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Green and brown algae are eukaryotic multicellular organisms belonging to different kingdoms. Brown algae, such as kelp and Sargassum, are mostly marine and use fucoxanthin to absorb sunlight. Green algae, the most primitive group of Plantae, are more common in freshwater. Both are important producers in aquatic ecosystems and provide food for many fish.
Green and brown algae are two groups that together make up the majority of algae in the world, although they are quite different. Along with red algae, both brown and green varieties are sometimes referred to using the colloquial term “algae”. Although both are eukaryotic (complex-celled) multicellular organisms, they belong to different kingdoms, with green algae belonging to Plantae and brown algae to Chromalveolata. Plantae and Chromalveolata are two of the six major eukaryotic divisions, the others being Fungi, Animalia, Amoebozoa, Rhizaria and Excavata. Both groups are mostly marine, but green is better suited to freshwater than brown.
Brown algae are more familiar to people as kelp, a seaweed with a very high growth rate, and Sargassum, a surface-floating variety found in the Sargasso Sea that provides a unique habitat for eels and other animals. Although seaweed and Sargassum are the best known varieties, there are over 1,500 species in total and they are especially common in the colder Northern Hemisphere. Brown algae are often found along rocky shores. Along with their mostly unicellular relatives in the phylum Heterokontophyta, they are autotrophs (photosynthesizing organisms) with chloroplasts covered by four membranes. This seaweed uses a pigment called fucoxanthin to absorb sunlight, giving it a brownish-green color. The cells within it often have holes used to share nutrients and free carbon.
Green algae are a little more common than brown algae from a human perspective, as they grow more frequently in and near lakes and rivers, which people tend to see more often than in the open ocean. It is famous for being the most primitive group of the kingdom Plantae and the life form from which land plants (embryophytes) evolved about 500 million years ago, during the Ordovician period. About 6,000 species of green algae are known, most of them unicellular, although the most visible species live in colonies structured in long chains or filaments. Only in the order Charales—stoneworts, a type of pond weed most closely related to land plants—does true tissue differentiation occur.
Both types of algae are extremely important as producers in aquatic ecosystems, and the diet of many fish, especially juveniles, consists mainly or exclusively of them. Some fish are also particularly good at cleaning algae from other fish. Together with corals, kelp forests create one of the richest and most complex aquatic ecosystems on the planet, home to tens or thousands of marine species.
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