The following points highlight the six main criterions on which modern concepts of classification of algae is based. The criterions are: 1. Nuclear Organization 2. Nature of Cell Wall Components 3. Pigmentation and Photosynthetic Apparatus 4. Nature of Reserve Food 5. Flagellation 6. Type of Life Cycle and Reproduction.
Criteria # 1. Nuclear Organization:
On the basis of nuclear organization algae can be prokaryotic or eukaryotic. Cyanophyceae or blue green algae are prokaryotic in nature whereas all other classes of algae are eukaryotic. In prokaryotic e.g., Cyanophyceae—nucleus is not organized as nuclear membrane is absent.
DNA fibrils are free in nucleoplasm and are not associated with histones. Cell division by mitosis and meiosis is not found. Membrane bound cell organelle like chloroplast, mitochondria and ER are absent. Eukaryotic algae have well differentiated nucleus, mitochondria, chloroplast and endoplasmic reticulum in their cell structure.
Criteria # 2. Nature of Cell Wall Components:
The cell wall in algae is generally made up of polysaccharides. In some cases lipids and proteins are also present in them. The inner layer of cell wall in algae is generally made up of cellulose, which is insoluble polysaccharide and the outer layer is made of pectic substances.
The cell wall in Chlorophyceae is made up of cellulose. In Xanthophyceae pectic substance is more common.
In Chrysophyceae cell wall is non-cellulosic which is silicified or calcified. In Phaeophyceae cell wall contains alginic acid and fucinic acid. In Rhodophyceae the cell wall is made of non-cellulosic polysaccharides like xylans and galactans. In Cyanophyceae or blue green algae the cell wall is more like bacteria and is made up of mucopeptides.
Criteria # 3. Pigmentation and Photosynthetic Apparatus:
The pigment is one of the most important criteria used in differentiation of classes in algae, as algae were initially and primarily separated on the basis of colour e.g., green algae, red algae, brown algae or blue-green algae. The pigments in algae can be chlorophylls, carotenoids and biloproteins.
These pigments are present in sac like structures called thylakoids. The thylakoids are arranged in stacks in granum of the chloroplasts. Different groups of algae have different types of pigments and organization of thylakoids in chloroplast.
The pigments in algae are:
The chlorophylls in algae are chlorophyll a, b, c, d and e types.
Chlorophyll a is present in all classes of algae.
Chlorophyll b is primary pigment of Chlorophyceae and Euglenineae.
Chlorophyll c is found in Phaeophyceae and Cryptophyceae.
Chlorophyll d is found in Rhodophyceae.
Chlorophyll e is confined to Tribonema of Xanthophyceae.
The carotenoids are of two kinds: Carotenes and Xanthophyll’s.
Among the carotenoids, β carotene is found in all classes of algae.
∝ Carotene is found in Rhodophyceae.
ϒ Carotene and lycopene are found in Chlorophyceae.
e Carotene is present in Bacillariophyceae.
There are about 20 types of xanthophyll’s commonly found in algae e.g., Neoxanthin, neo-fucoxanthin, fucoxanthin, chaetoxanthin, siphonoxanthin, oscillatoxanthin. As these xanthophyll’s are restricted to certain classes, the xanthophyll’s are important diagnostic characteristics of algae.
The biloproteins are water soluble pigments and can be phycocyanin, phycoerythrin and allophycocyanin. These are common only in Rhodophyceae and Cyanophyceae.
In Rhodophyceae, R-phycoerythrin is the chief pigment and in Cyanophyceae, C-phycocyanin is the chief pigment.
The chromatophores of different classes of algae differ in number of thylakoids per granum. In Chlorophyceae there are 2-6 thylakoids per granum and the pyrenoids are covered with starch plates.
In Xanthophyceae, Phaeophyceae and Bacillariophyceae there are 3 thylakoids per granum and pyrenoids are without starch plates.
In Rhodophyceae thylakoids are single and widely separated in chromatophores and pyrenoids are naked.
In Cyanophyceae the thylakoids are free in cytoplasm as chloroplasts are not found and pyrenoids are also absent.
Criteria # 4. Nature of Reserve Food:
The primary product of photosynthesis i.e., starch is same in all groups of algae but due to accumulation of food over long period the nature of insoluble reserve food may be different. The nature of reserve food can be a criterion for distinction of different groups of algae.
In Chlorophyceae the reserve food is starch. In Xanthophyceae oil and leucosine are reserve food materials. In Rhodophyceae rhodophycean or floridean starch and in Cyanophyceae myxophycean starches are the reserve foods. In Phaeophyceae the reserve food material is laminarin or mannitol.
Criteria # 5. Flagellation:
The type, number and position of flagella are important basis for separation of different classes of algae. (Fig. 1.A-D).
In Cyanophyceae and Rhodophyceae flagella are completely absent in vegetative and reproductive structures. In all other classes the basic flagellar structure is similar. The flagella have 9 + 2 pattern of component fibrils.
The flagella can be acronematic (Fig. 2B) or whiplash, pleuronematic or tinsel and prasionate in type.
In Chlorophyceae flagella are 2 (Fig. 2A), 4 or indefinite in number, apical or sub-apical in position and acronematic type i.e., isokontic.
In Xanthophyceae flagella are two, unequal apical one acronematic and one pantonematic i.e., heterokontic. tonematic flagella, (C)
In Phaeophyceae flagella are two lateral, one acronematic and one pantonematic and unequal in size. flagellum.
In Prasinophyceae, prasionate type flagella are found. These are pantonematic and covered by minute hairs.
Criteria # 6. Type of Life Cycle and Reproduction:
The presence or absence of sexual reproduction, complexity of reproductive organs, method of sexual reproduction i.e., isogamy, anisogamy and oogamy are important criteria of classification in algae. Haplontic life cycle, diplontic life cycle and triphasic life cycles are characteristic of different groups.
For example, sexual reproduction is completely absent in Cyanophyceae. The reproduction is oogamous and life cycles are usually complex in Rhodophyceae and Phaeophyceae. In Chlorophyceae the reproduction can be isogamous, anisogamous and oogamous, the life cycle can be simple or complex.