Microorganisms have flourished on the earth for at least 3.8 Ga (Ga= 10⁹ years) of its 4.5 Ga existence. They formed the first biosphere and continue to carry out a multitude of essential geochemical activities that prepared the Earth for the eventual evolution of macroscopic life. They have a wide taxonomic distribution and comprise of protozoa, algae, fungi, bacteria and viruses. Microorganisms are well associated with all plants and animals and are also responsible for their well being. Due to their interaction with all plant and animals groups and their abundance (in terms of biomass) in virtually all habitats, there has been an upsurge in the interest of research on microorganisms in recent years. Because of theirpotentially high metabolic and growth activities, they play an important role in the microbial loop where it increases food chain efficiency through the utilization of both the very smallest size fractions of particulate organic materials (POM), as well as of dissolved organic matter (DOM) which is usually measured as dissolved organic carbon (DOC).