9 Major Taxonomic Groups of Algae

List of nine major taxonomic groups of Algae:- 1. Chlorophycophyta 2. Xanthophycophyta 3. Bacillariophycophyta 4. Phaeophycophyta 5. Rhodophycophyta 6. Chrysophycophyta 7. Euglenophycophyta 8. Cryptophycophyta 9. Pyrrophycophyta.

Taxonomic Group # 1. Chlorophycophyta:

The green algae form one of the largest groups of algae. They show a wide diversity of form, structural organization and reproduction. The algal body may consist of a single cell or many cells. The unicellular green algae may be motile (Chlamydomonas, Sphaerella) or non-motile (Chlorella, Protococcus). Some unicellular forms may consist of two symmetrical half-cells (Cosmarium).

Others may form colonies which may be motile (Volvox, Pandorina), or colonies may be non-motile (Pediastrum, Hydrodictyori). The multicellular green algae may have uniseriate un-branched (Ulothrix) or branched filaments (Cladophora, Oedogonium). The multicellular green algae may also have one- cell thick prostrate thalloid structure (Coleochaete) or a heterotrichous filamentous form with a prostrate and an erect system (Chaetophora, Trentepohlia).

There are also coenocytic siphonaceous green algae (Caulerpa, Bryopsis). Acetabularia, another siphonaceous alga has a stalked umbrella-like structure. In Ulva, the algal body is a thin parenchymatous structure. A highly complex thallus with a multiseriate axis and leaf-like appendages is present in Chara and Nitella. The morphological features of some of the genera mentioned above have been shown in Fig. 5.31.

In both unicellular and multicellular green algae, the cell is bound by a cell-wall composed of generally an inner cellulose layer and an outer pectic layer. Most of the green algae have a single nucleus per cell, except the coenocytic siphonaceous ones which have many nuclei.

Also, each cell normally has a single chloroplast having one too many pyrenoids. The chloroplasts are of many shapes, characteristic of the genus (Fig. 5.32). The plastidial pigments are chlorophyll a and b, and β -carotene and xanthophyll’s. Starch is the main photosynthetic reserve substance.

Majority of green algae are aquatic, growing in fresh-water or marine habitats. There are some terrestrial representatives also, like Protococcus, Trentepohlia etc. The aquatic forms may be attached to a substratum or free-floating.

Green algae reproduce both sexually and asexually. The unicellular members multiply by cell division, or by production of biflagellate zoospores (Chlorococcum) or sometimes by formation of aplanospores (Chlorella). In Ulothrix, a filamentous green algae, quadriflagellate zoospores are formed in some vegetative cells.

Similar type of asexual reproduction takes place also in several other filamentous genera, like Cladophora, Fritschiella, Draparnaldia etc. In Oedogonium asexual zoospores are multi-flagellate and they are formed singly in a cell. In Hydrodictyon, a coenobial green algae, zoospores are produced in large numbers in a mother cell, but they are not released.

The zoospores remain within the mother cell and fuse with each other to form the characteristic net-like organization. In the siphonaceous green algae, like Acetabularia, asexual reproduction does not occur and they reproduce only by sexual means. Asexual mode of reproduction by formation of zoospores or aplanospores is also unknown in the conjugates, like Spirogyra, Zygnema etc. However, multiplication may occur by fragmentation of the filaments.

Some forms of asexual spore formation in the green algae are shown in Fig. 5.33:

Sexual reproduction in green algae occurs by several different ways, like isogamy, anisogamy, conjugation and oogamy. In Chlamydomonas, depending on species, sexual reproduction takes place by fusion of two morphologically similar or dissimilar gametes.

In Ulothrix, union of two biflagellate isogametes results in the formation of a quadriflagellate zygote. The zygote forms haploid zoospores which germinate to form vegetative filaments. Sexual reproduction in Oedogonium is oogamous. The multi-flagellate antherozoids enter into the oogonium containing a single ovum through a pore in the wall of the oogonium. The fusion product is an oospore. The oospore is subsequently released and the diploid nucleus divides reductionally to produce four haploid zoospores.

The zoospores on germination produce the filamentous vegetative body. In Spirogyra and other members of Conjugales, two filaments of opposite sexuality come close to each other and they lie parallel. Conjugation tubes are formed in a series of cells and the protoplasts of the male filament are transferred in an amoeboid manner into the cells of the female filament.

