Because they are the essential micronutrients for the different metabolites of living organisms?
 
        
             
        
        
        
Photsynthesis to make food
        
             
        
        
        
Cyanobacteria, also known as blue-green algae, are found in vast quantities in fresh and salt water. Cyanobacteria are able to conduct photosynthesis. By utilising energy from the sun, they produce carbohydrates from water and carbon dioxide. As a byproduct, they produce oxygen. So cyanobacteria provided oxygen to the atmosphere that allowed other lifeforms to develop.