Rhizopus

Rhizopus is a genus of filamentous fungi found in the phylum Zygomycota, commonly known as bread molds. With around 50 species, these fungi are widespread and thrive in environments rich in decaying organic matter. While some species act as weak parasites, others are known for their ability to produce industrial products. One well-known species is Rhizopus stolonifer, often referred to as "bread mould." 

1.0Classification of Rhizopus

2.0General Characteristics of Rhizopus

Rhizopus fungi is recognized by its white, cottony mycelium, extensively branched and featuring coenocytic and multinucleated hyphae. These hyphae contain oil droplets, glycogen bodies, and vacuoles in the cytoplasm. The hyphal wall is composed of chitin. Three types of hyphae are identified in the mycelium: stoloniferous hyphae, rhizoidal hyphae, and sporangiophores.

(i) Stoloniferous Hyphae: Slightly branched hyphae that extend horizontally above the surface and form clumps of rhizoids where they connect with the substratum.

(ii) Rhizoidal Hyphae: Often branched hyphae that link the growing organism to a substratum, facilitating the absorption of water and nutrients.

(iii) Sporangiophores: Vertical, unbranched hyphae that grow upward in clusters from the stolons. These hyphae serve a reproductive function as they bear sporangia.

3.0Diagram of Rhizopus

4.0Reproduction in Rhizopus

Vegetative Reproduction in Rhizopus 

Vegetative reproduction in Rhizopus primarily occurs through a process known as fragmentation. This process involves the separation of hyphae from the main aerial mycelium, resulting in distinct units called fragments. Each fragment grows into new mycelium. 

Asexual Reproduction in Rhizopus

It occurs by the formation of sporangiospores and chlamydospores. 

Formation of Sporangiospore

under favorable conditions, Rhizopus forms non-motile spores within singular sporangia situated at the tip of upright structures called sporangiophores. As the sporangium develops, the tip of the sporangiophore swells, and continuous nuclear division takes place. The sporangium's contents differentiate into a central zone with vacuolated cytoplasm and a peripheral zone containing dense cytoplasm with multiple nuclei. 

These zones are separated by a layer that forms a dome-shaped septum known as a "columella." The cytoplasm then divides into several portions, forming non-flagellate spores called sporangiospores with thin, smooth walls. The pressure in the columella, caused by water absorption in the sporangium, leads to the bursting of the thin, smooth wall, dispersing the spores. Upon finding a suitable substrate, the sporangiospores germinate, producing a germ tube that develops into a new mycelium.

Formation of Chlamydospore

In unfavorable conditions, certain Rhizopus species, facing a shortage of water, produce specialized, thick-walled resting cells called chlamydospores. 

During this process, protoplasmic contents accumulate at specific points in the hyphae and lose water, leaving empty spaces in between. Each gathered protoplasmic mass secretes a protective thick wall around it, forming a chlamydospore. These chlamydospores are resistant to drying out and contain ample reserve food. They act as resting cells, surviving when the mycelium perishes due to a drying substrate. When favorable conditions for growth return, these resilient chlamydospores germinate, giving rise to new mycelium and allowing the fungus to overcome unfavorable periods. 

Sexual reproduction in Rhizopus 

Rhizopus undergoes sexual reproduction through the development of two similar-looking but physiologically different multinucleate gametangia. Most species of Rhizopus are heterothallic, requiring mycelia of compatible strains (+ and -) to produce zygospores. 

In the case of homothallic Rhizopus species, zygospores form from mycelia derived from a single spore. During sexual reproduction, mycelia of opposite strains come close, forming small outgrowths called "progametangia." These structures have swollen apical ends filled with protoplasm. As gametangia matures, the common wall disappears, allowing mixing of contents through nuclear pairing and fusion of (+) and (-) strains. 

This results in the formation of diploid nuclei (2n), while other nuclei degenerate. The zygospore enlarges, secreting a five-layered structure and entering a resting period. After this period, the zygospore germinates, producing germ sporangiophores or promycelium. The promycelium develops a germ sporangium at the tip, liberating numerous spores. Reduction division occurs during zygospore germination, and each liberated spore germinates to form a new mycelium.

5.0Life cycle of Rhizopus

6.0Common Species of Rhizopus 

• Rhizopus stolonifer: Commonly known as black bread mold, it is widespread and often found on various organic substrates, especially bread.
• Rhizopus oryzae: This species is involved in the fermentation 
• Rhizopus arrhizus: It is associated with human infections and can cause mucormycosis, especially in immunocompromised individuals.

7.0Economic Importance of Rhizopus 

Food Fermentation: Certain species of Rhizopus, such as Rhizopus oligosporus, are used in the fermentation of food products. For example, they play a crucial role in the fermentation process of tempeh, a traditional Indonesian food made from soybeans.

Enzyme Production: Some Rhizopus species are utilized in the industrial production of enzymes. These enzymes find applications in various industries, including the food, pharmaceutical, and biofuel sectors. They are used in processes such as starch hydrolysis and the production of organic acids.

Decomposition and Nutrient Recycling: In natural ecosystems, Rhizopus participates in the decomposition of organic matter. This activity is crucial for nutrient recycling, contributing to soil fertility and overall ecosystem health. The breakdown of organic material by Rhizopus releases nutrients back into the soil, benefiting plant growth.