The protoplasts fuse to form zygotes. In some genera, e.g. Mougeotia, the zygote is formed in the conjugation tubes. The zygote nucleus divides by meiosis to form four haploid nuclei, of which three degenerate. From the zygote a haploid filament develops to initiate the vegetative phase.

In Chara, sexual reproduction occurs by oogamy and the sex organs are highly developed having complicated structures. The male sex organ is known as globule and the female one as nucule. They develop at the node of the multiseriate axis of the filament. The female organ (nucule) at maturity consists of an oogonium containing a single ovum enveloped by spirally coiled cells (tube cells). At the tip of each tube cell is a coronal cell.

The male organ is globular in shape, covered by shield cells. Inside the globule are long antheridial filaments. Each cell of such a filament produces a single antherozoid. Several thousand of spirally coiled biflagellate antherozoids are produced in each globule.

The antherozoids swim to the oogonium and enter into it through slits at the corona. One antherozoid fuses with the single ovum to produce the zygote (oospore). The oospore on maturity is a thick walled structure. Its diploid nucleus divides by meiosis to produce four haploid nuclei of which three degenerate. The zygote germinates to produce a haploid Chara plant.

Different types of sexual reproduction met with in green algae are illustrated in Fig. 5.34:

Taxonomic Group # 2. Xanthophycophyta:

Members of this algal division are commonly known as yellow-green algae. The cells contain chlorophyll a and c, as well as in some forms chlorophyll e. The cell wall is made chiefly of pectic substances, but in coenocytic forms cellulose is also present. The algae may be unicellular (listeria, Heterochloris), filamentous (Tribonema), coenocytic (Botrydium) or a branched siphonaceous thallus (Vaucheriu).

The organisms do not produce starch as photosynthetic reserve material. The main storage product is chrysolaminarin. Oils are also present. The yellow-green algae are mainly aquatic, but some may grow on land, particularly on drying mud, e.g. Botrydium, Capitulariella. Also, some species of Vaucheria grow extensively on moist soil.

The pigment-bearing organelles are generally known as chromatophores. There are numerous small round to oval chromatophores in the coenocytic, filamentous or siphonaceous members. The chromatophores generally lie towards the periphery and the central portion of the algal body is occupied by a vacuole. A characteristic feature is production of motile cells with two unequal flagella, although zoospores of Vaucheria are multi-flagellate.

The algae of this group reproduce both asexually, as well as sexually. Asexual reproduction generally takes place by formation of zoospores, whereas sexual reproduction may occur by fusion of isogametc as in Botrydium, Tribonema etc. or by oogamy involving a motile male gamete (antherozoid) and a non-motile egg contained in an oogonium (Vaucheria).

The vegetative thalli of a few members of the group are shown in Fig. 5.35:

Asexual reproduction by formation of zoospores and sexual reproduction by isogamy and oogamy are depicted in Fig. 5.36:

Taxonomic Group # 3. Bacillariophycophyta:

This algal division includes the diatoms which are exclusively unicellular organisms with beautiful ornamentation on their outer shell. The organisms are predominantly single, but some species form colonies e.g. Licmophora flabellata (see Fig. 5.31). Diatoms grow in fresh water and marine habitats, as also on moist soil.

They are particularly abundant in the Arctic seas, where they grow as planktons and provide food for marine animals. They are important primary producers contributing significantly to the food-chain. Their shell is highly resistant to decomposition and the remains of dead marine diatoms are deposited year after year at the ocean beds leading to formation of diatomite or diatomaceous earth, also known as kieselgur.

The diatom cell is known as frustule. The vegetative cell is usually diploid. The protoplast is enclosed by the cell membrane and a cell-wall which is mainly composed of pectic materials. Outside the cell wall, there is an outer shell consisting of two tightly fitting overlapping valves, like the two halves of a Petridish. This outer shell is composed of silicified substances and it is variously ornamented by geometrically arranged very fine pores. Generally, the protoplast contains a single diploid nucleus and numerous small brownish-yellow chromatophores, often provided with pyrenoids.

The photosynthetic pigments are chlorophyll a and small amount of chlorophyll c, but no chlorophyll b. Accessory pigments include P-carotene, lutein and fucoxanthin. Diatoms do not form starch. The main photosynthetic reserve substances are fats and oils, and a laminarin-like polysaccharide, called leucosin.

