Structure of Cyanobacteria:
Recent studies with electron microscope by Wilden and Mercer revealed astonishing details about the structure of cytoplasm and the nature of cell envelope. Many of the protoplasmic structures found in the bacteria occur in the Cyanobacteria. In the central part are present circular fibrils of DNA, which lacks basic proteins (histones). The peripheral protoplasm is composed principally of thylakoids and their associated structures, the phycobilisomes or phycobiliproteins and glycogen granules. 70-S ribosomes are dispersed throughout the cell but are present in the highest density in the central region around the nucleoplasm.
Cyanophycin granules are large bodies composed of stored proteins in the form of polypeptides, usually containing asparatic acid and arginine in a ratio of 1 : 1. Polyhedral bodies are usually associated with the DNA in the central parts of the cells and they have been postulated to contain RNA.
Polyphosphate bodies (Volutin granules):
Contain stored phosphate; polyglucan granules (a-granules) are common in the space between the thylakoids in actively photosynthesising cells. These granules contain a carbohydrate, composed of 14 to 16 glucose molecules, that is similar to amylopectin.
The cell wall is made up of peptidoglycan, i.e., mucopeptide and constitutes 50% of the dry weight. Outside the cell wall is a layer of extracellular mucilage in most Cyanobacteria, called Slime layer’br Sheath or capsule. The sheath is fibrillar, protects die cells from drying, and is involved in gliding. Sheaths are often colored :
(a) Red sheaths found in those Cyanobacteria, which grow in acid soils.
(b) Blue sheaths characteristic of those forms which grow in baste soils.
(c) Yellow and brown sheaths are common in specimens from habitats of high salt content, particularly when the algae undergo drying.
Ultra structure of Cell:
Prokaryotic cell of Cyanobacteria reveals following structures in electron microscopic studies:
1. Mucilage sheath:
Cells of most of Cyanobacteria have thin (Anacystic) or thick (Anabaena) mucilaginous sheath all around. This sheath is made up of microfibrils. These microfibrils are also scattered uniformly in matrix. It contains peptic acid and mucopolysaccharides.
2. Cell wall:
Cell wall has got four layers, known as LI, LII, LIII, and LIV. The LIV layer is outermost layer, which is in contact with mucilaginous sheath while the innermost layer is LI and is in contact with cytoplasm. Cell wall is made up of mucopeptide, muremic acid, Leopolysaccharides, glucosins, glutamic acid and a-diaminopimalic acid.
Cytoplasm is covered by protoplasmic membrane. It is a semi-transparent membrane made up of protein. Cytoplasm can be divided into two parts:
It is the outermost part of the cell, which contains thylakoids or lamellae. They are arranged in parallel rings or scattered. They are flat, sac-like structures, enclosed by unit membrane. Each membrane is 70-80 A thick. At their surface phycobilisome and biliproteins are present.
Apart from these structures, cytoplasm also contains ribosomes, cyanophycean granules, polyglucan granules, carboxysome, polyhedral bodies, Gas vacuoles, polyphosphate granules etc,
Central transparent part of the cell is called centroplasm. Genetic material is found in this portion.
Genetic material is in the form of DNA. Nuclear membrane, nucleoli and histones are absent. In this part besides DNA, RNA is also present. Thus, in Cyanobacteria organised nucleus is not present and this nucleus is called incipient nucleus.