of surviving adverse conditions.
• Asexual reproduction by spores in the sporangia.

Asexual reproduction in Rhizopus

• Asexual reproduction is by spores produced in the sporangium.
• In this process, a vertical hypha called sporangiophore
• The tip of the sporangiophore then enlarges into a sporangium.
• During the development of sporangium, dense cytoplasm containing several nuclei migrate to the periphery and develop into spores.
• The central portion contains a vacuole and is called the columella.
• Young sporangium is white or colourless.
• As sporangium matures, it becomes black and dries.
• The walls crack and release the dry powdery spores by air.
• Spores land on suitable substrates such as moist bread or fermented kenkey.
• Under favourable conditions such as the presence of moisture, a spore develops into a hyphae and a hyphae later develops into mycelium

Sexual reproduction

• Sexual reproduction in Rhizopus is called conjugation.
• Conjugation in Rhizopus involves the union of two different strains of mycelia (plus (+) and minus (-) strains).
• When the hyphae of (+) and (-): strains come together, they develop short swollen side branches called progametangia (progametangium which is singular)
• When the progametangia develop and meet a cross-wall, it develops between their tips.
• Each tip then develops into a gametangium (plural – gametangia) with a stalk called Suspensor.
• Each gametangium contains several haploid nuclei.
• Later, the cross-wall breaks down.
• The gametangia fuse and their contents mix.
• Their nuclei pair up as + and – which fuse to form many diploid nuclei.
• The fused gametangia becomes a zygospore.
• The zygospore grows and develops a thick black and resistant wall, after which it is disconnected from the parent hyphae.
• The zygospore can withstand adverse conditions.
• In favourable conditions, the zygospore undergoes meiosis, germinates and develops into a haploid hypha which is either (+) or (-).

Factors that enhance growth in Rhizopus.

• Suitable temperature for enzymatic activities.
• Moisture to activate enzymes to digest food.
• Presence of dead organic matter/nutrients for the growth of mycelium.

Adaptations of Rhizopus to dispersal.

• Spores are light in weight.
• Numerous spores to ensure survival.
• Thickened walls of spores for protection against unfavourable environmental conditions.

Mode of nutrition.

• Rhizopus has no chlorophyll and undergoes a saprophytic mode of nutrition.
• The hyphae penetrate the substrate
• Secrete digestive enzymes on the food to digest it extracellularly converting it into a soluble form.
• The end products are absorbed into hyphae by diffusion.

Economic importance of Rhizopus

• Contaminates food or food spoilage
• Causes decay or decomposition
• Improves soil fertility
• Causes diseases in plants.
• Natural recycling of wastes from the ecosystem.

Ecological importance

• Decomposition
• Natural recycling

Questions for revision.

1. State four parts of Rhizopus (bread mold) and their functions.
2. Name two habitats of Rhizopus.
3. Explain briefly how Rhizopus can be cultured in the laboratory.
4. Describe how Rhizopus obtain food.
5. State the effect of the presence of the following on Rhizopus.

• High humidity
• Temperature
• Dead organic matter

6. Briefly explain how Rhizopus recycle the soil nutrient by decomposition.

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