Chlorophyta (Green Algae)

CHLOROPHYTA

  • G.M. Smith proposed a new classification (in 1955) with certain modifications, supporting the classification proposed by Pascher.
  • He divided algae into seven divisions and further into classes.
  • The first division of Smith’s classification is Chlorophyta.
  • Chlorophyta comprises most of the described species of green algae.
  • Chlorophyceae (eg: Volvox) and Charophyceae (eg: Chara) are the two classes under the division Chlorophyta.
  • Below are the general characteristics possessed by the classes of Chlorophyta.

OCCURRENCE

  • They are mostly aquatic and predominantly freshwater algae.
  • 10% of the aquatic green algae are marine.
  • Some are terrestrial.
  • Many species of Chlamydomonas, Volvox, and Chlorella are frequently found in freshwaters.
  • The terrestrial forms grow on moist aerated soils, rocks, and cliffs.
  • Hormidium and Chlorella are found on loam cultivated soils.
  • Subaerial forms occur on moist tree trunks, moist walls, and rocks.
  • Ulothrix and Zygogonium are sub-aerial.
  • Some are found as planktonic organisms in marine environments.
  • They occur in dense populations causing blooms in tidal pools or bays.
  • Examples of marine green algae are Ulva (sea lettuce), Codium, Enteromorpha, Cladophora.

CHLOROPHYTA IN SMITH'S CLASSIFICATION

RANGE OF THALLUS ORGANISATION

  • Green algae are a heterogenous
  • They exhibit a wide range in their thallus structure from simple microscopic motile unicellular forms through multicellular flagellated or non-flagellated colonies.
  • Unicellular forms may be motile or non-motile.
  • In unicellular motile thallus form, vegetative cells have two or four flagella and are motile. Eg: Chlamydomonas, Sphaerella.
  • Unicellular non-motile thallus does not possess flagella, meant for locomotion. Eg: Chlorella.
  • The colonial habit is achieved by aggregation of the products of cell division within a mucilaginous mass. They possess three different types of colonial forms.
    • Coenobium: A colony with a definite shape, size and arrangement of cells. Maybe motile with flagella (Eudorina, Volvox) or non-motile without flagella (Pediastrum, Hydrodictyon)
    • Palmelloid: The number of cells, their shape and size are not definite. Eg: Chlamydomona, Tetraspora.
    • Dendroid: The colony appears like a microscopic tree. The number, shape and size of cells are indefinite. Eg: Ecballocystis.

CHLOROPHYTA IN SMITH'S CLASSIFICATION

  • Filamentous forms are of seven types:
    • Filamentous unbranched – Ulothrix, Oedogonium
    • Filamentous branched – Cladophora
    • Heterotrichous (The thallus is very much evolved and differentiated into prostrate and erect systems) – Fritschiella, Coleochaete
    • Siphonaceous (The thallus is made up of branched, aseptate, coenocytic, tubular filaments as the nuclear divisions are not accompanied by wall formation) – Protosiphon, Codium
    • Parenchymatous (Thallus is formed by the divisions of cells) – Ulva, Enteromorpha
    • Complex forms (The thallus is highly developed with well-differentiated tissues) – Chara

CHLOROPHYTA IN SMITH'S CLASSIFICATION

CELLULAR STRUCTURE

  • Green algae are eukaryotic
  • Have a well-organized nucleus
  • The cell wall is made up of non-living material i.e.,
  • The flagella have a precise number and orientation of microtubules.
  • Flagella are of equal length and are inserted at the anterior or apical end of the motile cells.
  • The flagella are of whiplash-type.
  • The thylakoids of green algae are much larger.
  • The thylakoids contain all the chlorophyll pigment of the chloroplast.

PIGMENTS AND RESERVE FOOD

  • Chlorophyll a, chlorophyll b, and carotenoids have been described from green algae.
  • Both carotenes and xanthophylls are present in green algae.
  • Green algae store starch as their reserve food in the plastids.

REPRODUCTION

  • There are three common ways of reproduction in green algae: vegetative, asexual and sexual.
  • In the vegetative mode of reproduction, the algal body cuts off or break and gives rise to new individuals. The process is known as fragmentation.

CHLOROPHYTA IN SMITH'S CLASSIFICATION

  • Vegetative reproduction also occurs by the formation of akinetes which are specially thickened vegetative cells.
  • In some filamentous forms, fragmentation is common.
  • Asexual reproduction takes place by zoospores. Common in Ulothrix, Chlorococcum.
  • Sexual reproduction in green algae takes place by fusion of gametes or by the exchange of genetic information through conjugation.
  • Sexuality in green algae is largely controlled by environmental factors such as light, temperature and nutrition.
  • Sexual reproduction can be seen in Chlamydomonas and Pandorina.

LIFECYCLE

  • Green algae have a haplontic life cycle.
  • All phases except zygote are haploid.

Zygotic meiosis

 

REFERENCES

  1. https://microbewiki.kenyon.edu/index.php/Chlorophyta#:~:text=Chlorophyta%20are%20photosynthetic%20organisms%2C%20obtaining,isogamous%2C%20anisogamous%2C%20or%20oogamous.
  2. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/chlorophyta