Algae are a fascinating group of organisms that exhibit a wide array of characteristics in their pigments, reserve food composition, and cellular structures. These variances have led researchers to classify algae into distinct categories based on their morphological and physiological traits. In 1935, Fritsch proposed an elaborate classification system, dividing algae into eleven classes according to factors such as pigment type, reserve food material, and reproductive modes.
Class 1: Chlorophyceae (Green Algae)
- Occurrence: Predominantly found in freshwater environments, with some species also thriving in marine habitats.
- Pigments: Primary pigments include chlorophyll a, chlorophyll b, and carotenoids (yellow pigments).
- Reserve Food: Starch
- Structure: Ranging from unicellular motile forms to heterotrichous filaments, with cellulose cell walls and starch-surrounded pyrenoids. Motile cells typically possess two equal flagella.
- Reproduction: Varied sexual reproduction modes, from isogamous to advanced oogamous types.
- Examples: Chlamydomonas, Volvox, Chlorella, Scenedesmus, Pediastrum
Class 2: Xanthophyceae (Yellow Green Algae)
- Occurrence: Primarily found in freshwater with a few marine species.
- Pigments: Abundantly contains yellow xanthophyll.
- Reserve food: Storage in the form of oil.
- Structure: Ranges from unicellular motile to simple filamentous. Cell walls are rich in pectic compounds and composed of two equal pieces overlapping at their edges. Motile cells have two highly unequal flagella.
- Reproduction: Sexual reproduction is rare and always isogamous.
- Example: Vaucheria.
Class 3: Chrysophyceae
- Occurrence: Mostly inhabits cold freshwater with some marine species.
- Pigments: Chromatophores are brown or orange colored, with fucoxanthin serving as the primary accessory pigments.
- Reserve food: Fat and leucosin.
- Structure: Ranges from unicellular motile to branched filamentous. Flagella are unevenly attached at the front end. Cells typically contain one or two parietal chromatophores.
- Reproduction: Sexual reproduction is rare but is of the isogamous type.
- Example: Chrysodendron, Phaeothamnion.
Class 4: Bacillariophyceae (Diatoms)
- Occurrence: Found in various freshwater, marine, soil, and terrestrial habitats.
- Pigments: Chromatophores are yellow or golden brown. Additional pigment nature is not well defined.
- Reserve food: Storage includes fat and volutin.
- Structure: Members are unicellular or colonial with cell walls composed partly of silica and partly of pectic substances. The ornamental cell wall consists of two halves, each made up of two or more pieces.
- Reproduction: Diploid forms. Unique sexual reproduction involves the fusion of protoplasts.
- Example: Pinnularia.
Class 5: Cryptophyceae
- Occurrence: Present in both marine and freshwater environments.
- Pigments: Chromatophores exhibit diverse pigmentation, often in shades of brown, usually parietal.
- Reserve food: Consists of solid carbohydrates or starch in some cases.
- Structure: Ranges from motile cells to coccoid advanced forms with slightly unequal flagella.
- Reproduction: Isogamous in reported cases.
- Example: Chroomona.
Class 6: Dinophyceae
- Occurrence: Widely distributed as sea water plankton with a few freshwater forms.
- Pigments: Chromatophores are dark yellow, brown, and contain numerous special pigments.
- Reserve food: Starch and oil.
- Structure: Unicellular motile to branched filamentous forms.
- Reproduction: Isogamous sexual reproduction, rare and not clearly defined.
- Example: Dinoflagellate, Ceratium.
Class 7: Chloromonadineae
- Occurrence: Freshwater formations.
- Pigments: Chromatophores are bright green with an abundance of xanthophyll.
- Reserve food: Storage primarily in oil.
- Structure: Motile, flagellate plants with two nearly equal flagella.
- Reproduction: Lacks sexual reproduction, cells divide by longitudinal means.
CLASS 8: EUGLENINEAE • Habitat: Found exclusively in freshwater environments • Pigments: Chromatophores are green in color. Each cell contains multiple chromatophores. • Reserve food: Polysaccharide and Paramylon • Structure: Motile flagellates with one or two flagella emerging from a canal-like invagination at the front end. They have a complex vacuolar system and a large nucleus. • Reproduction: Sexual reproduction is not well understood, but it is of the isogamous type. • Example: Euglena
CLASS 9: PHAEOPHYCEAE (BROWN ALGAE) • Habitat: Primarily found in marine environments • Pigments: Contains chl a, c, carotenes, xanthophylls, but lacks chl b • Reserve food: Mannitol, laminarin, and fats • Structure: Ranging from simple filamentous forms to bulky parenchymatous structures. Some species can grow to giant sizes with both external and internal differentiation. • Reproduction: Sexual reproduction varies from isogamous to oogamous. Motile gametes have two flagella attached laterally. Different species display various types of alternation of generations. • Example: Ectocarpus, Sargassum
CLASS 10: RHODOPHYCEAE (RED ALGAE) • Habitat: Some species are found in freshwater while others are marine • Pigments: Chromatophores contain red phycoerythrin, blue phycocyanin, Chl a, d, carotenes • Reserve food: Floridian starch • Structure: Ranges from simple filamentous forms to complex structures. No known motile structures • Reproduction: Sexual reproduction is advanced oogamous type. Male organs produce non-motile gametes, while female organs have a long receptive neck. Special spores (carpospores) are produced after sexual reproduction • Example: Batrachospermum, Polysiphonia
CLASS 11: MYXOPHYCEAE (CYANOPHYCEAE OR BLUE GREEN ALGAE) • Habitat: Found in both freshwater and marine environments • Pigments: Contains chlorophyll, carotenes, xanthophylls, phycocyanin, and phycoerythrin. The ratio of the last two pigments gives them a blue-green color • Reserve food: Sugars and glycogen • Structure: Ranges from simple cells to filamentous forms, some of which show false or true branching. They have a rudimentary nucleus and no proper chromatophores, with photosynthetic pigments spread throughout the cell periphery. No motile stages • Reproduction: They do not reproduce sexually • Example: Oscillatoria, Nostoc.
