In this article we will discuss about the phylogeny and interrelationship of cyanophyta.
There is no positive evidence to indicate the phylogenetic origin of the division Cyanophyta. There are, however, certain intriguing characters which quite often give plenty of scope for speculation. The complete absence of flagellate stages might lead one to assume the origin of the Cyanophyta from unicellular forms that were devoid of flagella.
This ancestral stock may or may not be related to the flagellate unicellular forms which apparently gave rise to the other groups of algae. But this assumption lacks evidence. Many authorities support the idea that their abundance of terrestrial forms suggests a terrestrial rather than an aquatic origin for the taxon as a whole which again needs further investigation.
Some have sought an affinity with the Rhodophy to owing to the presence of similar pigments (phycobilins), the absence of motile reproductive cells, and the occurrence of protoplasmic connections in some species of Cyanophyta.
Such a view will not stand the test of a critical enquiry as it entirely overlooks quite apart from essential differences in metabolism, the fundamental contrast in cell structure in both the taxa and by the absence of any type of sexual repdocuction in Cyanophyta.
The simplicity of cell structure, mode of reproduction, and the varied morphological peculiarities, such as, presence of heterocysts, akinetes, hormogonia, etc., more or less eliminate any affinity with other algal taxa.
The Cyanophyta are more primitive than many other organisms, may be except the bacteria (division Schizophyta) with which there are similarities. Both groups possess procaryotic cells and primitive nucleus which is different from all other organisms. Neither group has been demonstrated to have typical sexual process, although parasexual process is known in some bacteria.
There is presence of similar cell wall components mucopeptide and storage food diaminopimelic acid in both the two groups. Both groups usually have very simple vegetative body which is again enclosed in a gelatinous sheath, although similar sheath occurs among some other algae as well. Certain blue-green algae and certain bacteria differ from all or nearly all other organisms in being able to fix atmospheric nitrogen.
Again certain blue-green algae and bacteria thrive at temperatures far higher than that can be withstood by cells of other organisms. The fact that some blue-green algae may be infected with viruses (often designated as cyanophages) which resemble bacteriophages emphasizes further the similarity between blue-green algae and bacteria.
In fact, in their method of genetic recombination the blue-green algae resemble bacteria. Some chemosynthetic bacteria are so similar in appearance to the blue-green algae that Stanier and van Niel (1941) have suggested that the organisms in question are more closely related to the Cyanophyta than the Schizophyta.
It is also strongly held that the Cyanophyta have been derived from the Schizophyta. Pringsheim (1949) questioned the significance of this statement.
Again many botanists believe that the close relationship between the bacteria and the blue-green algae should be reflected by putting the two divisions together in a single division, the Schizophyta apart from other plants.
Stanier and van Niel (1962) grouped blue-green algae with bacteria in the Procaryota. Echlin and Morris (1965) prefer to emphasize the differences between the two groups, consider the blue-green algae to make up a division Cyanophyta, containing a single class, the Cyanophyceae.
1. Order Chroococcales:
The Chroococcales include the most primitive members of the Cyanophyceae. Almost all are fresh-water. Unicellular plant body solitary or colnial, multiplication is by cell division and by endospores.
The Chroococcaceae include algae having unicellular spherical (coccoid), ovoid, or cylindrical plant body either solitary or large number of cells united into spherical to flattened colonies. Each individual cell is surrounded by mucilage sheath and the colonies with homogeneous or lamellated sheaths.
Reproduction of the cells takes place by means of fission in three planes. Generic distinctions are largely based upon a combination of cell shape and planes of division.
This alga is abundantly found in wet rocks and other damp places. Its cells occur in small irregular groups within wide lamellate mucilage envelopes forming masses of jelly (Fig. 19A) which may be colourless or coloured yellow, brown, blue, violet, or red. It is an unicellular organism (Fig. 19B).
