Algae | Definition, Characteristics, Classification, Examples, & Facts

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• Introduction & Top Questions
• Physical and ecological features of algae
  • Size range and diversity of structure
  • Distribution and abundance
  • Ecological and commercial importance
  • Toxicity
• Form and function of algae
  • The algal cell
  • Flagella
  • Mitosis
  • Cellular respiration
  • Photosynthesis and light-absorbing pigments
  • The effects of water on light absorption
  • Nutrient storage
  • Alternative methods of nutrient absorption
  • Reproduction and life histories
• Evolution and paleontology of algae
• Classification of algae
  • Diagnostic features
  • Annotated classification

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• Are algae toxic?
• Why are algae important?

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External Websites

• Smithsonian National Museum of Natural History - Algae
• Frontiers - Algae as New Kids in the Beneficial Plant Microbiome
• Pennstate College of Earth and Mineral Sciences - What are Algae?
• National Center for Biotechnology Information - PubMed Central - Algae as Nutritional and Functional Food Sources
• IndiaNetzone - Algae
• Lenntech - Algae description and types
• OpenStax - Microbiology - Algae
• LiveScience - What Are Algae?
• Nature - Scientific Reports - Algae

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• algae - Children's Encyclopedia (Ages 8-11)
• algae - Student Encyclopedia (Ages 11 and up)

Quick Summary Ask the Chatbot a Question Also known as: Phycophyta, alga, algas Written by Robert A. Andersen Director, Provasoli-Guillard National Center for Culture of Marine Phytoplankton, Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, Maine. Coeditor of Chrysophytes: Aspects and Problems. Robert A. AndersenAll Fact-checked by Britannica Editors Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... Britannica Editors Last updated Dec. 10, 2025 •History Table of Contents Table of Contents Quick Summary Ask the Chatbot a Question Top Questions

What are algae?

Algae are defined as a group of predominantly aquatic, photosynthetic, and nucleus-bearing organisms that lack the true roots, stems, leaves, 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.

What organelles do algae contain?

Algae are eukaryotic organisms and 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.

Are algae toxic?

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.

What size are algae?

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.

Why are algae important?

Algae produce up to half of the oxygen in Earth’s atmosphere, and algae help keep carbon dioxide out of the atmosphere by storing it. Algae are also the food base for almost all aquatic life, and are economically important as a source of crude oil and as sources of food and a number of pharmaceutical and industrial products for humans.

algae, members of a group of predominantly aquatic photosynthetic organisms of the kingdom Protista. Algae have many types of life cycles, and they range in size from microscopic Micromonas species to giant kelps that reach 60 metres (200 feet) in length. Their photosynthetic pigments are more varied than those of plants, and their cells have features not found among plants and animals. In addition to their ecological roles as oxygen producers and as the food base for almost all aquatic life, algae are economically important as a source of crude oil and as sources of food and a number of pharmaceutical and industrial products for humans. The taxonomy of algae is contentious and subject to rapid change as new molecular information is discovered. The study of algae is called phycology, and a person who studies algae is a phycologist.

In this article the algae are defined as eukaryotic (nucleus-bearing) organisms that photosynthesize but lack the specialized multicellular reproductive structures of plants, which always contain fertile gamete-producing cells surrounded by sterile cells. Algae also lack true roots, stems, and leaves—features they share with the avascular lower plants (e.g., mosses, liverworts, and hornworts). Additionally, the algae as treated in this article exclude the prokaryotic (nucleus-lacking) blue-green algae (cyanobacteria).

Beginning in the 1830s, algae were classified into major groups based on colour—e.g., red, brown, and green. The colours are a reflection of different chloroplast pigments, such as chlorophylls, carotenoids, and phycobiliproteins. Many more than three groups of pigments are recognized, and each class of algae shares a common set of pigment types distinct from those of all other groups.

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.

Singular: alga (Show more) Key People: Ferdinand Cohn Sylvia Earle Sir William Jackson Hooker Nathanael Pringsheim Per Teodor Cleve (Show more) Related Topics: Chromophyta Euglenophyta hymenial algae Botryococcus Tasmanites (Show more) On the Web: National Center for Biotechnology Information - PubMed Central - Algae as Nutritional and Functional Food Sources (Dec. 10, 2025) (Show more) See all related content

This article discusses the algae in terms of their morphology, ecology, and evolutionary features. For a discussion of the related protists, see the articles protozoan and protist. For a more complete discussion of photosynthesis, see the articles photosynthesis and plant.

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