Chlorella

The name “Chlorella” originates from the Greek word ‘chlōros’, meaning ‘green’, combined with the Latin suffix ‘ella’, which means ‘small’.

Chlorella is a unicellular green algae. They are generally found in freshwater.

Kingdom: Plantae

• Chlorella belongs to the kingdom Plantae, which includes all plants. This classification indicates that Chlorella is a photosynthetic organism, using chlorophyll to capture energy from sunlight, much like higher plants. Photosynthesis in Chlorella is a fundamental process that allows it to convert light energy into chemical energy, essential for its growth and survival.

Division : Chlorophyta

• Within the plant kingdom, Chlorella falls under the division Chlorophyta. This phylum consists of green algae, which are characterized by their green coloration due to the presence of chlorophyll, similar to terrestrial plants.

Class: chlorophyceae

• Chlorella is a member of the class chlorophyceae, a group of green algae that includes a wide range of unicellular and colonial forms. Photosynthesis in Chlorella plays a vital role in Trebouxiophyceae, enabling these organisms to harness energy from sunlight for their metabolic processes.

Order: Chlorococcales

• The order Chlorellales contains various green algae, including Chlorella. It further refines the classification to group related species based on specific characteristics and evolutionary relationships.

Family: Chlorellaceae

• Chlorellaceae is the family to which Chlorella belongs. This level of classification groups Chlorella with other genera that share common traits and evolutionary histories.

Genus: Chlorella

• Chlorella is the genus, which is the most specific level of classification before getting to the species. Chlorella is the name of the genus that encompasses various species of single-celled green algae. These species rely on photosynthesis to produce their own food and energy.

1.0Structure of Chlorella

• Size- The small cell (2 to 10 µm)
• Shape- Round or oval-shaped. 
• Cell Wall: Chlorella cells are enclosed by a rigid cell wall, which is composed of a complex mixture of polysaccharides, including cellulose and other compounds. The cell wall of Chlorella can vary among species and can be relatively thick compared to other unicellular algae.
• Cytoplasm: It contains various organelles and all the necessary components for the cell to function, including ribosomes for protein synthesis and enzymes for metabolic processes.
• Nucleus: Chlorella, like eukaryotic cells, contains a well-defined nucleus that houses the genetic material (DNA) and controls the cell's activities.
• Chloroplast: Chlorella contains a single large cup-shaped chloroplast, which contains the photosynthetic pigments (Chlorophyll a & Chlorophyll b) required for photosynthesis. These chloroplasts are responsible for the green coloration of the cells and are the sites where photosynthesis takes place.
• Pyrenoid: Some species of Chlorella have a pyrenoid, a specialized region within the chloroplast that plays a role in the storage of starch.
• Flagella: No flagellation so nonmotile.

2.0Photosynthesis (Carbon fixation in Chlorella)

• Chlorella was used by Melvin Calvin to study the light-independent ("dark") photosynthetic reactions. The chemical processes of photosynthesis occurring independent of light is called dark reaction. It takes place in the stroma of chloroplast. The dark reaction is purely enzymatic and it is slower than the light reaction. 
• The dark reactions occur also in the presence of light. In dark reaction, the sugars are synthesized from CO₂. The energy poor CO₂ is fixed to energy rich carbohydrates using the energy rich compound, ATP and the assimilatory power, NADPH₂ of light reaction. The process is called carbon fixation or carbon assimilation. 
• Calvin cycle or C₃ cycle It is a cyclic reaction occurring in the dark phase of photosynthesis. In this reaction, CO₂ is converted into sugars and hence it is a process of carbon fixation. 
• The Calvin cycle was first observed by Melvin Calvin in Chlorella, unicellular green algae. Calvin was awarded Nobel Prize for this work in 1961. Since the first stable compound in Calvin cycle is a 3 carbon compound (3 phosphoglyceric acid or 3phosphoslycerate), the cycle is also called as C₃ cycle. 

The reactions of Calvin’s cycle occur in three phases. 

(i) Carboxylation phase 

(ii) Reduction phase 

(iii) Regeneration phase

3.0Reproduction in chlorella

The sole method of reproduction in species of chlorella is asexual reproduction, however, there is no sexual reproduction reported in chlorella. 

When condition is favorable the protoplast of chlorella divides into 2, 4, 8 daughter protoplasts, these protoplasts round off to form non-motile spores. These spores have the shape as the parent cell due to this they are also known as autospore. These autospore release and grow to become a cell.  

The life cycle of Chlorella involves a simple pattern of asexual reproduction. Here's an overview of the typical life cycle of Chlorella:

• Growth phase : during this phase the cell grow in size by using photosynthetic products
• Ripening phase : this phase is characterized by maturation of cells and preparing itself to divide. 
• Post ripening phase : in this phase each mature cell divides twice. 
• Division phase : in this phase parent cell rupture and new daughter cells released. 

4.0Importance of Chlorella

• Warburg (1919) used Chlorella in the experiments on photosynthesis.
• In 1961 Melvin Calvin studied the pathway of carbon dioxide assimilation in plants using Chlorella. 
• An antibiotic chlorellin is also obtained from chlorella.
• Chlorella contains high amount of protein, fats and contain essential vitamins (carotene, riboflavin, vit-B₁₂ etc.)  so it is also taken as food supplements by space travelers.