Cells are rather spherical in shape without any polarity and are surrounded by well-defined stratified envelopes which, although distinct, are more or less confluent with one another (Fig. 19C).
The sole method of reproduction is by simple cell division or fission (Fig. 19D to F). In this process, the chromatin material divides into two parts, and the cell constricts in the middle forming two new plants. In a group, however, cell division is along three planes which is rather a primitive condition, the products becoming irregularly arranged. Resting spores with firm thick walls are often formed in a number of species.
Some Indian species: Gloeocapsa atrata (Turp.) Kutz.; G. calcarea Tilden; G. decorti- cans (A. Br.) Richter; G. gelatinosa Kutz; G. punctata Nag.
1. Unicellular thick-sheathed plant body.
2. Mostly cells are held together by a mucilage sheath.
3. Reproduction by fission.
2. Order Nostocales:
This order includes filamentous un-branched algae often showing false branching. Majority of them are fresh-water. Heterocysts are present only in some genera. Reproduction is effected by hormogonia, hormocysts and akinetes.
(i) Family Oscillatoriaceae:
The Oscillatoriaceae are by far the largest family in the blue-green algae and include all the relatively simple forms in which the trichomes are unbranched and develop neither heterocysts nor hairs. Although in some genera such as Oscillatoria, the trichomes appear to be naked, a diffluent mucilage envelope formed during movement is probably present even though it is often indiscernible. Under favourable conditions, the sheath material is secreted copiously, sometimes in a well-hydrolyzed, sometimes in a solidified state.
The most popular system of classification of the Oscillatoriaceae has been that of Gomont, which is based in large part on the structure of the sheath. Thus, in Lyngbya the sheath is discrete and contains a single trichome (Fig. 21D). Symploca is similar, but the filaments adhere in fascicles at the surface of the colony. In Phormidium the sheaths are commonly coalesced. Hydrocoleum has several trichomes within a single sheath; Microcoleus has filaments containing a single bundle of many trichomes (Fig. 21E).
Sirocoleum has several such trichome bundles within a common sheath. Drouet’s recent work, however, has shown that a single species may, under certain changing environmental conditions, modify its sheath characteristics to resemble species of Oscillatoria, Lyngbya, Phormidium, Hydrocoleum, and Microcoleus.
Trichome characters are also used in classification. Spirulina is distinguished by spirally coiled trichomes (Fig. 20H). In Crinalium the trichomes are flattened. Various species, especially of Oscillatoria and Lyngbya are distinguished by characteristics of the apical cells of mature trichomes. The apigal cell may be attenuated or may possess a thickened hood, calyptra (Fig. 20E).
In Oscillatoria and Lyngbya, hormogonia are liberated soon after formation, while in Hydrocoleum and Microcoleus they germinate in situ to produce the numerous trichomes within a sheath.
False branching of filaments results in Hydrocoleum and Schizothrix from trichomes tending to grow out laterally and to continue to secrete a sheath as they extend.
Occasionally, Lyngbya majuscula occurs in abundance, notably in distributed shallow waters in the tropics. In Hawaii it has been responsible for a severe toxic reaction of the skin known as “swimmers’ itch”. Lyngbya and other blue-green algae are the prime suspects in the chemical toxicity chain in fish poisoning in certain tropical Pacific areas.
Oscillatoria commonly occurs on the surface of damp earth or on the sides of flower pots and also on stones. The muddy banks of streams and ponds are frequently covered with extensive sheets of Oscillatoria. Patches of algal layer with adherent mud are often found floating on the surface of water. Oscillatoria can thrive in waters where other algae cannot survive, and it often grows luxuriantly in pools of stagnant putrescent water.
The plant body is of un-branched, cylindrical trichome being devoid of sheath (Fig. 20A), or with barely perceptible, evanescent sheath may be free or interwoven to form a thin stratum.
