Thallus Organisation in Algae

• Algal thallus ranges from unicellular to multicellular form and size ranges from a few microns to some meters.
• The smallest unicellular algae is Micromonas pusilla which is 1 μm (0.00004 in.) in size and the giant kelps contain the longest thalli that reaches up to 60 m (200 ft) in length.
• The algal unicellular forms remain solitary as a single unit that is capable of completing their life cycle with the help of all physiological, biochemical, genetic requirements and may be motile or non-motile.
• The unicellular form constitutes a colonial structure when held together in a common gelatinous matrix, this is known as the intermediate stage of unicellular and multicellular structures.
• There are present various intermediate stages in thallus organization such as palmella, dendroid, palmelloid, coccoid, filamentous, siphonaceous, heterotrichous, uniaxial, multiaxial etc.

Types of thallus organization in Algae

The algal thallus organization can be classified in these following groups such as;

1. Unicellular

• These algae are also known as the acellular algae and they function as living organisms.
• Unicellular algae can be motile or non-motile.
• Unicellular algae are classified into three categories based on the presence or absence of flagella.

A. Motile form of Unicellular Algae

The motile form of unicellular algae can move from one place to another place. They can be either rhizopodial or flagellates.

• Rhizopodial: 
  • They are unicellular
  • They lack a rigid cell wall.
  • They contain a naked protoplast.
  • Their cell envelope is periplasmic and soft and permits extensive changes in the shape and size of the thallus.
  • Instead of flagella, they contain cytoplasmic projections known as Pseudopodia and Rhizopoda.
  • They follow the Ameboid movement for their locomotions.
  • Examples: Chrysamoeba (Chrysophyceae), Rhizochloris (Xanthophyceae), and Dinophyceae.
• Flagellates
  • They are unicellular.
  • These are looks like motile gametes and zoospores
  • They are spherical, elongate, ovoid or round in cross-section.
  • They contain one or two or many, equal or unequal, and tinsel or whiplash-type flagella.
  • Examples: Except Myxophyceae, Phaeophyceae, Rhodophyceae the Flagellated forms are present in almost all groups of algae.

B. Non Motile form of Unicellular Algae

 The Non Motile form of Unicellular Algae can not move from one place to another place. Non Motile form of Unicellular Algae are divided into two groups such as;

• Coccoid
  • They are unicellular.
  • Coccoid has a rigid cell wall and are non-flagellated
  • They lack motility but they can motile during the reproductive stages.
  • Most of them are belonging to algal classes and predominate in Xanthophyceae (70 %).
  • Example: Prochloron, Aphanocapsa, and Synechococcus
• Spiral
  • They are unicellular with spiral filament.
  • Example: Spirulina

2. Multicellular Forms

Different types of unicellular algae aggregate and formed multicellular algae. There are four types of multicellular forms of algae such as;

A. Colonial Aggregation

• When the unicellular form of algae loosely aggregates within a common mucilaginous investment they are called Colonial Aggregation.
• A cytoplasmic thread connects each cell within this aggregation.
• Colonial Aggregation divided into two classes such as –

(i). Coenohium

• The coenobium colony contains a defi­nite number of cells with a definite shape and arrangement.
• Coenohium colony is divided into two different groups such as;
  • Motile: They are cellular and flagellated and can move by an organised beating action of flagella. Example: Volvox, Pandorina, Eudorina etc.
  • Non-Motile: They lack flagella hence they are no-motile. Example: Scenedesmus, Hydro- dictyon.

