In this article we will discuss about:- 1. Distribution of Cyanophyta 2. Organisation of Cyanophyta 3. Reproduction.
Distribution of Cyanophyta:
The division Cyanophyta or Myxophyta, commonly known as blue-green algae, consists of a single class Cyanophyceae or Myxophyceae or Schizophyceae whose plants are extremely simple in several respects. Members of this division enjoy a wide range of distribution in all kinds of habitats, from the tropics to the polar regions, including marine and fresh-waters, and on tops of mountains.
The fresh-water forms either inhabit damp terrestrial places or are aquatic, reaching their best development in Stagnant waters to give the water a green or yellow-green colour, the familiar appearance ‘water blooms’. A number of blue-green algae have a capacity to change their colour in relation to the wave-length (colour) of the incident light.
This change in colour is often described as complementary chromatic adaptation or Gaidukov phenomenon.
Several species grow in hot springs where the temperature of the water may be even about 85°C. Blue-green algae may also .cause the precipitation of calcium carbonate from lake water.
A considerable number of species live in association with other organisms; examples are afforded by their occurrence as algal constituents of Lichens, as symbionts in the cells of diverse unicellular animals and different groups of plants and as ‘space-parasites’ within the tissues of various higher plants.
Whereas, a few species are mild parasites in the digestive tracts of human beings and other animals. Again others have been shown to be definitely able to fix atmospheric nitrogen. ‘The capacity to thrive under such diverse circumstances indicates a great degree of adaptability.
Organisation of Cyanophyta:
The plants are diverse in structure, such as, unicellular, colonial to filamentous un-branched, falsely branched and branched forms. The single isolated cell with no polarity is the simplest condition, although by cell division the individuals remain combined to form palmelloid aggregates or colonies of different kinds.
Cells are spherical to ovoid and are similar in structure being surrounded by a gelatinous sheath probably represent the most primitive types (Fig. 8B). There are also almost similar forms being unicellular but with marked polarity (Fig. 10E) which probably are the direct descendants from the previous forms which lack polarity.
The simplest filamentous forms are made up of a long series of cells placed one upon the other to form a trichome (Fig. 21E). The trichome may be straight or spirally coiled (Fig. 20A, H). The trichomes often secrete mucilaginous material of varying consistency which may be homogenous or lamelloid or striated and (Fig. 23E & F). The trichome with the sheath together is termed a filament.
In many filamentous forms cell division and growth are generally diffuse or spread over the entire length of the trichome. The most advanced forms have a heterotrichous condition of habit. The threads of many of the filamentous forms include structures known as heterocysts (Fig. 22B to E), specialized enlarged cells formed by differentiation of vegetative cells.
One of the most conspicuous features of the heterocysts is the strong external thickenings of the cell wall. These thickenings are composed of three distinct layers, the outermost fibrous, the middle homogeneous and the innermost lamenated.
A mature heterocyst contains granular structures which are evenly, distributed throughout the cytoplasm, giving a homogeneous appearance under the light microscope from which earlier workers concluded that heterocysts are ’empty’ cells having no cytoplasmic content.
The position of a heterocyst in a filament may be terminal (Fig. 10B), or intercalary (Fig. 22E) and depending on its position it bears one or two polar nodules which behave like plugs filling the pore channels of the heterocyst, thereby heterocyst loses its protoplasmic connections with the adjacent cells.
There has been much speculation as to the nature and functions of the heterocysts. No doubt, in some cases they break-up the filament into hormogonia (sing, hormogonium) (Fig. 23E). It has been suggested that the heterocyst is a form of food storage organ or that it represents reproductive organ that is now functionless.
In some species, however, it behaves as a reproductive body and is capable of germinating into a new filament. Fritsch (1951) suggested that the heterocysts, during vegetative period, secrete substances that stimulate growth and division of the adjacent cells.
Present day workers are of opinion that heterocysts are involved in nitrogen fixation and may be the sites of effective actual nitrogenase activity.
Besides heterocysts, a filament can also bear a structure much larger than the vegetative cells with appreciably thick wall enclosing accumulated food reserves in the form of cyanophycean granules. Such a thick-walled structure is known as an akinete (Fig. 22B, D & E) which being thick-walled is highly resistant to water shortage and unfavourable temperatures.
Under favourable conditions the akinete germinates into a new filament. Akinetes may be formed in specific positions, especially in relation to the heterocyst (Fig. 10B). In others, they are formed in short or long chain. Only a few of the trichomatous forms of Cyanophyta exhibit various types of rapid movements, whereas, in filamentous genera movement is usually restricted in hormogonia.
But the unicellular forms show only slow movements. The motion consists of creeping or gliding of the trichome backwards or forwards in the direction of its longitudinal axis accompanied by rotation either clockwise or anticlockwise depending on the species.
Reproduction in Cyanophyta:
The Cyanophyta are characterized by the complete absence of sexual reproduction; no sexual organs and no motile reproductive bodies have been observed. Propagation takes place by simple division, by spores (akinetes, endospores, and exospores) or else by fragmentation (fission). The multiplication of unicelluar and colonial forms is brought about mainly by simple cell division.
In all the filamentous forms multiplication is largely effected by the formation of hormogonia.
The hormogonia are the fragments of the filament delimited by the occasional death of cells at intervals along the length of the filament. In certain forms, the hormogonia are enveloped by thick sheath and thus are modified as organs of perennation, known as hormocysts or hormospores (Fig. 10C), which on germination grow out into new filaments.
Many of the filamentous forms produce heterocysts (Fig. 23D, E), which break up the filament into hormogonia. At times the heterocysts may behave as spores to germinate into new filaments. In some Cyanophyta, the akinetes germinate immediately after formation, while in others they remain dormant for long time behaving as resting spores.
On germination the protoplast divides producing germling. In a number of genera, particularly in certain unicellular members, small spores, known as endospores are formed endogenously within a cell (Fig. 10D). They arise by successive division of the protoplast along three planes. The endospores forming cell behaves as a sporangium.
The endospores are generally naked, but a thin wall is secreted after their liberation from the sporangium.
In some epiphytic forms the delicate cell wall ruptures apically exposing the protoplast from which spherical spores are abstricted successively from its tip, these are called exospores (Fig. 10E’). The abstricted spores are surrounded by a delicate membrane.
The protoplast is continuously active. The exospores may germinate when already attached to the parent .protoplast giving rise to new individuals.
True sexuality does not exist in the blue-green algae, but a kind of parasexual phenomenon designated as genetic recombination has been demonstrated in Anacystis nidulans by gene transfer and gene recombination through blue-green algal virus. Genetic recombination differs from true sexuality in that it is not through syngamy or meiosis, and yet the function of true sexuality is achieved.
Genetic recombination has also been reported in Cylindrospermum majus and Anabaena doliolum. It is likely that genetic recombination is brought about by conjugation between donor and recipient cells, as in bacteria. It is also possible that gene recombination may be caused by transduction, a process in which the virus acts, as a vector of certain genes transferring them from donor to recipient cells.
Large number of blue-green algal viruses have already been discovered to play the above role, of which mention may be made of the blue-green algal virus, cyanophage LPP-1 having host range (Lyngbya, Plectonema and Phormidium).