Spirogyra are free-floating green algae present in freshwater habitats such as ponds, lakes, etc. Spirogyra are commonly known as “water silk or pond silk”. They have a filamentous and unbranched vegetative structure. There are around 400 species of Spirogyra found. The genus Spirogyra is named after the unique spiral chloroplast present in the cells of algae.
Spirogyra are photosynthetic and contribute substantially to the total carbon dioxide fixation carried out. They increase the level of oxygen in their habitat. Many aquatic organisms feed on them.
Classification of Spirogyra
Spirogyra is classified under Chlorophyta due to the presence of chlorophyll. The genus contains around 400 species.
Structure of Spirogyra
- The vegetative structure of Spirogyra is an unbranched filamentous thallus
- The thallus is multicellular with each cylindrical cell joined end to end
- They are 10-100 µm in width and may grow several centimetres in length
- They are present as a slimy mass due to the presence of mucilage sheath around the filament
- The cell wall is made up of two layers, inner cellulose and outer pectose. The slimy mucilage sheath is due to the dissolution of pectose in water
- In each cell, there is a nucleus, cytoplasm, a large central vacuole and spiral chloroplasts
- Chloroplasts are ribbon-shaped and arranged spirally. There may be 1-16 chloroplasts present in a cell
- Chloroplast contains many pyrenoids in a row. Pyrenoids store starch and protein
Spirogyra undergo vegetative, asexual and sexual reproduction. The life cycle of Spirogyra is haplontic, i.e. the dominant stage is free-living haploid (n) gametophyte and the sporophyte is represented only by the diploid zygote (2n)
Vegetative reproduction is by fragmentation. Under the favourable conditions, vegetative reproduction is the preferred mode of reproduction.
- The vegetative filament after fragmentation develops into a new filament. Each fragment undergoes multiple division and elongation to form a new filament
- Fragmentation can be due to mechanical injury or dissolution of the middle lamella with a change in the salinity and temperature of the water
- Sometimes the middle lamella of one cell protrudes into an adjacent cell resulting in the breakage of the filament
Asexual reproduction is found in few of the species of Spirogyra. Asexual reproduction is by the formation of azygospores, akinetes or aplanospores.
- Formation of aplanospores occurs under unfavourable conditions. The protoplast shrinks and forms a wall around it. This results in the formation of aplanospores
- Akinetes are also formed similarly but they have a thicker cell wall of cellulose and pectin
- Akinetes and aplanospores are non-motile spores, which develop into a new filament under favourable conditions after the decay of the parent filament
- Azygospores are also known as parthenospores. These are the gametes, that failed to fuse during sexual reproduction and develop into a new filament asexually
Sexual reproduction in Spirogyra is isogamous, i.e. male and female gametes of similar size fuse together in the sexual reproduction.
Sexual reproduction is by conjugation. Conjugation is of two types, Scalariform conjugation and lateral conjugation.
In scalariform conjugation, two filaments of Spirogyra sp come together and lie side by side. The structure formed looks like a ladder, so it is named as scalariform conjugation or H-shape conjugation.
- Tube-like structure develops from each cell of the two filaments lying together
- Conjugation canal is formed between two cells after fusion of the developing tube
- The male gamete fuses with a female gamete of the other filament and one of the filaments become empty and the other has zygotes
- These zygotes are released after the decay of the parent filament and germinate under favourable conditions
In lateral conjugation, adjacent cells of a Spirogyra sp function as male and female gametes. Conjugation tubes are formed between cells of the same filament. Lateral conjugation is of two types:
- Direct lateral conjugation: Passage is formed between two adjacent cells through the middle lamella. Male gametes fuse with female gametes. Zygotes are formed in alternate cells
- Indirect lateral conjugation: The conjugation canal is formed by the cell having male gamete and joins the adjacent cell having female gamete
The entire protoplast of a Spirogyra act as a gamete. They are known as aplanogametes. Aplanogametes are formed in the gametangia, which are formed at the end of the growing season of Spirogyra.
The zygote in Spirogyra is known as zygospores. Zygospores are diploid (2n) and formed by the fusion of male and female gametes. Zygospores are the only diploid stage in the life cycle of Spirogyra.
The zygospore has a thick cell wall made up of three layers; exosporium (outer layer of cellulose), mesosporium (middle layer of chitin and cellulose) and endosporium (inner layer of cellulose).
The zygospore remains dormant until favourable conditions are available.
At the time of germination, the zygospore undergoes meiosis to form 4 haploid (n) nucleus, of which only one survives and others disintegrate.
Developing zygospore burst open to form germ tube. The germ tube divides repeatedly by transverse division and develops into a new haploid filament of Spirogyra.