(ii). Aggregated Forms

• Aggregated forms of algae are aggregated irregularly and show a colonial mass of various sizes and shapes.
• The Aggregated Forms of algae is divided into three groups such as;

(a). Palmelloid:

• In palmelloid all the non-motile cells remain embedded within an amorphous gelatinous or mucilaginous matrix.
• All the cells present in the palmelloid are independent and perform their functions individually.
• Example: Tetraspora and Palmodictyon (Chlorophyceae), Gleochloris and Chlorosaccus (Xanthophyceae), Phaeocystis (Chrysophyceae) and Microcystis (Cyanophy­ceae)

(b). Dendroid:

• In Dendroid the cells are variable in number, shape, and size.
• They resemble microscopic trees.
• They show polarity due to the presence of a mucilaginous thread at the base of each cell.
• Example: Prasinocladus, Ecballocystis, Chrysodendron

(c). Rhizopodial: 

• The cells are united by rhizopodia.
• Example: Chrysidias­trum

B. Filamentous Forms

When cells are divided repeatedly in a single plane and in a single direction they form a filamentous plant body. In this type, the cells are firmly attached to each other — end to end forming a chain or a thread-like structure.

Filamentous Form is two types such as;

(i). Unbranched Filament

• They are free-floating or attached to the substratum.
• In the case of Unbranched filaments which remain attached to the substratum are differentiated into base and apex whereas the free-floating one is not differentiated into basal and apical ends. 
• Example: free-floating (e.g., Spirogyra), attached to the substratum (e.g., Ulothrix, Oedogonium, etc.).

(ii). Branched Filament

• When a filament occasionally starts division in a second plane they form a Branched Filament.
• Branched Filament is two type such as;

(a). Falsely branched

• The trichome is generally fragmented due to the degeneration of an intercalary cell (or by the formation of biconcave separation discs) after which one or both of its ends adjacent to the dead cell grows out of the parent sheath, giving the resemblance of branching.
• Example: Scytonema

(b). Truly branched

• True branching occurs when repeated transverse divisions of the lateral outgrowths produced by a few or many scattered cells of the main filament.
• The truly branched thalli are of four types:
  • Simple Filament: In Simple Filament, the whole thallus is attached to the substratum with the help of a basal cell and the branches may emerge from any cell of the filament except the basal cell. Example: Cladophora.
  • Heterotrichous Habit: In Heterotrichous Habit, the whole thallus is divided into a well-developed prostrate and erect system. Example: Fritschiella, Ectocarpus, Draparnaldiopsis, Stigoclonium.
  • Parenchymatous forms: This occurs when cells of the primary filament divide in all directions, any essentially filamentous structure is thus lost early. Example: Porphyra , Ulva , Enteromorpha.
  • Pseudoparenchymatous Habit: The Pseudoparenchymatous is formed when one or more central or axial filaments get together with their branch fuses and develop a parenchymatous structure. Example: Batrachospermum, Polysiphonia.

C. Siphonocladous Organization

• This is restricted to members of Chlorophyceae.
• In this type the unbranched ( Urospora, Chaetomorpha ) or branched ( Acrosiphonia, Cladophora ) filaments are composed of multinucleate (semi – coenocytic) cells.
• Valonia is an example of Siphonocladous which is a spherical vesicle up to 10 cm in diameter and has been described as the largest plant cell.

D. Siphonous Organization

• Few marine Chlorophyceae (order Siphonales, Bryopsidales,Dasycladales) and some Xanthophyceae ( Botrydium , Vaucheria ) enlarged and elaborate their thallus in absence of septa. The Nuclear divisions do not follow the cytokinesis (free nuclear division) and which results in a coenocytic, multinucleate thallus and a saphenous organization.
• They range from Saccate (e.g. Botrydium ) to uniaxial (e.g. Vaucheria , Bryopsis ) and multiaxial (e.g. Codium ) forms.

Further Reading

• What is bacteria? Types, Structure, Shapes, Morphology, Nutrition, Growth, Habitat, Reproduction, Examples.
• Prions: Definition, Structure, Function, Disease, Prevention, Control, Treatment, Transmission.
• Viroids Definition, Infection, Structure, Examples, Diagnostic, Origin, Disease.
• What is Algae? Characteristics and Occurrence of Algae.