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  Classification of Algae-I, Biology tutorial

Introduction to Classification of Algae:

Categorization indicates clustering of organisms according to similarity in characters. It is not farfetched but true that organisms displaying similar morphology, life cycle physiololgy and biochemistry are genetically connected from evolutionary point of view (phyllogenrtically related) and one is justified is grouping them together. Algae could be classified according to the common feature in 8 division of kingdom protista.

Criteria for categorization of Algae:

Criteria utilized by phycologists are fairly diverse. Usually the number of characters are utilized together ranging from enzymes, external morphology, nature of cellular storage Products, pigment composition, ultra structure, DNA homology, chromosomes number and morphology, isoenzymes, and DNA banding etc. as novel methods are developed they are utilized to decide more exactly relatedness (or absence of it) organisms that appear otherwise associated to each other.

Significant characters of each of divisions of algae are given below. Each division is again in genera, orders, families, and species.

Prokaryotic Algae:

Division CYANOPHYTA (Ccyanobacteria or Blue-green algae):

Prokaryotic algae are positioned in Division Cyanophyta. Algae of the division may be unicellular, colonial and filamentous, with or lacking branches, branching may be true or false type. Many forms are fixed in mucilaginous or galetinous sheaths. Composition of cell wall is similar to bacteria cell wall. It is composed of distinctive mucopeptides and muramic acid. Ultra structure of cell shows no organism organized nucleus, photosynthestic lametae, mitochondria or chloplasts, and ribosomes of 70s types are present in cytoplasm of cells. Few filament forms have specialized cells known as 'heterocysts'.

Main photosynthetic pigments are chlorophyll a and phycolbilins- (phycocyanin and phycoerythrin). The number of caroteniods comprising Bcarotene are also present, some of which are specific to division.

Carbon is reserved in cells as glycogen granules and nitrogen as cyanophycean granules. Other granules similar to polyphosplate granules, few enzyme aggregates similar to carboxysomes may be present. Reproduction happens by simple cell division. No motile cells are discovered in cyanobacteria and they don't have sexual method of reproduction. Thick walled cells known as akinetes or spores are present in few forms for perannation and asexual reproduction. Cynobacteria are dispersed all over earth in diverse habits, rivers, brine salt pans, fresh water lakes ponds, Antarctic areas, arctic, hot water springs, desert soils subaerial surfaces such as trees trunks, building terraces and rock surfaces.

For e.g.: Microcysts, Stigonema, Spirulina, Nostoc, Anacysts, Oscillatoria, Tolypo thrix, Gleotrichia, Anabaena, Lyngbya, Calothrix, and Scytonema.

Eukaryotic Algae:

Kingdom Protista includes 8 divisions of algae. Few phycologists make 9 divisions treating Bacillariophyta separate from Chrysophyta. They are explained in detail.

Division CHLOROPHYTA (Green algae):

This comprise unicellular to multicellular forms of green algae. Multicellular forms may be in form of branched or unbranched, thalloid, filamentous, tubular or sheet like arrangement of cells. Few of the green algae are colonial in form. Cell structure is eukaryotic type as in higher plants with membrane bound organelles plastids, nucleus, mitochondria, and cytomplasmic ribosomes of 80s type.

Cell wall is usually composed of cellulose. At times cells are also enclosed with chitin. Principal photosynthetic pigments are chlorophyll a and b, carotenes and xanthophylls found in thylakoids. The storage products of cell are generally starch, butin some algae lipids. Reproduction happens by asexual and sexual methods. Asexual reproduction is by biflagellate or guadri-flagellate zoospores while gametes (sexual reproduction) are biflagellate. Flagella are anterior and of whiplash type. Sexual reproduction comprises isogamy, anisogmy, anisogamy and oogamy. Green algae are distributed in flesh water and marine habitats; few may be subaerial on wet soil or bark of trees.

For e.g.: Chlamydomonas, Caulerpa, Pediastrum, Spirogyra, Cladophora, chlorella, Acetabularia, Trentephohlia and Micrasterias.

Division PHAEOPHYTA (Brown algae):

They are most complex in morphology in structure. They range from simple branched filaments to huge bodies. Cell wall composition is complex, besides cellulose, it may have fucoidin, algin. Principal photosynthetic pigments are chlorophyll a and c and carontenoids Fucoxanthin (colour is brown) os present in large amount which give alga brown color by masking green color of chlorophyll. Photosynthetic storage product is mannitol, sometimes laminarin, hardly ever, lipid droplets may be discovered in cells. Sexual reproduction ranges from isogamy to oogamy. Motile swarmers contain two unrequal laterally inserted flagella, one of the flagella is larger and anterior and the other is smaller and posterior. Most of brown algae are seaweed, extremely large in size, usually called as kelps. They are major source of iodine, agar and related products. For e.g.: Fucus, Padina, Laminaria, Alaria, Sargassum, Dictyota, Macrocysts, Ectarpus, and Nereocystis.

Division RHODOPHYTA (Red algae):

Many forms are multicellular and highly branched, few are thalloid and one alga Porphyridium is unicellular. Body may be enclosed with calcium carbonate incrustations.

Besides cellulose their cell wall has pectin, polysuphata, esters and great amount of polysaccharides on outside of surface. These polysaccharides are source of agar and carageenans. Red algae for e.g. coralline algae secrete calcium carbonate around the cells and form stiff thalli.

Major photosynthetic pigments are chlorophyll a, d and phycoerythrin. Few red algae have phycocyanin also. Algae appear red or pink in color due to large amount of phycorythrin.

Food reserve in cells is Floridian starch. No motile cells are discovered at any phase of reproduction. Sexual reproduction is advanced oogamous type. Male gametes - spermatia are passively transported through water movements to tip of trichogyne of female carpogonium, in any other division of algae. Many red algae are marine in habitat. Some are found in fresh water lakes, streams, rivers and ponds. Few are epiphytic or parasitic in nature. For e.g.: Portphyra, Porphyridium (unicellular), Polysiphyonia, Gracilaria Gelidium and Corallina.

Division XANTHOPHYTA (Yellow-Green algae):

Few forms are unicellular and motile where as others are filaments, with multinucleate cells.

Photosynthetic pigments are chlorophyll a, c, B-carontene that is present in large amount, and xanthophylls giving cells greenish-yellow colour. Food reserves comprise lipid and chrsolaminarin (B-1,3-linked polymer of glucose also called as leucosin)

Cell wall often comprises of 2 overlapping halves, comprising pectin, silica and little amount cellulose. Sexual reproduction is rare. Motile cells have 2 unequal flagella present on anterior end; one is tinsel and other whiplash type. Yellow-green algae are extensively distributed in aquatic, fresh water habitats. Few are sub-aerial and some are marine in distribution. For e.g.: Botrydium, Vaucheria.

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