Volvox

Volvox is a green algae representing a coenobial colony that grows in water bodies in the form of minute pin-head balls. Each alga is globose in nature, and cytoplasmic strands join thousands of cells to form the coenobial colony. It grows in freshwater habitats like ponds, pools, or tanks. During the monsoon, the colony multiplies rapidly, forming a green scum over the surface of the water.

Volvox are photosynthetic algae that make a substantial contribution to the total carbon dioxide fixation. They also increase the level of oxygen in their habitat. Volvox is a part of the food chain and thus an important part of the aquatic ecosystem. Overgrowth of Volvox could result in a harmful algal bloom in warm waters having high nitrogen content. The genus Volvox (Gr. volvare, to roll) is named for its rolling movement.


Volvox colony under a light microscope.	Algal bloom due to the overgrowth of Volvox sp.

Systematic Classification

Kingdom: Plantae

Phylum: Chlorophyta

Class: Chlorophyceae

Order: Chlamydomonodales

Family: Volvocaceae

Genus: Volvox

Name of Some Common Species

Volvox globator
Volvox aureus
Volvox prolificus
Volvox carteri
Volvox barberi
Volvox tortius
Volvox mononae

Structure of Volvox

A volvox colony is spherical in shape and mucilaginous in nature. The colony is composed of 500–60,000 unilayered cells attached by delicate cytoplasmic strands. It remains covered by a proteinaceous, gelatinous matrix that fills up the entire colony. Unique functional diversity is noticed among the cells of the colony, and the four types of cells that are observed are:

Vegetative cells contain chloroplasts and pyrenoids, which help in the production and storage of food.
Locomotory cells are pyriform and flagellated cells present at the periphery of the colony, which help in the locomotion of the entire colony.
Gonidia are the asexually reproductive cells that help in the production of daughter colonies.
Gametangia are sexually reproductive cells that produce gametes and are abundant in the posterior portion of the colony.

Structure of a Single Cell

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Each cell is conical in structure, and the peripheral cells are pyriform and consist of the following parts:

A gelatinous sheath that appears hexagonal due to lateral compression. The cells in the colony remain connected via the extension of the gelatinous sheath, in the form of mucilaginous cytoplasmic connections.
A cellulosic cell wall.
The cell membrane is lipoproteinaceous.
A distinct eukaryotic nucleus.
Cup-shaped chloroplast that occupies the entire posterior broader part of the cell. It consists of a pyrenoid with a central protein core surrounded by starch plates.
Eye spot is a hyperpigmented spot located in the anterior of the chloroplast, consisting of a biconcave photosensitive lens and a curved pigmented plate that helps Volvox in phototactic movement.
Peripheral cells consist of whip-like flagella that aid in locomotion.

Reproduction in Volvox

In Volvox, asexual reproduction occurs during favourable conditions, when there are sufficient nutrients and plenty of water. Towards the end of the growing season, as the conditions gradually become unfavorable, they resort to sexual reproduction.

Asexual Reproduction

Gonidia develop at the posterior part of the Volvox colony.
Each gonidium is a large, globose cell without any flagella and possesses a chloroplast containing an increasing number of pyrenoids.

Matured gonidia are increasingly vacuolated and divide mitotically at first, but later meiotically during zygote formation.
Gonidia undergo repeated divisions, forming an 8-celled plakea stage, and consist of three different cell tiers.
These cells rearrange themselves to form a U-shaped structure, which divides longitudinally, forming a 16-celled structure.
This 16-celled structure nearly forms a sphere except for a small opening at the upper part, known as the phialophore, and repeated mitotic division continues.
When the cell division stops, the cells of the sphere pass through the phialophore and turn inside out.
The individual cells develop flagella and become part of a new colony, which may or may not be liberated from the parent colony.

Sexual Reproduction

Gametangia are the gamete-producing cells and give rise to both antheridium and oogonium.

Antheridium

The protoplasts of antheridial cells divide, forming 16–64 elongated, biflagellated antherozoids with yellow-green chloroplasts.

Formation of antherozoids also passes through the plakeal stage and the inversion process.

Antherozoids are liberated from the globose mass by rupturing the antheridial wall.

Oogonium

The entire protoplast develops into a spherical oogonium.

It contains a parietal yellow-green chloroplast.

It matures to a flask-shaped structure and has a beak that opens at the surface of the coenobial colony.

Antherozoids swim to the beak of the female colony.
Antherozoids release proteolytic enzymes, break the layer, and enter the oogonial chamber.
An antherozoid fuses with an egg, forming a zygote or oospore.
Oospore secretes a thick three-layered wall with red/orange pigment haematochrome, which provides resistance.
The colony ruptures to release the oospore. It is the perennial body of the Volvox.
In germination, the diploid oospore undergoes meiosis to form haploid nuclei. Only one functional nucleus survives that gives rise to a new haploid coenobium.

Frequently Asked Questions (FAQs)

Q1. Why is Volvox considered a colonial alga instead of a multicellular organism?

Because all cells are similar and loosely connected in a gelatinous matrix, showing division of labour but no true tissues.

Q2. How does Volvox contribute to the aquatic ecosystem?

It fixes carbon dioxide, increases oxygen levels, forms part of the food chain, and may cause algal blooms if overgrown.

Q3. What is the main difference between asexual and sexual reproduction in Volvox?

Asexual reproduction occurs through gonidia forming daughter colonies, while sexual reproduction involves antherozoids and oogonia producing zygotes (oospores).