Rhizopus

Rhizopus is the genus of common saprophytic fungi, but also includes some parasitic species. They are dark greyish-brown in colour, prefer moist or humid environments, with the ability to grow on a wide variety of organic substances, which include fruits and vegetables, bread, leather, etc. Rhizopus is multicellular, and some cause fungal infections and fatal diseases.

Rhizopus sp. Under a light microscope (LPCB stained)

Systematic Classification

Kingdom: Fungi

Phylum: Zygomycota

Class: Zygomycetes

Order: Mucorales

Family: Mucoraceae

Genus: Rhizopus

Name of some common species

Rhizopus stolonifer
Rhizopus oryzae
Rhizopus microsporus

Structure of Rhizopus

Rhizopus are fast-growing and have a cottony appearance.
The body of a Rhizopus consists of branching, known as mycelium, that is coenocytic.
Three types of hyphae (thread-like, branched filaments of a fungus) constitute the coenocytic mycelium of Rhizopus-

Stolon: the aerial, internodal region that forms an arch and touches the substratum to form the nodal region.

Rhizoids: branched structures at the nodes formed where the stolon touches the substratum. They anchor the mycelium to the substratum and absorb food. Their diameter not fixed; varies depending on growth conditions.

Sporangiophores: the unbranched, elongated, columellate, reproductive hyphae that grow vertically from the stolon.

A spherical or globose-shaped sporangium (100 µm in diameter) carries the sporangiospores, and the columella is sterile and provides support to the sporangium; it does not absorb water. Sporangiospores are unicellular, asexual spores of Rhizopus.

Each hypha consists of-

A cell wall composed of chitin, lipid, protein, etc.
A protoplasm containing nuclei, mitochondria, endoplasmic reticulum, and various cytoplasmic inclusions.

Reproduction in Rhizopus

Rhizopus reproduces by three methods-

Vegetative Reproduction

Due to the formation of small fragments or accidental breakage, the stolon of Rhizopus might break into multiple smaller units, and each unit is capable of growing as a mother mycelium.

Asexual Reproduction

It takes place in two ways

Formation of Sporangiospore

Sporangiospores are non-motile spores that form inside the sporangium.
With time, the protoplast of the sporangium is differentiated into sporoplasm (a thick, dense layer of multinucleated cytoplasm towards the peripheral region) and the columella plasm (the remaining part of cytoplasm with few nuclei towards the centre).
Gradually, a series of vacuoles appear between the sporoplasm and columella plasm, which become flattened and form a cleavage cavity.
A dome-shaped septum is developed, which forces its way into the sporangium.
After the maturation process, the sporangium wall dries and collapses to form a cup-shaped, irregular surface.
This surface gets ruptured and scatters the spores in the vicinity.

Formation of Chlamydospores

During unfavourable conditions, the protoplasm of a hyphal segment gets surrounded by thick and nutrient-rich walls.
The chlamydospore gets detached from vegetative hyphae, and they remain in the resting phase.
When this chlamydospore gets enough moisture, it undergoes the formation of a germ tube, which leads to the formation of the new thallus.

Sexual Reproduction

Positive and negative thalli come in contact with each other.
Conjugation occurs between the male and female thalli by means of the outgrowth of positive and negative thalli.
The conjugation septum is developed between the progametangia and plasmogamy, which results in the formation of coenogametes.
Due to the conjugation, karyogamy occurs, leading to the formation of a zygote.
The zygote undergoes maturation, is surrounded by a thick-walled zygospore.
Under favourable conditions, the zygospore forms a germ tube and germinates into a new vegetative body.

Economic Importance of Rhizopus

R. oryzae is used for biosorption of heavy metals in wastewater and also in the production of cortisone and lactic acid.
R. stolonifer produces lactic acid and fumaric acid; cortisone is produced industrially using Rhizopus species such as R. oryzae
R. delemar produces fumaric acid and biotin
Various Rhizopus species are used to make tempeh (a soybean fermented product)
Many traditional alcoholic beverages are also prepared using Rhizopus.

Frequently Asked Questions (FAQs)

Q1. What diseases in humans are caused by Rhizopus?

A. Certain species of Rhizopus are opportunistic pathogens that attack individuals with a compromised immune system. They can cause mucormycosis in humans, which typically affects the lungs, sinuses, and skin.

Q2. How does the mycelium of Rhizopus become coenocytic?

A. The hyphae that constitute the mycelium of Rhizopus are aseptate, which results in the coenocytic condition- a continuous filamentous tube containing multiple nuclei in a common cytoplasm. The structure helps in the efficient and rapid transport of nutrients through the fungal body.