They are photosynthetic and have pigments

The members of this phylum are referred to in literature by various names viz.

Cyanophytes, Myxophytes,

Cyanochloronts, Cyanobacteria,

Blue-green algae (BGA), Blue-green bacteria,

Blue-greens.

They are photosynthetic and have pigments

like chlorophyll a,

carotenoids, along with phycobilins. They have autotrophic mode of

nutrition.

These organisms have

resemblance with algae on one hand and bacteria on the

other. Many scientists place them with bacteria. But now majority of the workers keep them with algae.

Following is a table which shows the

resemblance between

BGA with general algae

& bacteria

Resemblance with algae Resemblance with bacteria

1. Presence of Chlorophyll a 2. Presence of special cells, akinetes and heterocysts, such cells never found in bacteria

3. O₂ is evolved in

photosynthesis while O₂ is never evolved in bacterial photosynthesis

1. Prokaryotic cell organization, i.e.

plastids, mitochondria, ER, Golgi complex etc are absent

2. Cell divide by fission method as in bacteria

3. Central aqueous vacuole is absent

Resemblance of BGA with Algae & Bacteria

Occurrence & Distribution:

There are about 150 genera and 1500 species of BGA. They have thrived in

almost every environment like freshwaters, algal blooms, moist soils etc. Some species are aerial or terrestrial. Some species are extremophiles i.e. they are found in

extreme environments hot springs, cold environment of Antarctica and salt

marshes.

Thallus structure in cyanobacteria. (a) Unicells such as Synechocystis, (b) colonies of individual cells such as Aphanothece, (c) unbranched filaments including Lyngbya, (d) unbranched forms with heterocytes and akinetes such as Anabaena, (e) aggregations of multiple trichomes in a common sheath, as in Microcoleus, (f) false-branched forms including Scytonema, (g) true-branched forms such as Stigonema.

Algal bloomAlgal bloom

Lichens are composite organisms consisting of a fungus that contains photosynthetic algal cells. Their fungal partner, generally belongs to the subdivision Ascomycotina, and the algal partners are either green algae or cyanobacteria.

Lichens growing on a

rock

Some species of BGA are endophytic, i.e. they are found in another plant, not growing parasitically rather they are symbiotic. For example, Anabaena species are found in Azolla (a water

fern). Nostoc species are found in the roots of Cycas.

Azolla (a water fern)

Nostoc growing in corolloid root of Cycas

BGA are Extremophiles:

Organisms that occupy extreme environments are called as

Extremophiles.

Algae in Hot Environments:

A number of species of cyanobacteria can tolerate temperature as high as 88°C in thermal pools of Yellowstone National Park (USA).

It is a national park located primarily in the U.S. state

of Wyoming although it also extends into Montana and Idaho.

Grand Prismatic Spring, Yellowstone National Park, Wyoming, USA

At the center of this aerial view of spring in Yellowstone National Park, water temperatures reach near 88°C which is too hot to support most life — but along the edges of the pool, blue-green algae can thrive.

Algae are also found in

Antarctica. Presently there are over 300 algal taxa

identified Antarctica, blue- green are predominant.

This Antarctic ground algae is the only green thing growing in Antarctica (-20 ᵒC), probably Nostoc commune

Range of Thallus Structure:

There is a great diversity in the range of structure.

BGA can be Unicellular (eg.

Synechocystis),

Synechocystis

Synechos = in succession Kokkos = berry

Filamentous

eg. Anabaena Nostoc

Oscillatoria Lyngbya

Spirulina, Rivularia

Scytonema (Psuedobranched)

Oscillatoria

Scytonema — Showing false branching

Lyngbya is named for a phycologist Lyngby. It causes human skin irritation called “Swimmer's itch”.

Lyngbya

Colonial Forms Gloeocapsa,

Chroococcus, Microcystis,

Aphanothece,

Aphanocapsa etc.

Gloeocapsa

Gr. micros=small,

kystis=bladder; a colonial blue-green alga. It is

abundantly found in

freshwater algal blooms, sometimes they appear as granular masses on lake bottoms.

Aphanothece

Cell Structure:

BGA are Gram positive

prokaryotes. They are devoid

of nucleus, chloroplast, Golgi

complex, ER, mitochondria.

A

Cyano- phycean Cell

The filament has mucin

covering. They have no flagella or cilia for locomotion. There is naked circular double

stranded DNA which is in the centre of cytoplasm known as centroplasm.

The cell wall is multi-layered. It includes peptidoglycan and

there are photosynthetic pigments in outer part of

protoplast. Some of them lie

freely in the cytoplasm. Mostly all the photosynthetic pigment

are present except chlorophyll b.

