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noun, singular: alga

A group of photosynthetic organisms that in general possess pigments such as chlorophyll but lack true roots, stems and leaves characteristic of terrestrial plants

algal (adjective)

Of, pertaining to, characteristic of, or relating to an alga


In general, algae are a group of photosynthetic eukaryotes (except for the prokaryotic blue-green algae). They lack true roots, stems, and leaves. The absence of true roots, stems, and leaves taxonomically separates them from higher plants. They have photosynthetic pigments though and are one of the distinctive features used to classify them into groups such as green algae, blue-green algae, yellow algae, red algae, brown algae, and golden algae. Most of them are eukaryotes and occur in different forms. Some of them are unicellular whereas others are multicellular. They may also form colonies. Most algae are aquatic. Others are terrestrial and may be found on moist soil, trees and rocks. Some algal species form symbiosis with other organisms. For example, a lichen is a symbiotic association between fungi and green (and occasionally blue green) algae.

In five kingdom scheme of classification, the algae, together with the protozoa, belong to Kingdom Protista. They are distinct from the protozoa by being photosynthetic. The algae are further grouped into various phyla: Euglenophyta (euglenids), Chrysophyta (diatoms), Pyrrophyta (dinoflagellates), Chlorophyta (green algae), Phaeophyta (brown algae), and Rhodophyta (red algae). The Cyanophyta or blue-green algae, which are prokaryotic organisms, are traditionally included in this group but in modern classification, they are now grouped together with bacteria under Kingdom Monera.

It should be noted, however, that the taxonomic classification of organisms is bound to change as further studies of the species would lead to newer system of classification, such as that in The NCBI taxonomy database.1

Word origin: Latin alga, from Proto-Indo-European *alg-, *alǵ- ‎(“to be dirty, duckweed”)

See also:

1 The NCBI taxonomy database. Retrieved from [1].