A variety of algae can be found from microscopic green algae (microalgae) to giant green algae over 30 meters in length (macroalgae). Blue algae, green algae, brown algae, and red algae are among the microalgae, including cyanobacteria (similar to bacteria). Microalgae (there are more types, but these are the most common) can be used to treat and purify industrial wastewater while providing nutrients.
The majority of microalgae grow through photosynthesis, converting sunlight, carbon dioxide, and certain nutrients, such as nitrogen and phosphorus, into biomass. Autotrophic growth is the result of this process. In addition, other algae can grow in the dark using sugar or starch (named heterotropy) or combine the two types (called mixotropy).
Algae are very diverse and play important roles in many ecosystems almost everywhere on earth, including the basis of the aquatic food chains that support all fisheries in the oceans and inland, and around 70 percent of all ecosystems produce plants that include various evolutionary lineages of mostly photoautotrophic organisms. Different groups of algae are of polyphyletic origin and embody most of the existing plant divisions. Algae are halophytic; their vegetative body is not organized in roots and leaf trunks as is the case with cormophytes. Algae live in single cells, colonies, filaments, or bodies of primitive vegetation and have no vascular system. In contrast to phanerogams (seed-producing plants), algae are cryptogams that spread through “hidden” or “hidden” reproductive strategies.
According to a concept of dividing living organisms into five kingdoms (Monera, Protoctista, Fungi, Animalia, and Plantae), prokaryotic algae (blue-green algae, cyanobacteria, cyanoprokaryota) are placed in Monera (Eubacteria) and eukaryotic algae in Protoctista. Therefore, algae do not belong to the realm of the Plantae. However, it is widely accepted (due to photosynthesis as a common trait) to interpret algae as "lower plants" as opposed to vascular "higher plants".