Spirogyra

Spirogyra, also known as "water silk" or "pond silk," is 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.  Spirogyra is a filamentous green alga widely found in freshwater habitats such as ponds, ditches, and slow-moving streams. Known for its beautiful spiral-shaped chloroplasts, Spirogyra has fascinated botanists, microbiologists, and environmental scientists for centuries. Apart from its striking appearance, it plays an important role in aquatic ecosystems, oxygen production, and research in cell biology.

 

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 musculature 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 holdfast is present (e.g., S. fluviatilis). The cells are elongated and cylindrical. The cell wall is two-layered, the outer layer is of pectic substance, and the inner layer r of cellulose.

The outer part (pectin) dissolves in water to form a mucilaginous sheath. For this reason, Spirogyra filaments are slippery. The transverse or septum can be plane, colligate (with an H-shaped piece), replicate (ring-like ingrowths), or unduloseptate (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, a thin layer of cytoplasm, a 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 the fragmentation of the filament. Later on, each segment gives rise to a 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 the contraction of protoplasm and the formation of a new cellulose wall around it. These nonmotile aplanospores are either round or oval. These later on, under favourable conditions they give rise to the new individuals. Aplanospores are known to occur in S. aplanospora, S. articulate, etc.
• Azygospores or parthenospores: If there is a sudden change in the environment, the gametes fail to fuse and each functions as a parthenospore. Since the structure of the 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 a papilla towards the other filament. It stimulates the formation of similar papillae 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 a gamete from one of the gametangia passes 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 in the formation of a zygospore. In the beginning, the zygospore is greenish, but simultaneously chloroplasts of male and female gametes disappear due so 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 the 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 the 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 a male gametangium from which a gamete passes through the tube into the female gametangium. By fusion, a 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 the 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 zygospore: The zygospores, on the arrival of favourable conditions, germinate. The nucleus undergoes meiosis to form four haploid nuclei (tetra-nucleate). Of these three nuclei degenerates and one functions. The exo and mesospores rupture, and the endospore protrudes out in the form of a 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 the dominant phase in the life cycle is haploid (n) and the diploid phase is represented by only the zygospore, and it undergoes R.D. or meiosis (zygotic meiosis).

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Spirogyra belongs to the group Chlorophyta (green algae) and the family Zygnemataceae. It is one of the simplest eukaryotic organisms with multicellular filamentous structures.

• The name Spirogyra is derived from the Greek words speira (spiral) and gyros (coil), referring to the spirally coiled chloroplasts that make it easily recognizable under the microscope.

• It is often seen as a slimy green mat on the surface of freshwater bodies during warm seasons. The slimy texture is due to the secretion of mucilage around the filaments.

• Because of its oxygen-producing ability during photosynthesis, it is sometimes called “water silk” or “pond scum.”

Taxonomy and Classification of Spirogyra

The scientific classification of Spirogyra is as follows:

• Kingdom: Plantae

• Division: Chlorophyta

• Class: Zygnematophyceae

• Order: Zygnematales

• Family: Zygnemataceae

• Genus: Spirogyra

There are more than 400 species of Spirogyra, making it a highly diverse genus.

Habitat of Spirogyra

Spirogyra is primarily aquatic and freshwater in nature.

• Found in ponds, ditches, rice fields, lakes, and slow-moving streams.

• Grows in clean, nutrient-rich water but may also appear in stagnant conditions.

• During the rainy season or in nutrient-rich environments, it often blooms massively, forming dense mats on the water surface.

• These mats sometimes deplete oxygen in water at night (due to respiration), leading to fish mortality.

General Characteristics of Spirogyra

Some of the defining features of Spirogyra include:

1. Multicellular Filamentous Structure – It forms long, unbranched filaments made up of cylindrical cells arranged end-to-end.

2. Cell Wall – Composed of cellulose and pectin. The outer pectin wall dissolves in water and makes the filaments slimy.

3. Chloroplasts – Distinctive spiral-shaped chloroplasts, each containing pyrenoids for starch storage.

4. Nucleus – Centrally located and suspended by cytoplasmic strands.

5. Reproduction – Can reproduce by vegetative, asexual (rare), and sexual methods (conjugation).

6. Nutrition – Photoautotrophic, using sunlight for photosynthesis.

7. Mucilage Secretion – Protects from herbivores and desiccation.

8. Buoyancy – Gas bubbles generated during photosynthesis often make the filaments float on the water surface.

Structure of Spirogyra

Each filament of Spirogyra is a chain of elongated cells. The structure can be divided into:

1. Cell Wall

• Double-layered, with an outer pectin layer and an inner cellulose layer.

