Algae Classification PDF

ALGAE CLASSIFICATION For many years it has been customary to classify algae according to their colour Therefore we speak of green, brown, golden-brown, red algae, etc. All algae contain at least one type of chlorophyll, but they also contain other typ...

ALGAE CLASSIFICATION For many years it has been customary to classify algae according to their colour Therefore we speak of green, brown, golden-brown, red algae, etc. All algae contain at least one type of chlorophyll, but they also contain other types of pigments and these may mask the colour of the chlorophyll  ALGAE Cont’d Unicellular motile algae are grouped by some biologists along with some multicellular, motile animals, in a separate kingdom (neither plant nor animal, but including attributes of both) called the Protista  Usually movement by algae cells is produced by the beating action against the water of one or more of the protoplasmic extensions from the cell called cilia or flagella PRIMARY CLASSIFICATION OF THE ALGAE Algae are basically classified into the following Divisions/Phyla: ✓ Cyanophyta (Blue-green algae) ✓ Chlorophyta (Green algae) ✓ Bacillariophyta (Diatoms) ✓ Phaeophyta (Brown algae) ✓ Rhodophyta (Red algae) ✓ Euglenophyta (Euglenoids) ✓ Chrysophyta (Golden algae)  CLASSIFICATION Cont’d In general details of vegetative structure and processes of reproduction are not particularly useful for the primary classification of algae Instead, the primary classification is based on five criteria of a different nature:  CLASSIFICATION Cont’d 1. Photosynthetic pigments 2. Nature of the food reserve 3. Nature of the cell wall component 4. Types of flagella 5. Details of cell structure  The final classification of the algae depends on a combination of several characteristics and not on any one single feature  1. PHOTOSYNTHETIC PIGMENTS Algae from the various Phyla/Divisions show striking differences in colour And these often afford a quick guide to the preliminary classification of an alga However the colour frequently varies with changes in environmental conditions  And an accurate classification depends on the chemical analysis of the photosynthetic pigments The distribution of these pigments is important in algal classification  There are three kinds of photosynthetic pigments in algae. These are ✓ the chlorophylls ✓ the carotenoids, and ✓ the biloproteins  i. CHLOROPHYLLS Chlorophylls extracted from different algae show different spectral properties On the basis of this a number of different chlorophylls have been recognized and termed chlorophyll a, b, c, d and e  The distribution of these chlorophylls among various algal groups is what results in the differences in colour Chlorophyll a is present in all algae as it is in all photosynthetic organisms except the photosynthetic bacteria Chlorophyll b, the other chlorophyll of higher plants, is found in the Euglenophyta and the Chlorophyta  They are not found in any other algal division Chlorophyll c is present in members of the Chrysophyta, Bacillariophyta, Cryptophyta and the Phaeophyta  Chlorophyll d appears to be present only in the Rhodophyta Chlorophyll e has been identified in only two species of the Xanthophyta  ii. CAROTENOIDS Carotenoids are of two kinds: ✓Carotenes, and ✓ Xanthophylls Carotenes are linear unsaturated hydrocarbons, and Xanthophylls are oxygenated derivatives of the carotenes  CAROTENOIDS Cont’d β-carotenes are present in most algae although they are replaced by α- carotenes in some members of the Chlorophyta and Cryptophyta and to a lesser extent in the Rhodophyta In the Chlorophyceae β-carotene is replaced by two carotenes which are characteristic of photosynthetic bacteria: lycopene and γ-carotene  CAROTENOIDS Cont’d There are many different xanthophylls in algae, and since many are unique to particular algal groups, they are important diagnostic features in the classification of algae  iii. BILOPROTEINS Chlorophylls and carotenoids are soluble in lipid solvents and cannot be extracted in aqueous solution However, water soluble pigments can be extracted from some types of algae These were called phycobilins During the extraction procedure the free pigment cannot be separated from the protein part  BILOPROTEINS Cont’d And the name of the pigment was therefore changed from phycobilins to biloproteins to indicate the existence of the pigment-protein complex Biloproteins are present in only three algal divisions: ✓the Cyanophyta ✓the Rhodophyta, and ✓ the Cryptophyta  BILOPROTEINS Cont’d Analysis of the spectral properties of these pigments shows that there are two kinds of biloproteins: Phycocyanin and Phycoerythrin Each of these biloproteins shows differences among the three groups of algae  BILOPROTEINS Cont’d Cont’d In general those of the Cyanophyta are of the C-type Those of the Rhodophyta are of the R-type, and Those of the Cryptophyta