Cyanobacteria

Definition: Cyanobacteria represents the major group of photosynthetic bacteria that carry out oxygenic photosynthesis. It releases oxygen and uses water as an electron-donating substrate, i.e. splits water molecule to release oxygen. Other than this, they are also capable of nitrogen-fixing.

Classification of Cyanobacteria
Kingdom: Monera
Division: Eubacteria
Class: Cyanobacteria
Examples: Nostoc sp., Oscillotoria sp., Spirulina sp., Lyngbya sp., Azolla sp., Anaebina sp., Gleotrichia sp. etc.

Content: Cyanobacteria

  1. History
  2. Characteristics
  3. Habitat
  4. Types
  5. Structure
  6. Specialised Structure
  7. Reproduction
  8. Economic Importance
  9. Harmful Effects

History of Cyanobacteria

Cyanobacteria were the first algae to appear as a fossil in the history of 3.5 billion years. Fossils of cyanobacteria were called as “Stromatolites“. These were the first organism that was responsible for releasing oxygen into the environment through their photosynthetic activities. Among prokaryotic organisms, cyanobacteria were the first organisms to use two photo-systems both I and II.

Firstly it showed resemblance to the algae group, by which it then included in the class Myxomycetes. But, after further study on their structure and characteristics features, it is now placed in the group Bacteria under the class Schizomycetes or cyanobacteria.

Characteristics of Cyanobacteria

Cyanobacteria are prokaryotic and gram-negative bacteria. These are aerobic and photoautotrophic organisms. Its size ranges from 1-10µm. It contains a primitive nucleus. Membrane-bound organelles are absent in cyanobacteria. Chlorophyll a, phycocyanin and phycoerythrin are the photosynthetic pigments that are present in this.

  • Chlorophyll a: Imparts green colour
  • Phycocyanin: Imparts blue colour
  • Phycoerythrin: Imparts red colour

As cyanobacteria also called blue-green algae, is due to the presence of chlorophyll a and phycocyanin. Chlorophyll involves in photosystem I whereas phycocyanin involves in photosystem II. Its photosynthesis is “oxygenic”. Cyanobacteria have the ability to fix atmospheric N2. The reserve food material is “Cyanophycean starch”.

Habitat

These are the most genetically diverse organisms that show a broad range of habitat. They can live in freshwater, marine water, damp places, moist soil, hot springs etc.

Types

Cyanobacteria can be of three kinds: Unicellular, colonial and filamentous.

  1. Unicellular: These are single-celled, having mucilage sheath covering.
    Examples: Chroococcus sp.
  2. Colonial: These are multicellular. It forms when single cells colonise themselves.
    Examples: Gleocapsa sp.
  3. Filamentous: These founds in the form of a chain, covered by a sheath. It consists of:
    Hetero-cysts: Helps in N2 fixation
    Akinete: Collects the food material.
    Examples: Nostoc sp., Oscillotoria sp. etc.
    types of cyanobacteria

Structure of Cyanobacteria

cyanobacteria structure

The structural components of cyanobacteria are:

Sheath

It acts as an envelope of cyanobacteria. It is mucilaginous, undulating and hygroscopic in nature. Its physical and chemical properties depend upon the environmental conditions. It contains “pectic substances”.  Its appearance is fibrillar under the microscope.
Function: sheath protects the cell from unfavourable conditions. It keeps the cell moist.

Cell Wall

It is present between the sheath and plasma membrane. The pores that are present on the cell wall helps in secretion of mucilage. It consists of four layers represented by L-I, L-II, L-III and L-IV and interconnects with each other by “Plasmodesmata“.

L-I: It is the innermost layer, electron transparent, 3-10nm thick
L-II: It is thin and electron-dense. It is made of mucopeptide, mumaric acid, alanine, glucosamine, glutamic acid and diaminopametic acid. Its primary function is to provide shape and mechanical strength to the vegetative cell.
L-III: It is electron transparent and 3-100nm thick.
L-IV: It is the outermost layer of the cell wall that is electron-dense and wrinkled.

Plasma Membrane

It is a bilayer membrane, also called “Plasmalemma“. It is 70Å thick, selectively permeable, maintains the structural integrity of the cell. There is a structure called “Lamellosomes” forms when plasma membrane invaginates and fuses with lamellae. It encloses cytoplasm and other cell inclusions.

