Spirogyra is a green alga that is mostly found in freshwater in the form of clumps. These are also called Water silk or Mermaid’s tresses.
- It is a unicellular organism but can be seen in freshwater bodies as it clumps together to form a multicellular structure.
- Spirogyra consists of chlorophyll which gives it a green appearance. As a result, it can also prepare its own food.
- Spirogyra is a filamentous type of freshwater green algae, which are most easily recognized due to their spirally coiled chloroplasts.
- Spirogyra spp. are filamentous, unbranched algae that have a unique mode of sexual reproduction.
Classification of Spirogyra
Around 400 species of Spirogyra are found throughout the world in various freshwater bodies.
The following is the taxonomical classification of Spirogyra described or proposed by E. Fritsch (1935) in his book “The Structure and Reproduction of the Algae”
Protista or Protoctista
Habitat of Spirogyra
- Green algae, Spirogyra (Chlorophyta), are found in a wide range of habitats including small stagnant water bodies, rivers, and streams.
- It is also found as a part of the vegetation found at the edges of large lakes and rivers.
- Spirogyra exists in a filamentous form which often forms masses that float near streams and ponds, buoyed by oxygen bubbles released during photosynthesis.
- Some Spirogyra species are found in slow-running streams and rivers where they appear as green clumps.
- These are also found in temporary ponds that last for a short duration of time, where they flourish during wet weather and dry up subsequently.
- These are photosynthetic with chlorophyll as the photosynthetic pigment, which makes them an essential part of the aquatic ecosystem.
- Spirogyra is a long, filamentous, multicellular green algae consisting of cells attached end-to-end to form long chains. These filaments or chains can be several centimeters in length.
- The vegetative structure of Spirogyra is termed thallus, which is the unbranched filamentous strand consisting of multiple cells.
- The cells in the filament are 10-100 µm in length and are cylindrical in structure. Each cell in the filament is attached to two cells along with its breadth except the cells at the two ends, which are attached to a single cell each.
- The terminal cell of the filament in some species might be non-green and is termed as a holdfast. Some species like Spirogyra longata also have root-like rhizoids arising from the holdfast that holds the filaments the substratum.
- The cells in the filamentous structure are characterized by one or more spiral chloroplasts that give the characteristic green color to the organism.
- The cell wall in each cell is composed of two layers; the inner layer composed of cellulose and the outer layer composed of pectin. These layers are responsible for the slippery texture of the organism.
- Under the microscope, Spirogyra appears surrounded by a slimy jelly-like substance which is the outer wall of the organism dissolved in water. Besides, the filaments are also surrounded by mucilage that holds the filaments together to form clumps in water.
- The wall between two cells is composed of the middle lamella, which can be either plane, replicate, or colligate.
- Most of the interior of the cell is occupied by the large central vacuole which surrounds the nucleus suspended by delicate strands of cytoplasm.
- Besides the large vacuole, the cytoplasm also consists of the chloroplast that forms a spiral around the vacuole and consists of specialized bodies called pyrenoids. Pyrenoids are unique structures with a central protein core that store starch.
- Except for the holdfast, all other cells in the filament can divide and increase the length of the filament.
Cultural characteristics of Spirogyra
- Morphological characteristics of algae are the essential characteristics for the identification of most algae. However, some cultural characteristics of algae might also be necessary for identification.
- Spirogyra, along with other green algae, can be cultured in BG-11 medium. However, Bold’s Basal Medium with triple nitrate and vitamins is also considered a suitable medium for the growth of Spirogyra species.
- Algae grow slower when compared to bacteria, and for Spirogyra, it takes several weeks to obtain a dense algae culture from one single cell.
- During the cultivation, concentrations of phosphate and nitrate are added twice a week to the algae suspension if necessary.
- The typical sigmoidal growth characteristics in Spirogyra consist of a short lag phase followed by exponential growth.
- As the density of the culture increases, the medium might turn dark green, and the supernatant of the centrifuged samples is found to become increasingly red and viscous, which most likely be caused by mucilage production of Spirogyra, which is a strategy of defending itself against epiphytes.
- Spirogyra grows in the form of green filaments that together form green clumps throughout the plates or bioreactors.
- Depending on the species, the length of the filaments can range from 100-600 µm with the width ranging from 10-100 µm.
- Besides, the shape of the gametangia and zygospores might also range between different species which helps in the identification of the organism.
Life Cycle of Spirogyra
- The life cycle of Spirogyra occurs via one of the three ways; vegetative, asexual, and sexual. The vegetative and sexual cycles are more common than asexual cycles.
- A form of alternation of generation characterizes the life cycle of Spirogyra. It is haploidic meaning, the haploid gametophytic structure of the organism is the prolonged structure followed by a brief diploid zygospore as the sporophytic structure.
- This is observed during sexual reproduction where the life cycle of the organism alternated between the haploid filament and the diploid zygospore.
- The zygospore is the only diploid phase in the sexual life cycle. After fusion, the female gametangia undergo decay to release the zygospore.
- The zygospore remains at the bottom of the pond until a favorable condition is present.
- The zygospore then divides meiotically to form four haploid nuclei, out of which only one survives.
- The zygospore then slowly grows in size and bursts to release the germ tube. It is followed by repeated transverse divisions of the germ tube to form a haploid filament.
Reproduction in Spirogyra
Vegetative and sexual reproduction in Spirogyra is common, whereas asexual reproduction occurs occasionally.
1. Vegetative reproduction
- Vegetative reproduction in Spirogyra is the shortest method of reproduction that occurs via fragmentation.