are of a third type The proportion of one kind of photosynthetic pigment to the other is variable  PHOTOSYNTHETIC PIGMENTS Cont’d The proportion of one kind of photosynthetic pigment to the other is variable For example cells of the Chlorophyta and Euglenophyta appear green because of an excess of chlorophylls over carotenoids  PHOTOSYNTHETIC PIGMENTS Cont’d Whereas the yellow-brown colour of groups such as the Pyrrophyta, Chrysophyta, Cryptophyta, Phaeophyta, etc. and the yellow-green colour of the Xanthophyta reflects an excess of carotenoids compared with chlorophylls Also, the characteristic colour of the Cyanophyta (blue-green) and the Rhodophyta (red) are due to an excess of the appropriate biloproteins  PHOTOSYNTHETIC PIGMENTS Cont’d However the proportion of one type of pigment to the other can vary considerably with changes in the environmental conditions And it is difficult to justify its use as a taxonomic feature  2. FOOD STORAGE The initial stages of CO2 fixation are probably the same in all photosynthetic organisms Thus the primary products of photosynthesis are the same in all algae However the insoluble products which accumulate over a longer period of time are more variable and they afford useful taxonomic criteria  FOOD STORAGE Cont’d The compounds which are most widespread and most useful in the primary classification of algae are various polysaccharides “True” starch, similar to that found in higher plants, is only found in one algal division, the Chlorophyta Two other divisions, the Rhodophyta and the Cyanophyta, accumulate characteristic starches:  FOOD STORAGE Cont’d ✓Floridean starch and Myxophycean starch respectively Both are polyglucose molecules identical to the amylopectin part of higher plant starch In some other algae such as the Phaeophyta, the storage carbohydrate is Laminarin Paramylum is present in the Euglenophyta  3. CELL WALL COMPONENT When a cell wall is present in an alga its chemical constituent varies from one group to the other And these are sometimes important indications of the taxonomic position of the particular alga The cell wall is generally made up of two kinds of materials: ✓an inner water insoluble material, and ✓an outer pectic or mucilageneous substance soluble in boiling water  CELL WALL COMPONENT Cont’d Although both inner and outer wall materials are mainly polysaccharides, lipid and proteinaceous materials are also present  CELL WALL COMPONENT Cont’d The commonest water insoluble polysaccharide of the inner layer is cellulose And this is present in walled species of all divisions except the Chrysophyta Other characteristic components of the cell wall includes polyuronic acid and alginic acid  4. TYPE OF FLAGELLA Apart from the Cyanophyta and the Rhodophyta flagella are found in other divisions of the algae And their nature, number and position are important characters for the primary classification of the algae  FLAGELLA Cont’d The detailed fibrillar structure of the algal flagella in transverse section resembles that of cilia and flagella of other organisms in showing a typical 9+2 pattern of component fibrils  FLAGELLA Cont’d Each flagellum is bounded by an extension of the plasmalemma Within the plasmalemma there is a ring of nine pairs of fused fibrils/tubules and a pair of unfused fibrils/tubules at the centre This is the basic pattern of plant and animal flagella  FLAGELLA Cont’d The macrostructure of the flagellum does not however show such uniformity For a long time algal flagella were thought to be of two kinds: ✓the acronematic (smooth), and ✓the pantonematic/pleuronematic (flimmer)  FLAGELLA Cont’d The acronematic type is smooth and whiplike While the pantonematic/pleuronematic type has longitudinal rows of fine hairs (flimmers or mastigonemata/ mastigonemes) arranged along the axis of the flagella  FLAGELLA Cont’d More recent work with the electron microscope has revealed one other type of flagella in which the flagella surface is covered with minute hairs (different from those of the pantonematic/pleuronematic flagella) and scales  5. DETAILS OF CELL STRUCTURE The important structural features of the cell of various algae normally are uniform throughout the division In most texts particularly on the Chlorophyta, chloroplast runs throughout the entire division while chromatophores are found in others  CELL STRUCTURE Cont’d The distinction between these two pigments is generally based on the differences of pigmentation The term chloroplast is used in species of algae possessing chlorophylls a and b (as in higher plants) And the term chromatophore is applied to algal species not having chlorophyll b, but having an excess of carotenoids over chlorophylls  CELL STRUCTURE Cont’d The position of chloroplast in the cell is very important They are termed parietal when located towards the periphery of the cell, and axiel when located towards the