Spirogyra

Spirogyra, also known as "pond silk" or "water silk," are green algae that float in freshwater like ponds and lakes. They have a thread-like, unbranched structure, and there are about 400 different species. The name "Spirogyra" comes from the spiral shape of the chloroplasts inside their cells. These algae make their food through photosynthesis, helping to fix carbon dioxide and increase oxygen in the water. Many aquatic animals also feed on them.

Classification of Spirogyra

Spirogyra belongs to the group Chlorophyta because it contains chlorophyll. The genus has about 400 species.

• Domain- Eukaryota

• Kingdom- Plantae

• Phylum- Chlorophyta

• Class- Zygnematophyceae

• Order- Zygnematales

• Family- Zygnemataceae

• Genus- Spirogyra

Structure of Spirogyra:

• The vegetative structure of Spyrogyra is an unbranched, thread-like thallus.

• The thallus is multicellular, with cylindrical cells joined end to end.

• Cells are 10-100 µm in width and can grow several centimeters in length.

• Spirogyra appears as a slimy mass due to the mucilage sheath around the filament.

• The cell wall has two layers: an inner cellulose layer and an outer pectose layer.

• The slimy texture is caused by the dissolution of pectose in water.

• Each cell contains a nucleus, cytoplasm, a large central vacuole, and spiral-shaped chloroplasts.

• Chloroplasts are ribbon-shaped and arranged spirally.

• There may be 1 to 16 chloroplasts per cell.

• Chloroplasts contain many pyrenoids arranged in a row, which store starch and protein.

Reproduction in Spirogyra

Spirogyra reproduces through vegetative, asexual, and sexual methods. Its life cycle is haplontic, meaning the dominant stage is the free-living haploid (n) gametophyte, and the diploid zygote (2n) is the only sporophyte stage.

Vegetative Reproduction

Vegetative reproduction in Spirogyra occurs through fragmentation. Under favorable conditions, this method is preferred. The filament fragments, and each piece grows into a new filament. Fragmentation can happen due to mechanical injury or changes in water salinity and temperature. Sometimes, the middle lamella of a cell protrudes into an adjacent cell, causing the filament to break.

Asexual Reproduction in Spirogyra

Asexual reproduction happens in some Spirogyra species through the formation of azygospores, akinetes, or aplanospores.

• Aplanospores form when conditions are unfavorable. The protoplast shrinks and forms a protective wall around itself, turning into aplanospores.

• Akinetes are similar but have thicker walls made of cellulose and pectin.

• Both aplanospores and akinetes are non-motile and develop into new filaments under favorable conditions after the parent filament decays.

• Azygospores (also called parthenospores) are gametes that failed to fuse during sexual reproduction and develop into new filaments asexually.

Sexual Reproduction in Spirogyra

Spirogyra reproduces sexually through isogamous conjugation, where male and female gametes are of the same size. There are two types of conjugation: scalariform conjugation and lateral conjugation.

• Scalariform Conjugation- In this type, two Spirogyra filaments align side by side, forming a ladder-like structure. Tubes develop from each cell of the filaments, creating a conjugation canal where male and female gametes fuse. The zygotes are formed and released after the parent filaments decay, germinating under favorable conditions.

• Lateral Conjugation- In lateral conjugation, adjacent cells function as male and female gametes. Conjugation tubes form between cells of the same filament, and this type can occur in two ways:

• Direct Lateral Conjugation- A passage forms between adjacent cells through the middle lamella, and male gametes fuse with female gametes, resulting in zygotes in alternating cells.

• Indirect Lateral Conjugation- A conjugation canal forms between the male gamete cell and an adjacent female gamete cell.

During sexual reproduction, the entire protoplast of Spirogyra acts as a gamete, known as an aplanogamete, which is produced in the gametangia at the end of the growing season.

Zygospore Formation

The zygote formed through conjugation is called a zygospore. Zygospores are diploid (2n) and are the only diploid stage in the Spirogyra life cycle. They have a thick, three-layered cell wall:

• Exosporium (outer cellulose layer)

• Mesosporium (middle layer of chitin and cellulose)

• Endosporium (inner cellulose layer)

Zygospores remain dormant until favorable conditions return. When conditions improve, the zygospore undergoes meiosis, producing four haploid (n) nuclei, but only one survives while the others disintegrate. The zygospore bursts open, and a germ tube forms. The germ tube divides and grows into a new haploid filament.

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