Class: Cyanophyceae - Biology Ease

Class: Cyanophyceae

General Characteristics

Cyanophycea exhibits a prokaryotic cell organisation.
It is the oldest oxygenic photoautotrophs on the earth
It is commonly known as blue-green algae as their principle pigment is bluish-green ie. C- phycocyanin
It consists of photosynthetic pigments including chlorophyll a, chlorophyll b or d (some may have), accessory pigments phycobiliprotein.
They may be unicellular or multicellular, free-living or colonial unbranched filamentous or branched filamentous, heterocystous or non-heterocystous, aquatic or terrestrial
They don’t have well-organized cell organelle and their pigments throughout the peripheral cytoplasm.
They have the primitive type of nucleus ie. they lack nuclear membrane and nucleolus.
Also, they do not have flagella and their movement is by gliding movement
They also lack sexual reproduction
Cyanophyceae can also contaminate surface water supplies mostly during warm summer months. The toxic component is microcystin, which gives the water a moldy, musty, grassy, or septic-tank odour.

Blue Green Algae

Cell structure

Sheath

It is covered by a hygroscopic mucilaginous sheath which provides protection to the cell
The nature and consistency of the sheath is influenced by the environment
it protects cells from drying

Cell wall

A double-layered rigid cell wall is present under the sheath providing shape to the cell.
The inner layer is made of mucopeptide and muramic acid.
It consists of 4 layers (LI, LII, LIII, LIV)
The second (L-II) layer has mainly peptidoglycan and first (L-I) lie next to plasmalemma. The cell wall in both is composed of mucopeptide together with carbohydrates, amino acids and fatty acids.
The cell wall is quite similar to gram-negative bacteria

Plasma Membrane

The bilayer plasma- membrane is present beneath the cell wall and is called plasmalemma
It is selectively permeable and encloses the cytoplasm

Photosynthetic Apparatus

Pigments are present which are embedded within the lamella known as thylakoids (double-membrane structure)
They appear as elongated, flattened sacs consisting of two unit membranes
Adjacent thylakoids are separated from each other by a space of 50 nm, occupied by contiguous rows of discoidal phycobilisomes that transfer light energy to photosystem II reaction centers like chlorophyll b, c, d.
The basic subunit of a phycobilisome consists of apoproteins α and β, each of which is attached to a chromophore
α and β are attached to phycocyanin in the outer rods whereas the core of the phycobilisome α and β are attached to allophycocyanins
Phycobilins are major light-harvesting pigments present in Cyanophyceae
they have 4 phycobiliproteins

All cyanobacteria contain the first two pigments whereas the last occurs only in few species.

Cytoplasmic Inclusions

Plasma-membrane surrounds various inclusions like the cynophycian granules, Polyhedral bodies, Polyphosphate bodies, Polyglucoside bodies, Poly – beta-hydroxybutyric acid
Cyanophycin is consisting of a restricted number of amino acids, that is synthesized by a nonribosomal synthetase and it serves as a nitrogen reservoir.
Carboxysomes (polyhedral bodies) contain the carbon dioxide-fixing enzyme (RUBISCO)
Polyphosphate granules are synthesised for both energy and phosphate storage and are degraded to produce nucleotide triphosphate or phosphate.
Polyglucoside bodies – These are known to function in blasting the cell and play a role in buoyancy regulation.
Poly – beta-hydroxybutyric acid is biopolymer found exclusively in prokaryotes in which it acts as a storage and is also used as proplastic e.g., Spirulina and Nostoc muscorum

Gas Vacuoles

They are made of protein cylinders which are hollow packets cylinders consisting of metabolic gases and function as gas vesicles
Gas vacuoles appear as bright, refractile areas in the cells.
Cyanobacteria gas vacuoles are found most commonly in the truly planktonic forms that grow freely suspended in lakes and are responsible for buoying them to the water surface
This it helps cells moving upward, towards the light, where which helps in photosynthesis

Nucleoplasm

Nucleolus and nuclear envelope are absent
DNA present inside the cytosol made up of lots of threadlike fibres or filaments and is in the centre of the cell

Habitat

Cyanobacteria are highly diverse and are found almost everywhere including ponds, lakes, rivers, oceans, temperate soils, geothermal waters, desert soils, rocks, polar regions, brackish habitats and hypersaline waters.
The freshwater cyanobacterial species can grow from 0 °C under the ice to 26–35 °C in the tropical zone.
The thermophilic cyanobacteria may resist up to 45 °C or more
The soil is the best terrestrial cyanobacterial habitat as light, humidity, temperature; nutrient and pH are easily available for their growth.
Cyanobacteria are capable of independent existence in various environments but they also form a symbiotic association with an extensive variety of plants and animals e.g., with algae (diatoms), fungi (lichens), bryophytes ( Anthroceros ), pteridophytes ( Azolla fonds ) etc.

Special Cells

1.Aknites

Akinetes function as an asexual resting state that is capable of resisting harsh environments
and later can germinate to form new cells when conditions improve.
This consist of storage compound such as glycogen & proteins

2.Heterocysts

3.Hormogonia

This consists of gliding motility and many hormogonia also produce gas vacuoles which provide buoyancy to the cell

Reproduction

Cyanophyceae follows the vegetative and asexual mode of reproduction. Vegetative reproduction includes cell division, fragmentation and hormogonia formation.
Unicellular cynophyceae follow binary fission, while filamentous multicellular forms and colonial forms exhibit disintegrate and later mature into new individuals

Economic Importance

Cyanobacteria are used in photoproduction of biofuels, NH 3 scrubbing of excess atmospheric greenhouse gases including CO 2, production of various secondary metabolites, vitamins, toxins, cosmetics, dairy products, food-grade dyes and other therapeutic substances.

Hazards of blue-green algae

A few species of blue-green algae, such as Microcystis, Aphanizomenon, and Anabaena, produce toxins capable of causing illness in humans and animals.
These toxins can cause gastroenteritis, neurological disorders, and possibly cancer.
Occasionally, the rapid increase in their production may lead to cyanobacterial blooms which lead to destruction of many marine species
Toxins like Cyanotoxins are produced by BGA.

References