centre A further feature of the chloroplast which is emphasized is the presence or absence of a deeply staining area of the chloroplast generally associated with deposits of reserved products, the pyrenoid  CELL STRUCTURE Cont’d The cells of archegonate plants (bryophytes)normally have numerous discoid chloroplasts, and the possession of such a feature by some algal cells is therefore emphasized  MORPHOLOGIC DIVERSITY OF THE ALGAE The body of an alga is a thallus Algae range in form from Unicellular through Colonial, Filamentous, Siphonaceous, to the complex Parenchymatous thalli of the larger seaweeds  UNICELLULAR FORMS Unicellular forms are among all groups of algae except the Rhodophyta and Phaeophyta, although even among these two groups unicellular stages are produced at various points in their life history The unicellular species may be motile (flagellated), non-motile (coccoid) or amoeba-like  UNICELLULAR FORMS Cont’d Flagellated solitary cells are considered primitive in most groups of eukaryotic algae and are believed to have given rise to the other types They vary in the number and arrangement of the flagella MULTICELLULAR FORMS  1. COLONIAL FORMS The association of organisms into groups of cells or colonies probably originated as it does in ontogeny (development of the individual) ✓by the failure of the cells to separate after cell division  i. COENOBIAL In this type of thallus the cells are either embedded in mucilaginous matrix or united by a more localized production of mucilage It is not merely an irregular aggregation of cells but it is a well defined colony with important reproductive features  COENOBIAL Cont’d The coenobium (colony) is of constant size and shape for any given species and the cells show no vegetative division Thus the number of cells of a coenobium is determined at its formation and does not increase during growth of the colony  ii. AGGREGATIONS Unlike the coenobium an aggregation of cells is not of constant size and shape Moreover vegetative cell division takes place so there is an increase in cell number during growth The most common type of aggregation is the palmelloid form in which the cells are embedded in an irregular mass of mucilage  AGGREGATION Cont’d The dendroid colony consists of cells which are united by localized production of mucilage to form a tree-like structure Another kind is the rhizopodial form of aggregation consisting of variable number of amoeboid cells joined together by a number of cytoplasmic processes  2. FILAMENTOUS FORMS Filamentous forms are also characterized by vegetative cell division but unlike the irregular aggregations the cells are arranged in linear rows with adjacent cells sharing a common cross wall Cytoplamic connections (plasmodesmata) may extend through the cross walls  FILAMENTOUS FORMS Cont’d In uniseriate filaments the cells are arranged in a single series Multiseriate filaments have more than one series of cells but still retain a thread-like appearance  FILAMENTOUS FORMS Cont’d Filaments may be branched or unbranched More complex filamentous algae may show differentiation among the branches Heterotrichous filaments have a distinct system of prostrate branches growing attached to the substrate and an erect system of more open branches extending free of the substrate  FILAMENTOUS FORMS Cont’d In the pseudoparenchymatous thalli the branches do not spread apart in an open branching pattern but form a compact mass that makes individual branches difficult to see Such a structure is the basis of all larger members of the Rhodophyta 3. SIPHONEOUS/SIPHONACEOUS FORMS In this kind the thallus is multinucleate but is not divided into cells apart from those associated with reproduction The thallus can be extremely elaborate and is generally considered more desirable to refer to such a thallus as acellular and not unicellular  4. PARENCHYMATOUS FORMS Vegetative cell division in filamentous forms occurs in one plane only so that a single row of cells is formed When cells divide in more than one plane a parenchymatous construction is produced Cell divisions in three dimensions produce a solid mass of cells rather than the threadlike linear arrangement of a filament  PARENCHYMENTOUS Cont’d Parenchymatous thalli may be blades, branching cylinders or hollow tubes The parenchymatous construction which is also characteristic of bryophytes and vascular plants is the most advanced form  PARENCHYMENTOUS Cont’d Growth of the filamentous and parenchymatous thalli can be: Diffused ie. all the cells are capable of division Intercalary ie. well defined dividing regions are not located terminally  PARENCHYMENTOUS Cont’d Trichothallic ie. a specialized meristematic region at the base of branches or filaments, or Apical ie. one or more well defined apical cells dividing to give the remainder of the thallus  METHODS OF REPRODUCTION IN THE ALGAE A particular