Diatoms have two types of shape and accordingly they are known as centric and pinnate diatoms. The pinnate diatoms have bilaterally symmetrical valves with ornamentations arranged symmetrically on both sides of a line running along the long axis of the diatom cell. In the centric diatoms, the two valves are circular, triangular or rarely irregular in shape. The surface ornamentations are usually arranged radiating from a central point, or are concentrically arranged around a central point.

The ornamentation types of centric and pinnate diatoms are schematically shown in Fig. 5.37:

The centric diatoms are mostly marine, while the pinnate types are inhabitants of both saline as well as fresh-water habitats. Some colourless diatoms, like Nitzschia putrida, presumably arise by mutation of photosynthetic forms. They live heterotrophically feeding on organic products of marine organisms.

Diatoms multiply by cell division. During cell division, the protoplast increases in size pushing the two outer shells apart. The diploid nucleus divides by mitosis and the protoplast is divided into two parts. The daughter cells separate. Half of the outer shells is retained by each daughter cell and the open parts of the protoplasts are covered by new shell synthesized by the protoplasts.

As a result of cell divisions one of the daughter cells in which the lower shell (hypotheca) of the mother diatom acts as the upper shell (epitheca) becomes smaller in size. Successive cell divisions cause gradual reduction of the cell size. This continues till a minimum size is reached, beyond which the viability is lost. The diatom can survive then only by formation of auxospores. Asexual reproduction by zoospore formation is unknown in diatoms.

Cell division and the consequences are schematically represented in Fig. 5.38:

Sexual reproduction takes place in two ways which are characteristic of the pinnate and centric diatoms. In the pinnate forms, meiosis occurs in the diploid diatom cell resulting in the formation of gametes. Generally, the gametes are morphologically similar (isogametes) and are non-motile.

The fusion of gametes restores the diploid nuclei. The fusion product is known as an auxospore. The gametes come from different diatom cells which come close to each other and are enclosed by a common mucilaginous envelope.

The mature gametes come out of their cells and fuse in the envelope. The zygote matures into an auxospore. The auxospore later forms a diatom by production of the outer shell. In some pinnate diatoms, two gametes are produced by each and their fusion with two gametes produces two auxospores from a pair of conjugating diatoms.

Sexual reproduction in pinnate diatoms is shown in Fig. 5.39:

In the centric diatoms, sexual reproduction is oogamous. The male gametes are motile with a single tinsel-type flagellum, while the female gamete is a non-motile egg. Fertilization leads to formation of a diploid auxospore. Fertilization is effected by passage of only the male nucleus into an egg.

The auxospore germinates to produce a diploid vegetative diatom (Fig. 5.40):

In some centric diatoms, auxospore formation occurs by autogamy. In this process, the diploid nucleus of a single diatom undergoes reduction division producing four haploid nuclei. Two of these degenerate, while the other two fuse with each other to form a diploid nucleus. The protoplast containing the diploid nucleus is liberated from the mother frustule and become eventually an auxospore.

The centric diatoms are more primitive than the pinnate types. The two types differ not only in their cell shape and orientation of ornamentation on their cells, but also in several other characteristics. The centric diatoms are non-motile, while many of the pinnate ones exhibit a non-flagellar jerky locomotion. The centric diatoms do not have a raphe which the pinnate diatoms usually possess. The difference in sexual reproduction.

There are more than 5,000 species of diatoms, some of which are extinct and known only as fossils. Majority of fossil diatoms are of centric type. Some of the common fresh-water diatoms are species of Navicula, Melosira, Synendra etc. Licmophora produces beautiful colonies.

Taxonomic Group # 4. Phaeophycophyta:

The brown algae belong to this division. They include multicellular, almost exclusively marine algae, often attaining very large size. The individual cells are enclosed by an inner cellulose layer surrounded by an outer pectic layer. The cells are generally uninucleate and vacuolated containing numerous brown chromatophores which are called phaeoplasts.

The plastidial pigments are chlorophyll a and c, β-carotene and several xanthophyll’s of which the main is fucoxanthin which imparts brown colour to the chromatophores. Small pyrenoids are usually present, but outside the plastids. They are involved in the synthesis of the main photosynthetic product, laminarin. It is a dextrin-type polysaccharide. Starch is not formed by the brown algae.

