Theory:

Classification of algae:
Based on the various pigments found in them, algae are classified into different classes as follows:
 
Screenshot_2.png
Example:
Blue Green algae:
  
blue green algae.jpg
Ocillatoria, Nostoc
 
Green algae:
  
green algae.jpg
Chara, Chlamydomonas and Spirogyra
 
Brown algae:
  
brown algae.jpg
Sargassum, Laminaria
 
Red algae:
  
red algae.jpg
Polysiphonia, Gelidium
Economic importance of algae:
Algae as food:
  • Algae such as Chlorella, Spirulina and Ulva are used as food supplements by humans.
  • These are mostly consumed as food by the people in Japan, England and India.
  • Algae such as Laminaria, Ascophyllum are used as food by domestic animals.
  • In food industries, algae such as the Gelidium and Gracilaria are used in preparing ice creams and jellies.
Algae in agriculture:
Some of them, such as blue-green algae, are organisms that fix the atmospheric nitrogen into the soil. This fixation of atmospheric nitrogen increases the fertility of the soil.
  
Agar-Agar:
Red algae such as Gelidiumand Gracillariaare used to extract Agar-Agar which is further used to prepare growth medium in laboratories and make ice creams and jellies in food industries.
  
Iodine:
It is obtained from brown algae like Laminaria.
  
Space travel:
Chlorella pyrenoidosa is used in space travel to get rid of the carbon dioxide and decompose the human waste.
  
Single Cell Protein (SCP):
Single-cell algae such as blue-green algae are used in the production of protein.
Example: Chlorella and Spirulina.
Reference:
https://upload.wikimedia.org/wikipedia/commons/f/fc/Oscillatoria_filaments.jpg
https://upload.wikimedia.org/wikipedia/commons/e/ec/Nostoc_commune_kp.jpeg
https://upload.wikimedia.org/wikipedia/commons/5/5b/CharaV2.jpg
https://upload.wikimedia.org/wikipedia/commons/d/d3/201410_chlamydomonas.png
https://upload.wikimedia.org/wikipedia/commons/8/85/The_freshwater_alga_Spirogyra.jpg
https://upload.wikimedia.org/wikipedia/commons/3/39/Sargassum_muticum_Yendo_Fensholt_1955_Lamiot_WimmereuxHautsDeFrance_Estran_Juillet_2016a4.jpg
https://upload.wikimedia.org/wikipedia/commons/f/f8/Laminaria_saccharina_IMG_1029_listafyr.JPG
https://upload.wikimedia.org/wikipedia/commons/e/eb/Gelidium_sp.jpg
https://upload.wikimedia.org/wikipedia/commons/1/13/Polysiphonia-elongata-19880602a.jpg