Cyanobacteria are the largest and most widely distributed photosynthetic prokaryotes. In 1960, when the differences between prokaryotes and eukaryotes were found it becomes clear that cyanobacteria resemble bacteria more than any other algae.
Similarities between cyanobacteria and bacteria
- The absence of nucleus in both.
- The absence of well-formed plastids.
- Both have a mucilaginous sheath around the cell wall.
- Both have similar chemicals like peptidoglycan compounds.
- Both are similar to sensitivity to antibiotics.
- Cell organelles like mitochondria, Golgi bodies, and lysosomes are absent in both.
- The ribosomes are of similar nature. The large ribosomal unit is 50s and a small ribosomal unit is 30s. When they associate they form a 70s complex. ( These ribosomes are similar to those of ribosomes in mitochondria and chloroplasts of eukaryotic cells).
- In bacteria and blue-green algae, true vacuoles are absent.
Differences between cyanobacteria and bacteria
- The size of cyanobacteria is comparatively larger than bacteria.
- Bacteria sometimes bear flagella but in cyanobacteria, they are always absent.
- Bacteria are autotrophic or heterotrophic but blue-green algae ar usually autotrophic.
- Cyanobacteria have chlorophyll-a as photosynthetic pigment while bacteria have bacteriochlorophyll and chlorobium chlorophyll.
- No accessory pigments are in bacteria while like phycocyanin and phycoerythrin are present in dominating form.
- Tn bacterial photosynthesis, hydrogen donor is not water, as a result oxygen is not evolved. Thus photosynthesis is anoxygenic and in cyanobacteria hydrogen donor is water, oxygen is evolved.The process is oxygenic.
- The reserve food material is glycogen in bacteria and cyanophycean starch in cyanobacteria.
- Spore formation is endogenous and not endogenous in bacteria and cyanobacteria respectively.
Blue-green Algae or Cyanobacteria or Cyanophyta are Gram-negative and photosynthetic. The blue-green algae live virtually in all environments that contain water. The organisms range from tropics to plains and they occur in soil, fresh water and ocean.
In lakes and in the ocean they form part of the plankton. Some cyanobacteria live in the icy waters of glaciers others in hot springs where temperatures reach 85oC or more. Some blue-green algae live as symbionts with other organisms. For example, they often are the algal members of lichens.
some common characters of cyanobacteria are:-
- Common forms are unicellular, colonial, filamentous or heteroicous.
- The cell wall possesses of an outer sheath which is jelly like, slimy and mucilaginous.
- The cell contents are divided into two regions, outer chromatoplasm having photosynthetic pigments and inner colorless Centroplasm.
- The cell wall is two layered whose inner wall is made up of peptidoglycan or mucopeptides built up from amino acid sugars (e.g. glucosamine) and amino acids (e.g. muramic, diaminopimetlic).
- Bacteria and blue-green algae laxk mitochondria, true vacuoles and endoplasmic reticulum.
- Sterols are absent in bacteria as well as cyanobacteria.
- There is no membrane-bounded chloroplast and photosynthetic lamellae or thylakoids are usually distributed in the peripheral cytoplasm.
- True nucleus absent. fine fibrils of DNA are either distributed throughout the cell or concentrated in the central part. Its chromosome resembles bacterial chromosome.
- Well marked reproductive organs, sexual reproduction, and motile reproductive bodies are absent in cyanobacteria.
- Gas vacuoles are often present to regulate the buoyancy in waters.
- The color ranges from green to deep purple often blue-green. Thus the name blue-green algae is misleading. These colors are produced by different proportions of several pigments like chlorophyll a, carotene, xanthophyll, blue phycocyanin and red phycoerythrin. The last two are tetrapyrrole derivative occurring uniquely in the cyanobacteria.
- Reserve food is in the form of cyanophycean(myxophycean) starch.
- Flagella are absent in vegetative as well as reproductive phase.
Cellular structure of Cyanobacteria
They are unicelled or filamentous. Cells may be oval or spherical in unicelled forms. The cell is comparatively larger than a bacterial cell. The cell wall bears slimy mucilaginous sheath (mucopeptide). It has peptidoglycan cell wall. The cell contents are divided into two regions.
