Detailed account of Nostoc along with its thallus and cell structure
Nostoc, the most common genus of the cyanobacteria family, is a gram-negative photosynthetic cyanobacteria. Paracelsus first coined the term Nostoc, derived from the English word nostril and the German word Nasenloch, meaning a nose hole. The name of this class of algae is based on the appearance of their colonies, like that of nasal mucus. Besides this, the genus is also known by other names, like star jelly, troll's butter, and witch's butter, due to its jelly-like appearance. In earlier times, it was thought that these jelly-like masses fell from the sky onto Earth.
Systematic Position
- Division: Cyanophyta
- Class: Cyanophyceae
- Order: Nostocales
- Family: Nostocaceae
- Genus: Nostoc
The division Cyanophyta comprises the prokaryotic organisms that derive energy through photosynthesis. It includes blue-green algae, or cyanobacteria. The organisms under this division have scattered chlorophyll across the cytoplasm and other characteristic pigments like carotenoids, phycoerythrin, and phycocyanin. They have an incipient nucleus with DNA and RNA.
The order Nostocales is a large group of cyanophyceae and is represented by the maximum number of filamentous forms of species. They are found in natural habitats as singles or bunches of filamentous material covered with a mucilaginous sheath (e.g., Nostoc), marine (e.g., Nodularia), or both (e.g., Anabaena). They are well-known for their nitrogen-fixing ability and symbiotic relationships with a variety of organisms.
Occurrence
Nostoc occurs in a wide range of habitats. They are generally found in freshwater bodies and soil. Nostoc species have also been reported to grow in moist soils, in the Arctic, and in alpine meadows. Besides this, they also occur on rocks, tree trunks, the bottom of water bodies, etc.
Nostoc is also known to occur in symbiotic relationships with other organisms, like plants, bacteria, fungi, and other animals. Nostoc is well-known for establishing symbiotic relationships with hornworts (Bryophytes). Many species of bryophytes comprise cavities in their thallus body, which bears Nostoc cells. These cavities are called cyanobacterial cavities. In Dendroceros crispatus, the cavities appear like dark spots and have a diameter of around 0.5 mm. Similar cavities have been observed in Blasia pusilla.
Bryophytes, like Blasia pusilla and Anthoceros punctatus, liberate certain chemicals that induce hormogonia formation in Nostoc when nitrogen is deficient in the environment. They also secrete hormogonia-attracting chemicals that guide the Nostoc colonies towards the cavities. In response to these secretions, Nostoc enters the cavities through hormogonia. Later, bryophytes also secrete hormogonia-repressing factors that suppress hormogonia formation and promote heterocyst formation. In this way, plants fulfill their nitrogen requirements through associations with cyanobacteria such as Nostoc. There are symbiotic associations of Nostoc with several other plants, e.g., N. azollae with water fern, N. punctiforme with Cycas, and with the underground stems of Gunnera manicata.
Nostoc is also a component in lichen, a symbiotic association of algae with fungi. There is a diversity of Nostoc-containing lichens, differing in structural organization, complexity, and morphology. In Peltigera, Nostoc is present only in specific regions of the thallus, while in bipartite (with one algal/photosynthetic component) lichen Peltigera, there is an even distribution of Nostoc below the upper cortex of the thallus. In P. aphthosa (tripartite lichen, having two algal/photosynthetic components), there is an even distribution of green alga Coccomyxa below the upper cortex, while Nostoc is present in special structures called cephalodia.
Thallus Structure
There is a wide diversity of Nostoc thalli based on shape, size, and hollow or solid structure. They can be round, jelly-like, or irregularly shaped. Their colonies are generally greenish or bluish-green, and their size is as large as eight centimeters in diameter. Young colonies are microscopic, but they grow with maturity and form a visible colonial mass.
Many moniliform or bead-like cells, called trichomes, enclosed within a gelatinous matrix form Nostoc colonies of varying shapes and sizes. Trichomes are composed of round or oval cells loosely joined together in chains. They are more numerous at the boundary of the colony than at the center.
