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. They are named after their beautiful spiral chloroplasts.
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.
Each cell of the filaments features a large central vacuole, within which the nucleus is suspended by fine strands of cytoplasm.
The chloroplasts form a spiral around the vacuole. The cell wall consists of an inner layer of cellulose and an outer layer of pectin, which is responsible for the slippery texture of the algae.
In spring, Spirogyra grows under water. When there is enough sunlight and warmth they produce large amounts of oxygen, which get stored as bubbles between the tangled filaments.
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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 shows as 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 consists of two layers viz. 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 can be found in few species of Spirogyra. Asexual reproduction is carried out 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
Zygospores are also known as parthenos pores. These are the gametes, that failed to fuse during sexual reproduction and develop into a new filament asexually
Sexual reproduction in Spirogyra is isogamous which means that male and female gametes of same size fuse together in the sexual reproduction. Sexual reproduction is by conjugation. Conjugation can be further divided into two types, Scalariform conjugation and lateral conjugation.
In scalariform conjugation, two filaments of Spirogyra come together and lie side by side. The structure formed looks like a ladder and thus 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 created between two cells after fusion of the developing tube.
The male gamete fuses with female gamete of the other filament and one of the filaments becomes empty and the other has zygotes.
These zygotes are released after the decay of the parent filament and germinate under favourable conditions.
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 cell having male gamete forms conjugation canal and joins the adjacent cell having female gamete.
The entire protoplast of a Spirogyra acts as a gamete. They are known as aplanogametes. Aplano Gametes are created in the gametangia, which are in turn formed at the end of the growing season of Spirogyra.
The zygote in Spirogyra is known as zygospores. Zygospores are known as diploid (2n) and are formed by the fusion of male and female gametes. In the life cycle of Spirogyra, Zygospores are the only diploid stage.
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 a germ tube. The germ tube divides repeatedly by transverse division and develops into a new haploid filament of Spirogyra.