Most of these algae grow attached to some substratum and some like Sargassum are free-floating. The large brown algae, commonly known as kelps, can attain very large size, often several meters.’ Such forms have a complex structural organization. Some, like Ectocarpus have a simple thallus consisting of branched uniseriate filaments.

A characteristic feature of all brown algae is that both asexual zoospores and sexual gametes are motile with two unequal flagella which are inserted laterally. One flagellum, the longer one, is of tinsel type directed anteriorly and the shorter is posteriorly directed.

Vegetative multiplication occurs by fragmentation of the thallus. Asexual reproduction takes place by zoospores (Fig. 5.41). The brown algae have alternation of generations. The haploid and diploid thalli grow independently. They may have similar morphology (isomorphic) or different (heteromorphic).

In Ectocarpus asexual reproduction occurs by formation of biflagellate zoospores in two types of sporangia which are known as pleurilocular and unilocular sporangia, depending on whether the sporangium is many-chambered or single-chambered.

Both types are produced exclusively by the diploid thalli. The pleurilocular sporangia produce only diploid zoospores which on germination yield a diploid thallus. In unilocular sporangia, meiosis of the diploid nucleus occurs and the zoospores are haploid which, in turn, give rise to haploid thalli. Ectocarpus has an isomorphic alternation of generation.

In Dictyota, asexual reproduction occurs by formation of non-motile tetraspores in tetrasporangia, so called because each sporangium contains four spores. Tetrasporangia develop only on diploid thallus and tetraspore formation is preceded by meiosis. The haploid tetraspores germinate to produce the gametophytic thalli. Dictyota also has isomorphic alternation of generations.

Sexual reproduction of brown algae takes place by fusion of two morphologically similar (isogamy) or dissimilar (anisogamy) gametes, or by fusion of motile antherozoids and non-motile eggs (oogamy). In all cases, the fusion of the gametes takes place outside the thallus.

In Ectocarpus, both isogamy and anisogamy are known to occur in different species. The gametes are formed within pleurilocular gametangia which are similar in appearance to pleurilocular sporangia, but they are developed only on haploid thallus originating from the haploid zoospores produced in unilocular sporangia (see Fig. 5.41).

The gametes are uninucleate with laterally inserted unequal flagella. The fusion of iso- or anisogametes results in the formation of a diploid zygote which germinates to produce a diploid vegetative thallus. Meiosis takes place only during the formation of haploid zoospores in unilocular sporangia. The haploid zoospores produce the haploid gametophytes forming pleurilocular gametangia. The diploid and haploid thalli of Ectocarpus are morphologically similar (isomorphic alternation of generation).

Dictyota reproduces sexually by oogamy. Male gametangia (antheridia), as also the female gametangia (oogonia) are arranged in groups, called antheridial sori and oogonial sori in male and female gametophytes, respectively. Each antheridium produces 32 to 64 small uninucleate pyriform laterally biflagellate antherozoids (male gametes). An oogonium contains a single large non-flagellate egg. On maturity, the egg is released from the oogonium and fertilization takes place in water. The diploid zygote develops into thallus.

In Fucus, asexual reproduction by zoospore formation is absent. These brown-algae reproduce only sexually and sexual reproduction is oogamous. The sexual reproductive structures, antheridia and oogonia are formed in specialized structures, called conceptacles.

These are flask-shaped structures, sunken in the multicellular thallus which is differentiated into an epidermal layer, several layered cortex and medulla. The antheridial diploid nucleus divides meiotically and then mitotically to produce 64 antherozoids which are pear-shaped and laterally biflagellate. The oogonial diploid nucleus also undergoes meiosis to produce eight large non-motile egg cells. The eggs are released from oogonia and fertilization takes place in water.

The sexual reproductive structures of ectocarpus, dictyota and fucus are shown in Fig. 5.42:

Taxonomic Group # 5. Rhodophycophyta:

The algae of this division are known as red algae and they are of red, purple, violet or brownish colour. They are mostly marine and can grow at considerable depths (up to 200 meters). There are also some fresh-water red-algae, like Batrachospermum, Compsopogon etc. Red algae are almost exclusively multicellular, though very few unicellular forms are also known e.g. Por-phyridium cruentum.

The algal body shows a great diversity in both form and size. Mostly the thallus is filamentous, generally branched. The filaments may be uniseriate as in Batrachospermum and Ceramium, or multiseriate as in Polysiphonia.

