noun, singular: alga
A group of aquatic, photosynthetic, eukaryotic organisms ranging from unicellular to multicellular forms, and generally possess chlorophyll but lack true roots, stems and leaves characteristic of terrestrial plants.
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 (dinoflaggelates), 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.