Spirogyra

Spirogyra, also known as "water silk" or "pond silk," are filamentous green algae thriving in freshwater environments like ponds and lakes. They boast a simple, unbranched vegetative structure and possess a distinctive spiral chloroplast in their cells. With approximately 400 species identified, Spirogyra play a crucial role in photosynthesis, significantly contributing to carbon dioxide fixation and oxygen production in their habitat. These algae serve as a vital food source for numerous aquatic organisms.

About Spirogyra

Systematic position

• Kingdom – Plantae
• Subkingdom – Thallophyta
• Division – Algae
• Class – Chlorophyceae
• Order – Conjugales (zygnematales)
• Family – Zygnemaceae
• Genus – Spirogyra (For MP PMT & UP CPMT Students Only)

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Spirogyra Habitat 

It is an unbranched filamentous green alga of stagnant fresh waters which forms floating masses (supported by bubbles of gases of oxygen) called pond scum. A sheath of muscilage occurs on the outside. It gives a silky touch. Hence Spirogyra is also called water silk or mermaid's tresses.

Classification of Spirogyra

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Spirogyra Structure 

The thallus is an unbranched and uniseriate filament where cells are arranged in a single row.  All the cells of a filament are similar. In some species hold fast is present (e.g., S. fluviatilis). The cells are elongated and cylindrical. The cell wall is two layered the outer is of pectic substance and the inner of cellulose.

The outer part (pectin) dissolves in water to form a muscilaginous sheath. Due to this reason Spirogyra filament's are slippery. Transverse or septum can be plane, colligate (with H-shaped piece), replicate (ring like ingrowths) and unduliseptate (undulate). The filaments are covered by a layer of mucilage. Each cell of a filament consists of a small amount of protoplast bounded by the cell wall.

The protoplast is differentiated into plasma membrane, thin layer of cytoplasm, single nucleus, one (e.g. S.sahnii and S. venkataramanni) or many (16 in S.rectispora) ribbon (spiral) shaped chloroplasts (wavy margin) with pyrenoids and a large central vacuole. Nucleus occurs inside the central vacuole where it is suspended by means of cytoplasmic strands.

Spirogyra Reproduction

Spirogyra reproduces by vegetative, asexual and sexual methods.

1. Vegetative reproduction : It takes place by fragmentation of filament. Later on, each segment gives rise to new plant.
2. Asexual reproduction : Normally asexual reproduction is absent in Spirogyra. It occurs only occasionally by the formation of akinetes, aplanospores and azygospores (Parthenospores).

• Akinetes : Under unfavourable conditions, the cells of the filament develop into thick walled structures, which are known as akinetes. On the onset of favourable conditions, these give rise to new plants. Their wall is made up of cellulose and pectin e.g., S. farlowi.
• Aplanospores : These are formed as a result of  contraction of protoplasm and formation of new cellulose wall around it. These nonmotile aplanospores are either round or oval. These later on under favourable condition they give rise to the new individuals. Aplanospores are known to occur in S. aplanospora, S. articulate etc.
• Azygospores or parthenospores : If there is sudden change in the environment, the gametes fail to fuse and each functions as parthenospore. Since the structure of parthenospore is similar to the zygospores produced after sexual reproduction, it is also known as azygospore.

3. Sexual reproduction : The sexual reproduction in Spirogyra is called conjugation, It involves the fusion of two morphologically identical, but physiologically dissimilar gametes. The conjugation is of two types: lateral and scalariform

• Scalariform conjugation : This is the most common and advanced type of conjugation. It involves two filaments of Spirogyra and takes place between two recently formed cells. The participating filaments come closer. The cells of one filament show the formation of papilla towards the other filament. It stimulates the formation of similar papilla in cells lying opposite to them. The two papillae fuse by enzymatic dissolution of the wall thus forming a conjugation canal.

The conjugation tube between the two filaments looks like a ladder, through which gamete from one of the gametangia passes through to fuse with the passive gamete of another filament. The gametes are formed singly and both active and passive gametes are considered male and female gametes respectively. The fusion of both kinds of gametes with each other results into formation of zygospore. In the beginning, zygospore is greenish but simultaneously chloroplasts of male and female gametes disappear due to which zygospore does not remain green.

The zygospore is differentiated into three layers, the outer exospore which is thin, the middle mesospore which is a thick layer of cellulose, chitinized and pale yellow to brown in colour, and inner endospore which is thin and cellulose in nature.

• Lateral conjugation : It takes place between two nearest cells of the same filament (homothallic). Both male and female gametes are found in same filament. It is of two types.

Indirect lateral conjugation : Two outgrowths appear on both sides of a transverse septum of two adjacent cells which later on form a conjugation tube. Of the two cells, one behaves as male gametangium from which gamete passes through the tube into female gametangium. By fusion, zygospore is formed. Thus in each second cell of a filament zygospore is formed. It is commonly seen in S. affinis and S. tenuissima.

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Direct lateral conjugation : In this type of conjugation, the male gamete after passing through an aperture in the transverse septum of adjoining gametangium enters the female gametangium and fuses to form a zygospore. The two gametes, though morphologically alike but differ in their behaviour. Hence this type of sexual reproduction corresponds to physiologically anisogamy. It is commonly seen in S. jogensis.

4. Germination of zygospores : The zygospores on the arrival of favourable conditions germinate. The nucleus undergoes meiosis to form four haploid nuclei (tetra nucleate). Of these three nuclei degenerate and one functions. The exo and mesospores rupture and the endospore protudes out in the form of germling. The new cell undergoes transverse division continuously to form a new filament. Thus in the life cycle of Spirogyra, there is no flagellate phase.

Life cycle in Spirogyra is haplontic as dominant phase in life cycle is haploid (n) and diploid phase is represented by only zygospore and it undergose R.D. or meiosis (zygotic meiosis).

Read More: Sexual Reproduction