Sometimes, the multiseriate thallus may assume a pseudoparenphymatous form as in Chondrus. Red algae generally grow attached to some substratum at considerable depth and they are well-adapted to a low light intensity. The red algae sometimes show chromatic adaptation, a phenomenon known to occur also in cyanobacteria.

In red algae, cells are generally uninucleate with numerous small chromatophores. The photosynthetic pigments include chlorophyll a and seldom chlorophyll d. The accessory pigments are β-carotene, zeaxanthin and occasionally α-carotene. Besides these pigments, all red algae contain a red fluorescent water-soluble phyco-bilirubin, called phycoerythrin. In some red algae another blue phyco-bilirubin, phycocyanin is also present.

These two pigments are characteristically present in cyanobacteria. However, the phycobilirubins of red algae and cyanobacteria have some differences in properties and to distinguish them the pigments of red algae are designated as phycoerythrin-r and phycocyanin-r and those of cyanobacteria as phycocyanin-c and phycoerythrin-c.

The phycobilin pigments often mask chlorophylls and impart the algae different colours in shades of red to violet. In the unicellular red alga, Por-phyridium, the phycobilin pigments are present in phycobilisomes which are spherical to discoid proteinaceous bodies. These bodies are arranged on the surface of photosynthetic lamellae. Pyrenoids are present only in some red algae.

The main assimilation product is a carbohydrate, called floridean starch which has characteristics in between true starch and glycogen. It is formed as small granules, often as layers on the surface of the chromatophores.

In addition, most red algae also have oil droplets as reserve material. The cells are surrounded by cell wall consisting of an inner cellulose layer and an outer pectic layer. The thallus of several red algae, like Gelidium, Gracilaria, Chondrus etc. contains gelatinous materials from which important commercial products, such as carrageenan and agar agar are produced.

An important feature common to all red algae is the complete absence of motile stages in their life-cycle, either in the form of asexual zoospores or motile male or female gametes. In many red algae, asexual reproduction takes place by non-motile spores, called mono-spores produced singly in sporangia (e.g. in Batrachospermum). Sexual reproduction is oogamous.

The non-motile male gametes, called spermatia are produced singly within spermatangia which are equivalent to antheridia. The corresponding female structure is called a carpogonium. Carpogonia are provided with trichogynes. The spermatia are carried passively by water current to the carpogonia and the spermatial contents pass into the trichogyne to reach the female gamete. Fertilization leads to formation of a zygote within the carpogonium.

The events following fertilization are different in different genera. In some red algae, like Batrachospermum, the diploid zygote nucleus divides meiotically soon after karyogamy to produce haploid nuclei. These haploid nuclei pass into protuberances developing on the carpogonial wall. From each protuberance a gonimoblast filament develops which is a septate, branched or un-branched structure.

The gonimoblast filaments developing from a carpogonium form a cluster and each cluster is surrounded by an envelope consisting of sterile filaments. The whole structure constitutes a cystocarp. The terminal cells of the gonimoblast filaments are differentiated into carposporangia bearing one carpospore each. Carpospores are haploid, non-motile and naked. These spores germinate to produce a protonemal form from which typical Batrachospermum thallus develops. The gametophytic thallus, so produced reproduces asexually by monospore formation.

The asexual and sexual reproductive structures and the life-cycle of Batrachospermum are shown in Fig. 5.43:

In the Floridean red algae, like Polysiphonia, reduction division of the diploid zygote nucleus is postponed and it divides mitotically to form carposporangia containing diploid carpospores unlike those of Batrachospermum which produces haploid carpospores. The diploid carpospores germinate to give rise to independent diploid thalli of tetra sporophytes.

In these thalli haploid tetraspores are formed by meiosis of the diploid nuclei. The tetraspores germinate to yield haploid gametophytes of Polysiphonia. The gametophytic thalli are of two types, male and female, but they are morphologically alike. Thus, in Polysiphonia and the allied genera, a diploid generation in the form of tetra sporophytes is interposed between the gametophytic generations due to postponement of meiosis of the zygotic nucleus.

