Microalgae are microscopic algae, typically found in freshwater and marine systems. They are unicellular species which exist individually, or in chains or groups. Depending on the species, their sizes can range from a few micrometers (µm) to a few hundreds of micrometers. Unlike higher plants, microalgae do not have roots, stems and leaves. Microalgae, capable of performing photosynthesis, are important for life on earth; they produce approximately half of the atmospheric oxygen. The biodiversity of microalgae is enormous. It has been estimated that about 200,000-800,000 species exist of which about 50,000 species are described. Most of these microalgae species produce unique products like carotenoids, antioxidants, fattyacids, enzymes, polymers, peptides, toxins and sterols. Some genera of microalgae :