Algae is an extremely diverse group of organisms that make up the lower phylogenetic echelons of the plant kingdom. A precise definition of this group is elusive and they share many obvious characteristics with higher land plants, whereas their distinguishing features from other plant groups are varied and more subtle according to the classification of Bold and Wynne 1985. Bold and Wynne (1985) recognized ten divisions of algae retaining the nomenclature given by Papenfuss (1946), except for blue-green algae.
(To know about the last three divisions of Bold and Wynne (Pyrrhophyta, Cryptophyta, Rhodophyta), follow this link below:
Bold & Wynne’s Divisions: Pyrrhophyta, Cryptophyta, Rhodophyta
In this article, the second three divisions
are briefly characterized.
eg: Euglena, Trachleomonas, Phacus, Astasia, Colasium etc
- Common name: Euglenoids
- Habit: Unicellular flagellates, cylindrical, ovoid to fusiform, microscopic, both heterotrophic & autotrophic organism.
- Habitat: They are commonly found in freshwater, especially when it is rich in organic materials, with a few marine and endosymbiotic members.
- Cell wall: Doesn’t have a considerable cell wall, but has a flexible outer layer called pellicle or periplast. Pellicle may be flexible or rigid and and composed of up to 80% protein, lipid and carbohydrates.
- Color: Usually grass green & colorless species also found.
- Pigment: Pigmented with Chlorophyll ‘a’ and ‘b’. Main secondary pigments are beta-carotene, several xanthophylls (neoxanthin, and diadinoxanthin). Minor pigments include echinenone, diatoxanthin, and zeaxanthin.
- Plastid organization: Thylakoids in groups of three, without a girdle lamella.
- Storage product: Starch & paramylon. Oil is also present.
- Flagella: Number of flagella is one, two or rarely three being attached to the anterior end of the cell. The flagellar arrangement is atypical in that there is a longer emergent flagella of the tinsel variety, but the hairs are located on only one side of the flagella.
- Reproduction: Asexual Reproduction by binary fission.
- Toxin: Euglenophysin an alkaloid toxin, known to cause fish kills and inhibit mammalian tissue.
- A thick green or red scum on the surface of the water is often from a euglenoid bloom.
- These are an indicator group for disgusting environmental conditions.
eg: Laminaria, Saccharina, Fucus, Sargassum, Dictyota, Macrocystis pyrifera, Cladosiphon.
- Common name: Brown algae
- Habit: Brown algae are a group of autotrophic, multicellular organisms. The simplest form is a branched, filamentous thallus. The length of thallus can range from a microscopic length to several meters.
- Habitat: Phaeophyta are commonly adapted to marine environment, only a few phaeophyta are freshwater species. In fact, majority of phaeophyta are predominant in the temperate zones of Northern Hemisphere, whereas some species are found in warm tropical waters.
- Cell wall: The cell wall consists of two layers; the inner layer bears the strength, and consists of cellulose; the outer wall layer is mainly of algin (a carbohydrate).
- Color: Members of phaeophyta exhibit a characteristic greenish-brown color.
- Pigment: They contain the fucoxanthin, chlorophyll a and c (c1 and c2). The brown colored pigment is very important for the adaptation of phaeophyta in deep seas and oceans. Beta carotene is also present.
- Plastid organization: Thylakoids in stacks of 2 to 6 forming grana. Chloroplast is enclosed by double unit membrane.
- Storage product: Upto 34% Laminarin (a beta 1,3 linked glucan polymer), Mannitol (a sugar alcohol)
- Flagella: No flagella present.
- Reproduction: They commonly reproduce by asexual heterokont zoospores, or zygotes formed by fusion of motile or nonmotile female gametes with heterokont male gametes.
- Toxin: Algal bloom cause toxicity in water.
- The Brown Algae play the ecological roles of a decomposer, producer and a food source for aquatic life.
- Some members like kelp used by human as food.
- Alginic acid from the cell wall of brown algae , used in aquaculture.
E. g.: Calcidiscus bulbous, Chrysamoeba, Ochromonas, Dinobryon etc.
- Common name: Golden brown algae
- Habit: Mostly unicellular flagellates but some tends to form colonies. Some are amoeboid. Other members are non-motile. Cells may be naked and embedded in mucilage.
- Habitat: Chrysophytes live mostly in freshwater.
- Cell wall : The walls are frequently in two equal or unequal portions which overlap, composed of pectic substance although some cellulose may occasionally be present.
- Color: characteristic golden brown color due to masking of chlorophyll by brown pigment.
- Pigment: chlorophyll a & c (c1 and c2), fucoxanthin prominent among several xanthophylls. Beta carotene present.
- Plastid organization: Three thylakoids in number.
- Storage product: Chrysolaminarin and frequent incorporation of silica.
- Flagella: Characterized by two unequal flagella, the longer active one often possessing delicate cilia, oriented toward the moving direction. The smaller smooth passive flagellum, oriented toward the opposite direction.
- Reproduction: Commonly asexual. Sexual reproduction is rare but at times known examples is isogamous. The principal mode of reproduction within the class is by means of zoospores and aplanospores.
Toxin: Single species Prymnesium parvum produce toxin prymnesin.
- Somudro Mahbub Nasir on 11 July 2021