Algae

Algae are defined as a group of predominantly aquatic, photosynthetic, and nucleus-bearing organisms (eukaryotic), in the kingdom Protista, that lack the true roots, stems, leaves—features they share with the avascular lower plants (e.g., mosses, liverworts, and hornworts), and specialized multicellular reproductive structures of plants. Their photosynthetic pigments are also more varied than those of plants, and their cells have features not found among plants and animals.

(We exclude the prokaryotic (nucleus-lacking) blue-green algae (cyanobacteria).)

Algae contain three types of double-membrane-bound organelles: the nucleus, the chloroplast, and the mitochondrion. In most algal cells, there is only a single nucleus, although some cells are multinucleate.

Algae range in size from picoplankton, which are between 0.2 to 2 micrometers (0.000008 to 0.000079 inches) in diameter, to giant kelp, which can be 60 metres (200 feet) long.

The algae are not closely related in an evolutionary sense, and the phylogeny of the group remains to be delineated. Specific groups of algae share features with protozoa and fungi that, without the presence of chloroplasts and photosynthesis as delimiting features, make them difficult to distinguish from those organisms. Indeed, some algae appear to have a closer evolutionary relationship with the protozoa or fungi than they do with other algae.

Some species of algae produce toxins that are lethal to fish, or render shellfish and finfish unsafe for consumption. The (taxonomically contentious) dinoflagellates are responsible for red tides, which not only release toxins into the water that may be lethal to aquatic life, but also wind-sprayed toxic cells that can cause health problems for air-breathing organisms.

The study of algae is called phycology, and a person who studies algae is a phycologist.