• Provides rigidity and shape.

• Pectin dissolves in water, giving it a slimy texture.

2. Cytoplasm and Nucleus

• A thin layer of cytoplasm lines the cell wall.

• The nucleus is suspended in the center by cytoplasmic strands.

3. Chloroplasts

• Ribbon-shaped and spirally coiled, unique to Spirogyra.

• Each chloroplast contains pyrenoids, which synthesize and store starch.

4. Vacuole

• Large central vacuole filled with cell sap.

• Maintains turgor pressure and stores nutrients.

Reproduction in Spirogyra

Spirogyra reproduces by vegetative, asexual, and sexual methods.

1. Vegetative Reproduction

• The most common mode of reproduction.

• Filaments break into fragments due to mechanical injury, water currents, or decay of old cells.

• Each fragment grows into a new filament.

2. Asexual Reproduction

• Rare in Spirogyra.

• Occurs through spore formation under unfavorable conditions.

• Aplanospores or akinetes (resting spores) develop and germinate later.

3. Sexual Reproduction (Conjugation)

Spirogyra reproduces sexually through conjugation, a unique process among algae. There are two types:

a) Scalariform Conjugation

• Two parallel filaments align side by side.

• Conjugation tubes form between opposite cells.

• The protoplast of one filament migrates into the other and fuses to form a zygospore.

b) Lateral Conjugation

• Occurs within a single filament.

• Adjacent cells form conjugation tubes.

• The protoplast of one cell fuses with the neighboring cell, forming a zygospore.

Zygospore:

• Thick-walled, resistant structure.

• Helps the algae survive harsh conditions.

• Germinates under favorable conditions to form a new filament.

Life Cycle of Spirogyra

The life cycle of Spirogyra is haplontic:

• Dominant phase = haploid vegetative filament.

• Diploid phase = only the zygospore.

• Zygospore undergoes meiosis upon germination, producing haploid cells that grow into new filaments.

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Photosynthesis in Spirogyra

Spirogyra performs photosynthesis efficiently due to:

• Large surface area of filaments.

• Spiral chloroplasts that maximize light absorption.

• Pyrenoids for starch storage.

Photosynthesis equation in Spirogyra:

6CO2+6H2O+Light  →  C6H12O6+6O26CO_2 + 6H_2O + Light \; \rightarrow \; C_6H_{12}O_6 + 6O_2

This makes Spirogyra an important oxygen producer in aquatic ecosystems.

Ecological Importance of Spirogyra

1. Oxygen Production – Contributes to dissolved oxygen in freshwater ecosystems.

2. Primary Producer – Forms the base of aquatic food chains.

3. Habitat Provider – Dense mats provide shelter for microorganisms and small invertebrates.

4. Nutrient Cycling – Helps in recycling carbon and other nutrients in water.

5. Indicator of Water Quality – Presence indicates nutrient-rich waters (eutrophic conditions).

Economic Importance of Spirogyra

1. Educational Use – Commonly studied in schools and colleges as a model alga.

2. Research – Used in studies on photosynthesis, cytoplasmic streaming, and cell biology.

3. Food Source – In some regions, Spirogyra is consumed as a protein-rich food.

4. Fertilizer – After decay, it adds organic matter to ponds and fields.

5. Biotechnological Applications – A Potential source of biofuels, antioxidants, and natural polymers.

Adaptations of Spirogyra

• Mucilage coating protects against desiccation and herbivory.

• Floating ability ensures access to sunlight.

• Conjugation enables survival through resistant zygospores during harsh conditions.

• Spiral chloroplasts maximize the efficiency of light absorption.

Spirogyra Under the Microscope

When observed under a light microscope:

• Filaments appear long, unbranched, and green.

• Chloroplasts show a characteristic spiral or helical arrangement.

• Cytoplasmic strands suspending the nucleus can be seen.

• During conjugation, conjugation tubes and zygospores are visible.

Spirogyra Blooms

In nutrient-rich water bodies, Spirogyra can form massive blooms, covering the entire surface of the water.

• Blooms may block sunlight penetration, affecting aquatic plants below.

• At night, when respiration dominates, oxygen levels can drop drastically, causing fish kills.

• Blooms also indicate eutrophication due to excessive nutrients (nitrates, phosphates) from agricultural runoff.