Cytoplasm

It consists of two regions:
Chromoplasm: It comprises of a vesicular structure called thylakoids, having a capacity of “Photophosphorylation”.
Centroplasm: It is colourless and regarded as the primitive nucleus (devoid of nuclear membrane and nucleolus). Some granular structures present inside this.

Cytoplasmic inclusions: It includes:

Cyanophycean granules: It is the nitrogenous reserve material. It is composed of equal molecules of arginine and aspartic acid. It constitutes about 8% of the total cell weight.
Gas vacuoles: These are hollow, tough and elongated in shape. Its diameter is 75nm and length is 200-1000nm. Protein layer covers these gas vacuoles, which is 2nm thick. Its ends are conical.
Carboxysomes: These are polyhedral bodies, contains 1,5-ribulose biphosphate carboxylase.
Phosphate bodies: These are the volutin granules.
Phycobilisomes: Phycobiliproteins when aggregates into a ball like structure form phycobilisomes. It is attached to the outer surface of the thylakoid.
DNA matrix: Naked DNA fibrils dispersed in the cytoplasm, lacks nucleoplasm. The base composition of DNA differs. Cyanobacteria contain covalently closed, non-functional, circular plasmid DNA.

Specialised Structure

Cyanobacteria comprise of a specialised structure known as “Heterocyst”.heterocyst structure

Heterocyst: These are the modified vegetative cells. Its formation depends upon the nitrogen concentration in the environment. These are a slightly enlarged cell, pale yellow. It produces singly or in chains and its position can be terminal or intercalary. It acts as a site of nitrogen fixation (converts N2 to NH3), under anaerobic conditions. Mostly found in filamentous cyanobacteria like the members of Nostoc, Oscillotoria etc.
Functions:

  • Helps in N2– fixation
  • Helps in vegetative reproduction
  • It secretes enzymes that aid in growth and cell division of thallus.

Reproduction of Cyanobacteria

In cyanobacteria, reproduction is of two types: Vegetative and Asexual reproduction.

Vegetative reproduction

It can be done by following methods
methods of vegetative reproduction

Binary fission: In binary fission constriction forms between the cells leads to cell enlargement, exchange of chromosomal material. At last, the cell produces two identical daughter cells.
binary fission

Fragmentation: In this, during unfavourable conditions thallus detaches and then it forms a new thallus.

By the formation of separator disc: Sometimes gelatinous material occupies between the cell, forms separator disc. This disc then separates the cell which later creates a new vegetative cell.
seperator discBy hormogones: Hormogones are the short fragments (5-10) found in the cell. These separate from the cell during the unfavourable condition and the latter show gliding motility develops into a new filamentous body.
hormogones

Asexual reproduction

It includes the following methods
methods of asexual reproduction

By hormospores: Hormospores releases into the environment when the cell ruptures during unfavourable conditions. The spore germinates to produce a new individual when the favourable condition returns.
hormospores

By akinetes: Akinetes forms in adverse conditions. Its size is larger than vegetative cells, surrounded by thick walls. When enough food material occupies in akinetes, by which it separates from the vegetative cell and lives in a resting stage. When favourable conditions return, it grows a new individual.
akinetes

By heterocyst: It also forms in adverse conditions. On favourable conditions, these germinate into new filament from either end or both the ends.
heterocyst

By endospore:  Endospore present inside the cell. It releases outside the cell when cell ruptures during unfavourable conditions. And, when favourable conditions return, it germinates into a new individual.
endospores

By exospore: Exospore present outside the cell. In adverse conditions, it releases into the environment and germinates into new individual on favourable conditions.
exospores

Economic Importance

  • Cyanobacteria improve the fertility of the soil.
  • It prevents soil erosion.
  • Cyanobacteria help in the reclamation of USAR soil.
  • An acidic chemical secretes by Cyanobacteria that reduces soil alkalinity.
  • Spirulina is an edible cyanobacterium, produces SCPs (single-cell protein).
  • Cyanobacteria colonise and add organic matter into the soil
  • Extract of Lyngbya used in the manufacturing of antibiotics.
  • Cyanobacteria like Oscillotoria used as “Pollution indicator”.
  • Some cyanobacteria show symbiotic association with protozoa and fungi.

Harmful Effects

  • When cyanobacteria present excess in water, it causes “Water bloom”.
  • It depletes the amount of oxygen in the water, leads to the death of other aquatic organisms.
  • Cyanobacteria produce mucilage that produces lousy odour as well as it degrades the quality of water.

Thus, cyanobacteria are a group of photosynthetic microorganisms that shows a huge ecological and biological importance.

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