The type of life-cycle in Polysiphonia is referred to as a triphasic alternation of generation, because the gametophytic thallus (haploid) first produces a diploid phase represented by the carposporangia bearing structure, known as carposporophyte which remains attached to the female gametophytic thallus. Then the carpospores (diploid) produce another independent diploid phase, the tetra sporophyte. The tetraspores (haploid) germinate to give rise to male and female gametophytes.

The reproductive structures and life-cycle of Polysiphonia are shown in Fig. 5.44:

Taxonomic Group # 6. Chrysophycophyta:

Chrysophytes are generally known as golden-brown algae. They are mostly unicellular with a few multicellular filamentous representatives e.g. Phaeothamnion. They are mostly inhabitants of fresh­water bodies. Some are marine with silicified envelops. The unicellular forms may be flagellated with two unequal flagella placed anteriorly. The longer flagellum is generally of tinsel type. Some are amoeboid and show motility with pseudopodia.

Some members, like Ochromonas can change form from flagellated cell to an amoeboid one during formation of cysts. The unicellular types are generally naked without a cell-wall. The marine species, like Distephanus possess a skeleton of silica covered by a slimy layer. Some other marine forms, known as coccoliths have a covering of chalky materials. Some of the flagellated species, like Uroglena and Dinobryon can form coenobia.

The cells usually have one or two parietal chromatophores containing chlorophyll a and a small amount of chlorophyll c, as well as carotene and xanthophyll’s, particularly lutein and fucoxanthin. The green pigments are masked by the xanthophyll’s which impart the golden-brown colour to these algae. The photosynthetic reserve materials include chrysolaminarin and oils. Starch is not formed. The amoeboid forms can ingest solid food particles by phagocytosis. Some forms, like Ochromonas can grow auto-trophically as well as heterotrophically, or in its amoeboid state can ingest solid food particles.

Chrysophytes generally reproduce by binary fission through normal cell division. Some species, like those of Dinobryon and several others can also reproduce sexually by gametic copulation. The gametes are vegetative cells which fuse with each other (isogamy). Cyst formation is known to occur in several genera, like Ochromonas, Dinobryon etc.

Some representative types are shown in Fig. 5.45:

Taxonomic Group # 7. Euglenophycophyta:

The members of this algal division are commonly known as euglenoids, because the best known genus of the group is Euglena. The euglenoids are regarded as both algae and protozoa. Probably, they evolved from some flagellated protozoa which engulfed a green alga. The non-pigmented mutants are indistinguishable from protozoans and such forms can grow saprophytically or by ingestion of solid food particles by phagocytosis. Some species are also known to be parasitic.

Euglena is a photosynthetic flagellate containing several discoid chromatophores having both chlorophyll a and b, β-carotene and xanthophyll’s. This pigment combination is shared only by the green algae. No other algal group has both chlorophyll a and b.

The photosynthetic reserve material is paramylum which is a starch-like polysaccharide. But other cellular characteristics of Euglena are like those of flagellated protozoa. The cell is not enclosed by a cell-wall. Instead, there is a proteinaceous pellicle beneath the cytoplasmic membrane. It is a part of the modified cell membrane. The cell is also characterized by the presence of a gullet, contractile vacuoles and an eye spot.

The mitochondria are provided with discoid cristae — a primitive characteristic shared also by the amoeboid flagellates and acrasid slime molds. The number of thylakoid stacks in the chromatophores is three which is same as in chrysophytes and pyrrophytes. The organism contains many chromosomes in the eukaryotic nuclei and polyploidy is common.

Euglenq is a fresh water alga, preferring nutrient-rich (eutrophicated) habitats, where they grow abundantly as planktons. Individual cells are flexible, pear-shaped and provided with two flagella. One of the two flagella comes out through the gullet arising from a basal granule situated at the base of the gullet.

The other flagellum originates from another basal granule, but it remains confined within the gullet. The two flagella are connected with each other within the gullet. The cells are dorsiventrally flattened. Multiplication of Euglena is effected through binary fission along the long axis of the pyriform cell. Also, cyst formation is common and the cyst germinates to regenerate a cell. Sexual reproduction has not been reported.

The cellular characteristics and binary fission of Euglena are shown in Fig. 5.46:

Taxonomic Group # 8. Cryptophycophyta:

The organisms are generally known as crypto-monads. They, like euglenoids are considered as photosynthetic flagellates. ‘This small group comprises unicellular brown to olive-green flagellates occurring in fresh water and marine habitats. The unicells are dorsiventrally flattened with two unequal tinsel type flagella originating from a groove situated at the anterior end.