There are minute

granules which are attached to the

photosynthetic pigment and are referred as

Phycobilisomes.

The photosynthetic pigments are known as Phycobilins.

Phycobilins pigments are of three types,

1. Phycocyanin (Blue),

2. Allophycocyanin (Blue), 3. Phycoerythrin (red).

These are the accessory pigments.

They have dwell role

(i) they protect chlorophyll

molecules from photo-oxidation

(ii) they absorbs those radiations of sun which cannot be absorbed by chlorophyll and ultimately the

energy is funneled to chlorophyll molecules.

Gas Vacuoles: Aqueous vacuoles as in the rest of algae and plants are not found in BGA, instead gas vacuoles are present which have small microscopic units known as Gas Vesicles. These gas vacuoles basically regulate buoyancy of the cell.

Gas vesicles

In

each gas

vacuole, there are many

Gas

vesicles

The buoyancy cycle of cyanobacteria having gas vesicles.

Under low-light conditions, gas vesicles form, causing the

cells/colonies to rise upward in the water column. At the

surface, enhanced

photosynthetic production may give rise to higher turgor

pressures and accumulation of dense storage particles, causing the cells to sink. As nutrients are utilized by cells at lower depths, gas vesicles may reform, reinitiating the cycle.

Surface of water

Akinetes: filamentous BGA bear a

special cell, called an akinetes. Akinetes are also called Resting spores or

Arthrospores. Akinetes are spherical, oblong or cylindrical in shape and

contain large reserves mainly

cyanophycin (which is a type of protein reserve). The wall is two layered and

resistant to desiccation and temperature and remain vital for a long period.

They can tolerate adverse

conditions such as the complete drying of a pond or the cold

winter temperatures, and, as a consequence, akinetes serve as

"seeds" for the growth of juvenile filaments when

favorable conditions return.

Akinete Vegetative cells

Heterocyst

Polar plugs

Anabaena filament

Heterocyst:

is large sized thick wall which occurs in

sideways position (terminal heterocyst) or in between vegetative cells (intercalary

heterocyst) of cyanobacteria. These thick walls are permeable to nitrogen and

impermeable to oxygen. The terminal heterocyst bears one polar plug/polar

nodule and intercalary heterocyst contains 2 polar plugs/polar nodules.

There is no photosynthesis but

photophosphorylation does occur.

It depends on adjacent cell for its survival. Heterocysts are

specialized nitrogen-fixing cells formed by some filamentous cyanobacteria,

such as Anabaena, Nostoc and

Cylindrospermum during nitrogen starvation.

50 species of cyanobacteria bear heterocyst and hence capable of fixing nitrogen. They fix

dinitrogen (N₂) using the enzyme nitrogenase.

Nitrogenase is inactivated by the excess of oxygen, so the heterocyst must create an

anaerobic environment. The mature heterocysts contain no functional photosystem II and cannot produce oxygen. Instead, they contain only

photosystem I, which enables them to carry out cyclic photophosphorylation and ATP

regeneration.

Cyanophycin particles. Polar nodules (arrows) consist of cyanophycin particles.

b Cy

).

Function of Nitrogenase in Heterocyst

Reproduction:

Sexual reproduction is altogether absent.

BGA reproduce thru

vegetative and asexual

methods

Vegetative Reproduction: ^{It takes}

place by

(i) Cell Division → There occurs a median constriction in the vegetative cell and

ultimately two daughter cells are formed by binary fission (eg. Synechocystis).

Median constriction

Daughter cells

Binary fission / cell division is the most common method in BGA

(ii) Fragmentation of

filaments→ This occurs in filamentous BGA. Due to

mechanical jerks the filaments are broken into pieces (eg.

Oscillatoria).

(iii) Fragmentation of colony → This occurs in colonial BGA. Due to mechanical jerks the colonies are broken into pieces. Then

each bit matures into a full-

fledged colony (eg. Microcystis).

(iv) By Hormogones formation → This occurs in filamentous BGA.

Some cells become dead and from there segments are separated.

Each of them is known as

hormogone which develops into a mature filament (eg. Oscillatoria).

(v) By Hormocysts or

Hormospores → This occurs in filamentous BGA under

unfavourable conditions.

(eg. Oscillatoria).

Thick walled Hormospores

Asexual Reproduction:

(i) By Endospores formation

→ As the name indicates, these spores are formed inside the cell

(eg. Aphanothece)

(ii) By Exospores formation

→ As the name indicates, these spores are formed outside the cell

(eg. Chaemisiphon).

(iii) Heterocyst → Some mes, heterocyst also germinates

into new filament (e.g.

Anabaena).