- Spirogyra can multiply by fragmentation where the vegetative filament of the organism breaks into fragments, each of which independently develops into a new filament.
- The fragment undergoes multiple divisions to form an elongated vegetative filament.
- Under favorable conditions, fragmentation is the most common method of reproduction in Spirogyra.
- The breakdown of the filament to form individual fragments might occur either due to mechanical injury, dissolution of the middle lamella, or the formation of H-shaped fragments.
2. Asexual reproduction
Asexual reproduction is less common among Spirogyra, but it occurs in some species under unfavorable conditions by means of the formation of asexual spores like aplanospores, akinetes, and zygospores.
- Aplanospores are formed under unfavorable conditions where the cytoplasm of the cell shrinks and a wall is formed around it.
- The aplanospore is non-motile and eventually leads to the formation of a filament once the condition is favorable.
- In S. aplanospora, the formation of aplanospores is the only method of reproduction.
- In some species of Spirogyra, the cell develops a thick wall around itself to protect itself from the unfavorable condition.
- Once the condition becomes favorable, the akinete develops to form a filament. It is common in S. farlowii.
- In S. varians, sometimes the gametes fail to fuse during sexual reproduction and get enclosed by a thick cell wall forming an azygospore.
- Like other asexual spores, azygospores also develops to form a new filament.
3. Sexual reproduction
- Sexual reproduction in Spirogyra occurs via alternation between a haploid filament and a diploid zygospore.
- Conjugation is the method of sexual reproduction in Spirogyra where the fusion of two gametes of opposite strains takes place. The entire protoplasmic content of the cell acts as the gamete.
- The gametes are morphologically identical, but during conjugation, one of the gametes becomes active (male gamete) while the other becomes passive or non-motile.
- Conjugation in Spirogyra is of two types; Scalariform and Lateral Conjugation
a. Scalariform Conjugation
- It is the more common mode of conjugation that occurs between two different filaments when the two filaments come close and lie parallel to each other.
- Then the opposite cells develop protuberances or outgrowths that extend and come in contact with one another.
- The top of these outgrowths then dissolves to form a conjugation tube between the two cells.
- This results in the formation of a ladder-like structure (scalariform) throughout the filament.
- Meanwhile, the protoplasm of the cells round up to form gametes, and the motile male gamete then moves through the conjugation tube to reach the female gamete.
- The fusion of these gametes results in the formation of a zygote which is diploid. The zygote develops a thick wall to form a zygospore.
b. Lateral Conjugation
Lateral conjugation is less often and occurs between two adjacent cells of the same filament. It might occur in two further ways; Indirect and Direct lateral conjugation
i. Indirect lateral conjugation
- During indirect lateral conjugation, outgrowths emerge on the sides of the septum, which eventually leads to the formation of an opening at the lateral side of the cells.
- One of the two adjacent cells acts as male gametangia, whereas the other acts as the female gametangia.
- The male gamete then moves through the tube and fuses with the female gamete to form a zygote.
- In species reproducing by indirect lateral conjugation, in every second cell of the filament, a zygospore is formed.
- It occurs in tenuissima, S. affinis, etc.
ii. Direct lateral conjugation
- During direct lateral conjugation, a pore is formed in the septum that becomes big enough for the male gametangia to pass trough.
- The male gamete is then transferred through the pore into the female gametangia where they fuse to form a diploid zygote.
- It occurs in S .jogensis.
Identification of Spirogyra
The primary method of identification of Spirogyra is by the observation of the morphological structure of the organism, but other methods of identification are also available.
1. Cultural identification
- The cultural identification of Spirogyra is possible by studying various morphological and physiological characteristics of the organism.
- The length and width of the filament, the structure of the transverse wall (septum), number of chloroplasts per cell, and number of turns of the chloroplast, and the type of the vegetative cells are the few characteristics.
- Besides, species can also be identified based on the structure of the reproductive spores and the type of conjugation.
- The size of the zygospore and the nature of the spore membrane are also studied to obtain proper identification.
2. Molecular identification
- For better and more detailed identification of the species, molecular identification methods can also be applied.
- The most common method of identification of the organism using a molecular technique is DNA sequencing.
- During DNA sequencing, the DNA is first extracted and purified, followed by amplification via PCR.
- The purified PCR products can then be sequenced directly using various sequencing tools.
- Based on the results from the molecular and molecular methods of identification, a molecular phylogeny of the species of Spirogyra can be prepared.
The following table provides some characteristics of some species of Spirogyra that can be used for their identification:
Uses of Spirogyra
- Some species of Spirogyra are used as a source of food in different parts of the world as they are rich in vitamins and minerals.
- Green algae like Spirogyra are also an important part of the aquatic ecosystems as they are photosynthetic and thus provide oxygen to other organisms in the water. These are also the producers of the ecosystem.
- Spirogyra is also considered as an essential source of various natural bioactive compounds that can be used for the antibiotic, antioxidant and anti-inflammatory purposes.
- Some Spirogyra species have been found to have the potential for the treatment of municipal wastewater and biomass production for biofuel applications.
- A certain Himalayan species of Spirogyra, S. porticalis, has been found to produce thirteen known bioactive chemotypes with phytopharmaceutical importance including fatty acid esters, sterols, unsaturated alcohols, and alkynes.
- Spirogyra neglecta is found to possess cancer chemopreventive compounds and activity against lesions (having a high risk of becoming malignant) in rat livers. This might provide significant implications for cancer study and treatment for similar liver cancers found in humans.