plant may take to one or more of the three methods of reproduction i.e ✓Vegetative ✓Asexual, or ✓Sexual  1. VEGETATIVE REPRODUCTION Vegetative reproduction commonly takes place by cell division or by fragmentation Many filamentous algae reproduce vegetatively by the fragmentation of the filament to liberate small pieces Among filamentous members of the Cyanophyta this is a specialized process and a number of short motile lengths of filaments are formed  VEGETATIVE REPRODUCTION Cont’d These entities are referred to as hormogonia They are short segments from the ends of the filaments that form when the walls between the cells split or when the cell in between dies  2. ASEXUAL REPRODUCTION Asexual reproduction involves the formation of reproductive cells that develop directly into new individuals (without sexual fusion) This is normally achieved by the formation of spores of various kinds Most groups except the Cyanophyta and Rhodophyta produce zoospores which are motile unicells  ASEXUAL REPRODUCTION Cont’d Non-motile asexual spores are also produced and these are called aplanospores When the non-motile asexual spores appear identical to the parent cell (ie. similar in form but a miniature of the parent cell) they are referred to as autospores  ASEXUAL REPRODUCTION Cont’d And if they acquire a thick wall around them they are referred to as hypnospores The term ‘‘swarmer’’ is commonly used for any motile cell formed when a vegetative cell reproduces and it indicates that it is unknown whether the swarmer behaves as a gamete or a zoospore  ASEXUAL REPRODUCTION Cont’d Among multicellular forms the spore may be formed in all vegetative cells or their formation may be restricted to well-defined sporangia In the Phaeophyta two specialized kinds of sporangia can be recognized These are the plurilocular and the unilocular sporangia  ASEXUAL REPRODUCTION Cont’d The plurilocular consists of an enlarged vegetative cell which divides into a number of compartments and the content of each compartment develops into a swarmer  ASEXUAL REPRODUCTION Cont’d In the unilocular type contents of the enlarged vegetative cell divide to form a number of swarmers without any previous division of the parent cell into a number of compartments Swarmers from the unilocular type are always asexual whereas either gametes or zoospores can be liberated from the plurilocular sporangia  3. SEXUAL REPRODUCTION In sexual reproduction the cells released by the parents are gametes Pairs of compatible gametes fuse to form a zygote Sexual reproduction is achieved by three basic means: ✓Isogamy ✓Anisogamy, and ✓Oogamy  SEXUAL REPRODUCTION Cont’d If both gametes of a pair are flagellated and similar in size they are isogamates Gametes that are flagellated but differ in size are known as anisogamates Isogamy involves the fusion of two identical gametes and anisogamy is the fusion of two morphologically dissimilar gametes  SEXUAL REPRODUCTION Cont’d Sometimes morphologically identical gametes behave differently and so show physiological anisogamy In oogamy only one gamete (the sperm) is flagellated and it fuses with a larger non-flagellated gamete (the egg)  SEXUAL REPRODUCTION Cont’d Oogamy also differs from anisogamy in that the female gamete (the egg) is not liberated prior to fertilization but is fertilized while within the oogonium  GERMINATION OF THE ZYGOTE The zygote formed by the three methods of sexual reproduction has an independent existence for a variable length of time Upon germination the content of the zygote divides to form a number of zoospores  ZYGOTE GERMINATION Cont’d These are liberated and after a period of swimming they germinate into a parent plant More seldom the zygote germinates directly into the adult plant During growth an alga passes through a number of distinct phases and the sequence of these is known as its life history  ZYGOTE GERMINATION Cont’d The life history has two aspects ✓the somatic or morphological, and ✓the cytological The somatic or morphological aspect involves whether in the life history the vegetative stages are morphologically alike or not  ZYGOTE GERMINATION Cont’d The cytological aspect is usually concerned with the chromosome number of each particular stage The type of life history thought to be most primitive is that in which the only vegetative stage is haploid and the zygote represents the only diploid stage  ZYGOTE GERMINATION Cont’d The opposite extreme is that in which the vegetative stage is diploid and in which the gametes represent the only haploid stage Intermediate between the two extremes are those life histories in which there is an alternation between two vegetative stages, one haploid and the other diploid  ZYGOTE GERMINATION Cont’d When the two stages are morphologically similar the alternation is isomorphic, and When they are morphologically different the alternation is heteromorphic