Algae (Latin: Algae, Greek: Phykos) – a morphological and ecological group traditionally composed of several unrelated evolutionary lineages of thalloid organisms, meaning those without true tissues.
The scientific study of algae is called algology (or phycology).
What connects the various phyla within this group is, in most cases, their autotrophic nature and their role as primary producers of organic matter in aquatic environments—or in areas with limited or temporary water presence, such as moist soil or tree bark—as well as their primitive, non-tissue body structure.
Due to the fundamental distinction between prokaryotes and eukaryotes, some sources define true algae as only the eukaryotic organisms, referring to the entire group more broadly as “algae and cyanobacteria.”
Estimates of the number of algal species range from 30,000 to over 1,000,000, with some outliers suggesting up to 350 million. According to AlgaeBase, as of mid-2012, around 32,500 species had been verified, and approximately 100,000 names were registered, either as taxonomic synonyms or with uncertain status. The database’s creator estimates the total number of algal species to be about 72,500, of which around 44,000 had been described by 2012.
According to estimates, the most numerous modern photoautotrophic organism on Earth is the cyanobacterium Prochlorococcus marinus, with a population of about 10²⁷ individuals. Populations of coccolithophores may be similarly vast.