Extremity of trichome is distinctly marked being straight (Fig. 20A) or curved like a sickle (Fig. 20B) or coiled more or less like a screw (Fig. 20D) and often attenuated (Fig. 20C); apical cell sometimes rounded (Fig. 20F), sometimes attenuated (Fig. 20C), or capitate (Fig. 20G), frequently furnished with a calyptra (slightly thickened hood) (Fig. 20E).
Trichomes of Oscillatoria can move from place to place, exhibiting a swaying back and forth or oscillating as well as a twisting or rotating movement. In narrow trichomes, the cells are cylindrical with length greater than the breadth (Fig. 20D, G). Whereas, the broader trichomes have cells length less than the diameter (Fig. 20A, B, E & F).
Depending on species, the protoplasts are homogeneous (Fig. 20C), granulate (Fig. 20E to G), or with numerous pseudovacuoles. In Oscillatoria princeps and O. tenuis, the structure of central body has been worked out with the help of electron microscopy. This study has revealed that the central body is made up of a thin limiting membrane containing a homogeneous matrix in which fine chromatin threads are arranged (Fig. 21 A).
Reproduction is by cell division, which again mainly effected by hormogonia. Hormogonia are formed by the death of cell’ or group of cells here and there in the trichome or by accidental breaking. At times double concave discs of gelatinous material—separation discs are formed between two adjoining vegetative cells in order to dilimit the hormogonia (Fig, 21B & C).
Oscillatoria differs from Lyngbya is not having a prominent gelatinous sheath surrounding the trichome (Fig. 21D). In Phormidium, however, numerous trichomes are embedded in an amorphous mucus formed by the gelatinization of the sheaths of the individual filaments.
Some Indian species:
Oscillatoria chilkensis Biswas; O. limosa Ag. ex Gomont; O. ,obscura Bruhl et Biswas; O. sancta (Kutz) Gomont; O. vizagapatensis Rao, C. B.
1. Plant body exhibits oscillating, bending or gliding movement.
2. Filamentous unbranched plant body bearing extremely thin sheath.
3. Cells are cylindrical having length shorter or greater than the breadth.
4. Reproduction by accidental breaking and by separation disc.
(ii) Family Nostocaceae:
The Nostocaceae resemble the Oscillatoriaceae in their diffuse growth and the unbranched structure of the trichomes, but are distinguished by the prevalent production of heterocysts and frequent reproduction by akinetes. The sheaths are well-hydrolyzed and do not function to restrict growth. The cells are uniseriate and generally ellipsoidal. Division may occur in all the vegetative cells of a trichome at the same time.
Hormothamnion is a common marine form of warm water seas. It has filaments resembling Anabaena, with subspherical vegetative cells and many heterocysts with discrete, cylindrical sheaths. It does not, however, produce spores. The sheaths are agglutinated to form superficial Wefts on the substrate, and from these, Symploca-like erect turfs may later arise.
This is an alga of both terrestrial and aquatic habitats. Terrestrial species grow commonly either on bare soil or intermingled with plant parts.
Sometimes aquatic species may be submerged lying on the bottom of the pools, or attached to a substratum, at other times floating freely, composed of a more or less firm jelly with denser limiting layer within which are innumerable contorted moniliform chains or strands of cells which resemble tiny beads in a necklace.
Besides their association with fungi to form lichens, they behave as ‘space- parasites ‘in the thalli of Anthoceros. There are certain subterranean species which can survive deep in the soil. Nostoc is very hardy and can be revived from dry herbarium sheet after prolonged drying.
The trichomes consist of a single series of uniform, often torulose ellipsoidal cells more or less depressed which are often contorted and sometimes form densely interwoven masses (Fig. 22A & C). The sheaths are thin and very mucilaginous, generally diffluent (Fig. 22B). Innumerable threads are aggregated together within mucilage, with a more or less firm boundary forming little balls, colonies of jelly—Nostoc balls(Fig. 22A).
