Algae are a diverse group of simple plant-like organisms ranging from unicellular to multicellular forms. They are typically photosynthetic and aquatic, lacking true roots, stems, leaves, and vascular tissue. Algae are classified into several divisions including green algae, red algae, brown algae, diatoms, dinoflagellates, and cyanobacteria (blue-green algae). Each division contains many distinct species that vary in habitat, structure, pigmentation and other characteristics. Algae play an important role as primary producers in many ecosystems.