Some special types of colorless, hollow, and spherical or barrel-shaped cells, called heterocysts, appear along the trichomes at regular intervals throughout the filament. They are the specialized sites of nitrogen fixation that are carried out with the help of the nitrogenase enzyme in an anaerobic environment. An anaerobic condition is maintained within them through a glycolipid layer impermeable to oxygen, the absence of photosystem II, and the presence of cyanoglobin protein that eliminates oxygen. Heterocysts are also involved in vegetative reproduction.
They differ from other cells, particularly vegetative cells, in that they have thicker walls and larger sizes. They may be single or in pairs. The trichomes, together with the mucilaginous sheath around itself, form the filaments. The sheath may be hyaline or colored. Each filament consists of intercalary heterocysts and akinetes. Based on the position of heterocysts in the filaments of cyanobacteria, they are of two types, viz., intercalary and terminal. The terminal and intercalary heterocysts have one and two polar nodules, respectively, at the polar ends of the cells, where they adjoin other cells. The two pores of each intercalary heterocyst help establish cytoplasmic connections with neighboring cells.
A mucilaginous envelope of individual trichome sheaths keeps the colony assembled. This envelope is surrounded by a firm pellicle-like membrane, which provides a definite shape to the colony. A compound colony, the macroscopic thallus of Nostoc, is formed from the fusion of individual gelatinous sheaths of neighboring colonies.
Cell Structure
The cells of Nostoc colonies are prokaryotic. Their cell wall is made up of peptidoglycan. It also comprises muramic acid and di-amino-pimelic acid. The cytoplasm has two regions: the outer chromoplasm and the central centroplasm. These regions are not separated by a membrane. Chromoplasm is a colored component due to the presence of pigments like chlorophyll a, phycoerythrin, and phycocyanin. It consists of photosynthetic lamellae, where photosynthesis takes place. Myxophycean or cyanophycean starch and proteinaceous cyanophycin granules are also present in it. Extracellular pigments, like nostocine, are also present for protection from UV radiation. The centroplasm is a colorless component that consists of an incipient nucleus. Within the incipient nucleus, or false nucleus, the genetic material, DNA, is present.
FAQs about Nostoc
What is Nostoc?
Nostoc is a genus of cyanobacteria, commonly known as blue-green algae, characterized by its gram-negative photosynthetic nature.
How did Nostoc get its name?
The term "nostoc" was coined by Paracelsus, derived from the English word "nostril" and the German word "Nasenloch," referring to its resemblance to nasal mucus.
What are some alternative names for Nostoc?
Besides Nostoc, this genus is also known by names such as star jelly, troll's butter, and witch's butter due to its jelly-like appearance.
What is the systematic position of Nostoc?
Nostoc belongs to the division Cyanophyta, class Cyanophyceae, order Nostocales, and family Nostocaceae.
Where can Nostoc be found?
Nostoc is found in a wide range of habitats, including freshwater bodies, soil, rocks, tree trunks, and the bottom of water bodies. It can also occur in symbiotic relationships with other organisms.
What are some examples of symbiotic relationships involving Nostoc?
Nostoc forms symbiotic associations with plants like hornworts (Bryophytes) and water ferns, as well as with fungi in lichen associations.
How does nostoc contribute to nitrogen fixation?
Nostoc contains specialized cells called heterocysts, which are involved in nitrogen fixation, allowing it to convert atmospheric nitrogen into a usable form for plants and other organisms.
What is the structure of Nostoc's thallus?
Nostoc thalli vary in shape, size, and structure, ranging from round and jelly-like to irregularly shaped. They consist of trichomes enclosed within a gelatinous matrix and may contain specialized cells called heterocysts.
What are some unique features of Nostoc's cell structure?
Nostoc cells have a prokaryotic structure with a cell wall made of peptidoglycan. They contain chromoplasm and centroplasm regions, with chromoplasm containing photosynthetic pigments and centroplasm housing genetic material.
How does Nostoc protect itself from UV radiation?
Nostoc produces extracellular pigments like nostocine, which serve as protection from UV radiation, aiding its survival in various environments.