Here and there in a thread is a different kind of cell—heterocyst, distinguished by its by thick walls, at times much larger in size than its fellow having transparent contents with two polar nodules situated at two ends (Fig. 22B & C). The heterocysts may be terminal or intercalary. Their function is very obscure. But when intercalary they serve to break the filaments up into hormogonia.
They may’ also occasionally function as spores.
Vegetative reproduction is effected by division of the cells, the threads thus formed remain surrounded by the sheath of the parent thread. Reproduction also takes place by hormogonia and spores. Spore formation does not take place until the colony is mature. Vegetative cells of the filament are metamorphosed into spores—akinetes (Fig. 22B).
Akinetes are spherical or oblong much larger than the vegetative cells and are developed centrifugally in series between the heterocysts. Production of hormogonia and heterocysts generally takes place almost simultaneously throughout a colony. Nostoc shows a slight advancement in the formation of resistant spores from some of the ordinary vegetative cells.
These spores germinate to form new filaments when the conditions become favourable.
Some Indian species:
Nostoc commune Vaucher ex Bron. et Flah.; N. hatei Dixit; N. muscorum Ag. ex Bron. et Flah; N. paludosum Kutz. ex Bron. et Flah. ; N-punctiforme (Kutz.) Hariot.
1. Filamentous much contorted un-branched plant body.
2. Filaments sheathed and are held together by soft mucilage.
3. Cells rounded.
4. Presence of heterocysts in the filament.
5. Reproduction by hormogonia, akinetes and sometimes by heterocysts.
The closely related genus Anabaena differs from Nostoc in that no firm colony is formed. Trichomes of Anabaena either occur singly (Fig. 22D), as in many of the plank- tonic species, or form an ill-defined stratum; but never form colonies of definite form, several trichomes may then occur within a soft mucilaginous sheath. An individual trichome is never contorted and sheath is absent or more or less diffluent (Fig. 22E).
The akinetes of Anabaena are generally much more elongate than those of Nostoc (Fig. 22E). Some species are symbiotic. Anabaena cycadearum lives in the root tubercles of species of Cycas, and Anabaena azollae is found in hollows in the leaves of water fern (Azolla). Species of both Nostoc and Anabaena are capable of fixing atmospheric nitrogen.
Some Indian speciss; Anabaena ambigua Rao, C. B.; A. fertilissima Rao, C. B.; A. gelatinicola Ghose; A. sphaerica Bron. et Flah.; A. spiroides Kleb.
(iii) Family Scytonemataceae:
The Scytonemataceae are characterized by cylindrical, untapered, unbranched filaments of uniseriate cells, but with characteristic false branching of firmly sheathed filaments. Heterocysts occur here and there, tending by their shape to hold the trichome in the sheath and to favour apical growth.
In the areas between heterocysts, however, cell division may occur which tends to cause pressures that burst the sheath. This allows one or both broken ends to protrude and to continue growth as a single branch or as twin branches.
Many of the Scytonemataceae suggest derivation from Lyngbya-like types by the acquisition of heterocysts and false branching. Reproduction may be by simple fragmentation of the trichomes or filaments, or by production of hormogonia.
The manner of branching is used in some generic separations. In Scytonema branching takes place well removed from heterocysts, so that twin branches are prevalent. In Tolypothrix the growth-break occurs at the heterocyst and results in a solitary branch.
Species of this genus usually prefer subaerial habitats. Some often occur on wet rocks as large felt-like masses even to the extent of an inch in thickness, others grow on damp soil, but very rarely submerged under water. The filaments interwoven into a felty mass (Fig. 23A).
The individual filament with distinct basal and apical region forming little turfs has a very wide hyaline or coloured sheath with prominently diverging strata of firm texture. The presence of heterocysts and the type of false branching are some of the characteristic features of this genus (Fig. 23C). False branching originates laterally as a result of the degeneration of ah intercalary cell (Fig. 23D).
