Spirogyra

Spirogyra is the most common, free-floating freshwater green algae, occurring universally in temporary or permanent water bodies. It grows as a partially submerged or free-floating filamentous mass and is known as ‘water silk’, ‘pond silk’, and ‘mermaids’ traces’. Spirogyra are photosynthetic algae that make a substantial contribution to the total carbon dioxide fixation. They also increase the level of oxygen in their habitat. The genus Spirogyra (Gr. speira, a coil; gyros, twisted) is named after the unique spiral chloroplast present in the cells of the algae.


Spirogyra sp. under a light microscope.	Spiral chloroplast of Spirogyra sp.

Systematic Classification

Kingdom: Plantae

Phylum: Charophyta

Class: Zygnematophyceae

Order: Zygnematales

Family: Zygnemataceae

Genus: Spirogyra

Name of Some Common Species

The genus contains around 400 species. Some of the common species are

Spirogyra elongata
Spirogyra maxima
Spirogyra neglecta
Spirogyra adnata
Spirogyra affinis
Spirogyra fluviatilis
Spirogyra dubia

Structure of Spirogyra

The vegetative body of Spirogyra is composed of unbranched filaments, having a silky appearance. The filaments are composed of an uniseriate row of an indefinite number of cylindrical cells, where the basal and apical cells are undifferentiated. In some species, the basal cells are modified to form the holdfast, while in some, the attachment occurs through a hapteron (a rhizoidal growth from the basal cell).

Structure of a Single Cell

Structure of a single cell of Spirogyra

The cell is cylindrical, comprising the following structures.

Cell wall consists of two concentric protective layers (the outermost pectose layer (forms mucilage) and the inner two cellulose layers). The pectin of the outermost layer gets dissolved in water and forms a slimy, mucilaginous sheath, which provides a slippery texture to the filament. The transverse walls are three-layered and might be oriented differently.
Cell membrane is a selectively permeable lipoprotein membrane, remains within the cell wall.
Cytoplasm is granular and vacuolated, remaining within the cell membrane. Strands of cytoplasm hold the nucleus in position.
Ribbon-like, spirally-coiled single or multiple chloroplasts are present with smooth or wavy margins. Each chloroplast consists of small spherical pyrenoids (starch plate enclosing a protein core), equidistant from each other.
The eukaryotic nucleus is held in position by cytoplasmic strands.
A large central vacuole nearly occupies the entire cell, and remains separated from the cytoplasm by a thin layer of tonoplast.
Cell organelles like mitochondria, ribosomes, endoplasmic reticulum, and Golgi bodies are also present.

Spiyorga

Reproduction in Spirogyra

Spirogyra reproduces by both vegetative and sexual means, and rarely through an asexual mode of reproduction.

Vegetative Reproduction

It occurs in Spirogyra through the process of fragmentation. Due to mechanical injury or extreme change in external conditions, the vegetative filament of the organism can dissociate into separate fragments, and each of these fragments develops into a new filamentous plant body.

Sexual Reproduction

It occurs in Spirogyra through the process of conjugation, where the two gametangia fuse. It occurs under unfavourable conditions, but also occurs during nitrogen deficiency or the presence of high concentrations of sugar. The two types of conjugation seen are-

Scalariform Conjugation

Takes place between two or three filaments in a ladder-like configuration.
After coming in close contact, they produce lateral outgrowths.
The wall of contact between the tubular outgrowths dissolves to form the conjugation canal or conjugation bridge.
The entire cytoplasm in the conjugating cells retracts to form the gametangia. Gametes are non-flagellated, amoeboid in nature, produced inside gametangia.
The male gametangium (-) passes to the female gametangium (+) through the conjugation tube by amoeboid movement; the male filament becomes empty.
The fusion of gametes occurs in the female filament, resulting in a diploid, dark coloured, thick-walled zygospore.
The zygospores are released after the parent filament has decayed, and germinate under favourable conditions.

Lateral Conjugation

This type of conjugation occurs between the adjacent cells of the same filament and can be

i) Indirect Lateral Conjugation

The male and female gametangia are formed in the adjacent walls of the same filament.
Lateral walls of each side of the transverse septum form papillae, which ultimately grow to form the conjugation tube.
The male gametangium leaves its cell through this tube to the female gametangium to form a zygospore.
The zygospore thus alternates with empty cells in the same filament.

ii) Direct Lateral Conjugation

A protuberance is formed at the centre of the transverse wall, between two cells.
The upper cell acts as the male gametangium, and the lower cell is the female gametangium.
An apical hole is formed at the head of the protrusion, through which the male gametangium passes to the female gametangium to produce a zygospore.

Asexual Reproduction

Spirogyra mostly reproduces vegetatively and sexually; reports of asexual spores are rare and species-specific.

Formation of aplanospore: Repeated cytoplasmic division produces non-motile aplanospores with thick cellulosic walls; after being liberated, these spores germinate to produce a new plant body.

Formation of azygospore: During unfavourable conditions, failing conjugation results in the female gametangia developing into azygospore or parthenospore; these are not true asexual spores, but germinate to form new plants under favourable conditions.

Economic Importance of Spirogyra

Serves as food for aquatic animals and is used in some regions traditionally, but not widely, as human food.
These photosynthetic organisms act as the producers of the aquatic ecosystem; moreover, they provide oxygen to the other aquatic organisms.
An essential source of various bioactive compounds that can be used for antibiotic, antioxidant, and anti-inflammatory purposes.
Some species have the potential to treat the wastewater and help in biomass production for biofuel applications.
Spirogyra neglecta possesses chemopreventive compounds as seen in rat livers.
Spirogyra porticalis produces thirteen known bioactive chemotypes with phytopharmaceutical importance.

Frequently Asked Questions (FAQs)

Q1. Why is Spirogyra called ‘water silk’?

Spirogyra filaments have a slimy, silky texture due to the mucilage around their cell walls, giving them the appearance of silky green threads floating in water.

Q2. Under what conditions does Spirogyra reproduce sexually?

Spirogyra reproduces sexually by conjugation, mainly under unfavourable conditions such as nutrient deficiency, stress, or environmental change.

Q3. What is the economic importance of Spirogyra?

Spirogyra plays a role in oxygenating water, serves as food for aquatic animals, provides bioactive compounds, and has potential uses in wastewater treatment and biofuel production.

*The highlighted text is factual data.