Cyanobacteria

The group cyanobacteria includes another successful group of prokaryotes namely blue green algae. They are photoautotrophic prokaryotes occupying a wide range of habitats. They are predominantly aquatic, found mostly in fresh water bodies. Some species occur in marine water. A few are terrestrial, found in moist soil and on moist rocks. Some species such as Phormidium and Mastigocladus occur in hot water springs at a temperature of 70⁰C to 75⁰C. Trichodesmium erythraeum is a marine, planktonic blue green alga, which gives the characteristic red colour to the "Red Sea".

The Thallus

The plant body or thallus of blue green algae ranges from unicellular to multicellular, branched forms. There are no flagella but some members exhibit a movement caused by gliding.

fig. 8.7 - Types of Cyanobacteria

Cell Structure

Each cell whether in a unicellular form like Gleocapsa or multicellular form like Nostoc, has a definite cell wall. Most often it is surrounded by a thin or thick mucilagenous sheath. The inner layer of cell wall has a chemical composition similar to bacterial cell, made up of peptidoglycans. The cell wall is followed by a cell membrane composed of lipids and proteins. The inner contents of the cell can be distinguished into an outer pigmented region called chromoplasm and a central clear, hyaline region called centroplasm. The centroplasm contains photosynthetic pigments chlorophyll - a, b, carotene and others located in broad sheet-like, structures called thylakoids. The central nucleoid has many irregularly arranged fine strands of DNA.

fig. 8.8 - Cell Structure of Cyanobacteria

The planktonic forms contain gas vacuoles in their cells to help in floating.

Some cyanophycean members possess special types of cells called heterocysts, as in Anabena and Nostoc. They are large, thick walled, round cells without a nucleoid. These cells represent the sites of protein synthesis. They also represent regions where filaments can break into fragments.

Nutrition

Blue green algae contain in addition to chlorophyll, other photosynthetic pigments, such as Phycobillins. Phycobillins are of three types phycocyanin (blue), allophycocyanin (blue) and phycoerythrin (red).

Reproduction

Cyanobacteria reproduce mostly by vegetative and asexual methods. Vegetative reproduction occurs by fission or fragmentation or by the formation of hormogonia. Unicellular forms exhibit fission while filamentous multicellular forms exhibit fragmentation.

Asexual reproduction occurs by the formation of thick walled cells called akinites, which can also store reserve food material. In some members endogenous or exogenous spores may be formed. There is no sexual reproduction, but genetic recombination as in bacteria has been reported in some species like Cylindrospermum majus and Anacystic ridulans. Conjugation has not been observed in cyanobacteria, but gene recombination is known to occur in some forms.

Spirochaetes

These are slender, helically coiled bacteria which move by rotation of the cell. They are mostly aquatic. Some are parasitic. The most familiar example is Treponema palladium, a human parasite which causes the disease Syphilis. It is a sexually transmitted disease.

Actinomycetes

These are a group of filamentous bacteria having a plant body resembling the fungal mycelia. They grow in the form of radiating colonies. They grow abundantly in water and soil rich in decaying organic matter. They are gram positive bacteria which constitute a major component of decomposers.

There are some parasitic species which are pathogenic to plants and animals, including humans. The most familiar example is that of Mycobacterium tuberculosis which causes tuberculosis (T.B.). Mycobacterium leprae which causes Leprosy, and Corynobacterium diptheriae, which causes diptheria.

Rickettsiasis

These are a group of coccoid or rod shaped bacteria much smaller than other typical bacteria. They are non-motile and reproduce by fission. Most of them are obligate parasites. Some familiar examples are Rickettsia prowazeki which causes typhus fever.

Mycoplasmas

They are tiny organisms found in soil and sewage water. Some are parasites. They are the smallest among living organisms (other than viruses) varying in size from 0-125 to 0-150mm in diameter. They exhibit a great deal of variation in their shape and form.

fig. 8.9 - A Generalised Cell Structure of Mycoplasma

They are unicellular, gram negative, non-motile prokaryotes which lack a distinct cell wall.

Some examples of mycoplasmas are pathogenic parasites in humans, animals and plants. The most familiar example is Mycoplasma pneumoniae which causes the disease pneumonia.