There are two basic types of algae, macroalgae and microalgae. The key difference between macroalgae and microalgae is that macroalgae are large and multicellular aquatic photosynthetic plant-like organisms, most people call it seaweed, while microalgae are small and unicellular aquatic photosynthetic plant-like organisms. It is estimated that there are about 800,000 species of algae, but only about 50,000 have been identified.