The intercalary growth produces strong pressure to the sheath, which finally ruptures as a result of which the trichome forms a loop outside. Twin branches are produced out of this loop due to further growth (Fig. 23E-H). Degeneration of a heterocyst followed by growth of the two filaments on either side may also initiate production of false branching.
Akinetes are of rare occurrence in a filament. The hormogonia usually constitute the only method of reproduction (Fig. 23B).
Some Indian species:
Scytonema amplum West et West.; S. fritschii Ghose; S. hansgirgi Schmidle; S. iyengari Bharadwaja; S. pascheri Bharadwaja.
1. Filamentous body with false branching.
2. Filaments bearing prominent sheath.
3. Presence of heterocysts in the filaments.
4. Reproduction by the development of hormogonia.
(iv) Family Rivulariaceae:
The Rivulariaceae are a well-circumscribed family including many marine forms that occur as gelatinous strata or hemispherical masses on rocks, plants, and marsh soil. The trichomes are un-branched and are oriented in their relation to the substrate to show a heterocyst terminating the basal end and a colourless hair terminating the distal.
The cells are uniseriate, and transverse division occurs mainly in the region just above the heterocyst.
The development of the marked difference between the base and apex of the trichome is secondary. Reproduction is commonly by hormogonia produced from portions of the trichome beneath the terminal hair. As the hormogonia germinate and grow, they become tapered at either extremity and ultimately break to form two or more trichomes, each with a basal heterocyst.
In the marine genus Isactis the filaments are erect and parallel, with their sheaths coalesced to form densely gelatinous colonies. Calothrix, which includes a number of species characteristic of the marine littoral, always has a single trichome in a sheath. Gardnerula has many trichomes packed inside a thin sheath.
Gloeotrichia always occurs in colonies of spherical, hemispherical or irregular gelatinous masses being attached to plant parts or on wet rocks of cliffs or may grow directly on soil or free-floating in water (Fig. 24A & B). Each colony is exceedingly firm in consistency and contains numerous radiating filaments with repeated false branching, of which, each branch terminating in a multicellular colourless hair (Fig. 24D & E).
The chief characteristic of this genus is the more or less gradual attenuation of the trichomes which terminate in a hair composed of elongate colourless cells (Fig. 24D).
One or two heterocysts are located at the broad basal end of the trichome (Fig. 24C & D). Only the lower and broader portion of the filament is covered by a sheath (Fig. 24G, D & G). Growth of an individual trichome may be trichothallic and is initiated by a meristem of flat cells situated at the base of the hair-like structure (Fig. 24E).
Many species exhibit false branching due to growth of the lower part of a trichome, mostly beneath an intercalary heterocyst. Hormogonia are formed from the upper part of the trichomes, the apical hair being shed. Reproduction is by hormogonia and akinetes. Number of akinetes may be one or more, and may be usually developed next to the heterocyst (Fig. 24G & H).
Some Indian species:
Gloeotrichia ghosei Singh, R. N.; G. indica Schmidle; G. natans Rabenhorst ex Bron. et Flah. G. pilgeri Schmidle; G. pisum Thuret ex Bron. et Flah.
1. Filamentous un-branched vegetative body differentiated into base and apex.
2. Presence of a heterocyst at the broad basal end of the trichome.
3. The narrow end of the trichome is terminated by a hair of colourless cells.
4. The heterocyst is usually followed by an akinete.
5. The filament is partly covered by a sheath.
Closely related genus Rivularia differs from Gloeotrichia in its absence of formation of akinetes. In Rivularia, sheaths of the individual filaments are often diffluent as against Gloeotrichia, where sheaths only conspicuous near the bases of the trichomes, being gelatinous and confluent in the outer parts of the thallus.
Some Indian species:
Rivularia beccariana (De Not.) Born, et Flah.; R. globiceps West, G. S.; R. hansgirgi Schmidle; R. manginii Fremy.