The outer chromatoplasm is having photosynthetic pigments and inner colorless centroplasm. In the cytoplasm photosynthetic lamellae are present. These lamellae or thylakoids contain dominating pigments phycocyanin and phycoerythrin (phycobilins) in addition to chlorophyll a.
The cells contain food material in the form of myxophycean starch or cyanophycian granules. Cells may contain gas vacuoles. The true nucleus is absent and replaced by circular DNA which resembles the bacterial chromosome and occupies the central position of the cell. Membrane bound organelles like mitochondria, endoplasmic reticulum, true vacuoles are absent.
Taxonomic position of Nostoc (Nostoc is a genus of cyanobacteria)
Division – Cyanophyta (=Myxophyta)
Class – Cyanophyceae (=Myxphyceae)
Order – Nostocales
Family – Nostocaceae
Genus – Nostoc
Nostoc punctiformae is found in corolloid roots of Cycas. The thallus of Nostoc is introduced in paddy fields to improve soil fertility since it has the ability to produce soil nitrates by nitrogen fixation. Some species of Nostoc are found as algal components of lichens.
The thallus is a mass of mucilage, each with a diameter upto 8cms. Within the thallus, there are numerous unbranched filaments. The cells of the filament are spherical and appear like a string of beads.A chain of cells is called a trichome. Each trichome is surrounded by a layer of mucilage sheath to produce a filament.
From certain cells of the filament, two types of modified cells are produced, namely,
The cell wall is covered by a layer of mucilage. The cells have a prominent cell wall. The cell wall has carbohydrates, mucopeptides, amino acids and fatty acids. Within the plasma membrane, two zones are present, an outer colored zone called chromoplasm (=colored cytoplasm) and inner colorless zone the centroplasm.
The photosynthetic pigments present in chromoplasm are:-
Chlorophyll a (green pigment)
C-phycocyanin (blue colored pigment)
C-phycoerythrin(red colored pigment)
unique xanthophylls (yellow colored pigment) like myxoxanthin
The reserve food material includes starch called Cyanophycean starch and a type of protein called cyanaphycin.
The cell organelles like a nucleus, mitochondria, flagella, Golgi bodies, endoplasmic reticulum, true vacuoles are absent. Ribosomes of 50s and 30s type are present. These ribosomes are similar to bacterial ribosomes.
In Nostoc, reproduction is by the vegetative or asexual method. Sexual reproduction is absent. The most common methods of reproduction are:-
As the thallus enlarges in size it breaks up into smaller thalli under the influence of water currents and other forces. The fragments enlarge by growth and the process of fragmentation is repeated.
These are enlarged thick walled cells which are produced under unfavorable environmental conditions. They are meant for survival or pernnation. On a return of favorable conditions, each akinete develops into a new filament by repeated cell division.
They help in hormogonia formation. In some species of Nostoc-like N.commune the contents of the heterocyst produce 2 to 8 spores and these spores come out of the heterocyst to produce new filaments of Nostoc. Heterocysts also help in nitrogen fixation. They contain enzymes that play a role in nitrogen fixation.
Economic importance of blue-green algae
- A large population of blue-green algae in the soil prevents soil erosion. This is due to the fact that the mucilage present in the algal thallus effectively binds the soil particles.
- Many blue-green algae grow well on alkaline soil and they reduce the high alkalinity making the soil fit to cultivate higher plants. This is an important step in the reclamation of alkaline soils.
- Many blue-green algae like Nostoc, Oscillatoria, and Anabaena have the ability to fix nitrogen and thereby forming nitrates in the soil. This makes the soil very fertile.
- In ponds and lakes blue-green algae sometimes grow very rapidly to produce water blooms. Blue-green algae like Microcystis, imparts an unpleasant odor making the water unsuitable for human consumption.
- The algal bloom consumes oxygen for respiration and at night the water is depleted of oxygen. This kills valuable aquatic organisms like fishes which decompose and pollute the water.
- Some blue-green algae like Anabaena lemmermanii secrete toxins which can kill fishes and also large animals like cattle which consumes the toxin containing water.