There are two comparatively large parietal chromatophores with or without pyrenoids. The photosynthetic pigments are chlorophyll a and c, a-carotene and alloxanthin. In addition, the organisms also contain phycoerythrin, phyco-cyanine and some other related phycobilin pigments, suggesting their probable origin from red algae. The marine crypto-monads, like red algae can grow at deeper layers. Photosynthetic reserve material is starch which is formed outside the chromatophores. Oils are also present.

Crypto-monad cells are either naked or invested in a cell wall consisting of cellulose. The naked cells are generally covered by a thin coating of a granular material outside the cell membrane. The organisms multiply by binary fission along the longitudinal axis of the uni-cell.

Asexual reproduction may occur by formation of zoospores. Crypto monas is also known to reproduce by sexual means. Cyst formation is absent, a feature which distinguishes the crypto monads from chrysophytes. Other distinctions between the two groups are absence of fucoxanthin in crypto monads and presence of phycobilins in at least some crypto monads.

A gross representation of the cell structure of the best known representative of the group viz. crypto monas is shown in Fig. 5.47.:

Taxonomic Group # 9. Pyrrophycophyta:

Commonly known as dinoflagellates, these algae are predominantly marine, though there are also fresh-water representatives. Together with diatoms and coccoliths (primnesiophytes), the dinoflagellates constitute the major part of marine phytoplankton’s. They may attain such a high population density that sea water becomes red, producing the so-called “red tide”.

Some members of the group, like Noctiluca miliaris which feed on diatoms, are bioluminescent and is one of the causes of luminescence of seas. Some members are known to be toxigenic and are responsible for large-scale fish mortality. Some dinoflagellates live symbiotically with marine animals e.g. giant clams.

The symbiotically existing dinoflagellates, known as zooxanthellae are coccoid in shape. They remain enclosed in double- membrane bound intracellular vacuoles. The dinoflagellates produce glycerol which is absorbed by the animal partner for its nutrition. The symbiotic dinoflagellates obtain CO₂, inorganic nitrogen, phosphates and vitamins from the animal.

The dinoflagellates are unicellular motile algae having typically flattened cells with an equatorial constriction, known as girdle. Although unicellular, they show a great diversity of form. In the more primitive types, such as Gymnodinium and allied genera, the cells are either naked or enclosed in a delicate cellulose membrane.

In the more advanced types, like Peridinium, Ceratium etc. the cell is covered with characteristic polygonal cellulose plates, called thecal plates. Such dinoflagellates are said to be “armoured”. The thecal plates are variously ornamented due to deposition of silica or calcium carbonate and are provided with numerous pores through which cytoplasmic strands protrude. The thecal plates are capable of intercalary growth. The equatorial girdle is also covered by the cellulose plates.

A characteristic feature of dinoflagellates is their flagella. Each cell is provided with two long flagella which are inserted in the girdle. One of the flagella encircles the girdle, while the other is projected posteriorly at right angles to the other. The flagella are covered with fine hair (tinsel type). With these flagella, dinoflagellates can swim at a very high speed and can traverse up to 100 times its length per second.

Dinoflagellates contain numerous yellow-brown chromatophores having chlorophyll a and c, β-carotene and several xanthophyll’s of which the most characteristic is peridinin. Photosynthetic reserve materials are starch and oils. Chrysolaminarin is also present in some members.

Dinoflagellates have unusual chromosomes which remain in a condensed state throughout. Unlike other eukaryotic chromosomes, histone content in chromatin is very low. The organisms are haploid. Multiplication occurs by cell division along the long axis of the cell.

In the armoured dinoflagellates, the armour is pushed apart to release the daughter cells which later build new armours. Asexual zoospores are formed in some species, as in Glenodinium. Sexual reproduction is rare, but is known to occur either by copulation of uniflagellate isogametes e.g. in Noctiluca or by fusion of micro- and macro- gametes.

The zygote nucleus divides meiotically to produce four nuclei, of which three degenerate. A haploid motile cell then comes out of the zygote, in some forms, e.g. Ceratium intracellular cyst formation is known. These are produced under unfavorable conditions and when favourable conditions return they germinate to produce new cells.

Morphological features of some representative dinoflagellates are diagrammatically represented in Fig. 5.48: