Red algae is a large group of algae found in the phylum Rhodophyta. The name Rhodophyta is derived from the Greek words for red and plant: ῥόδον (rhodon) and φυτόν (phyton). The exact number of described species varies from roughly 6,000 to 10,000 depending on which classification you prefer to adhere to.
The red algae belong to the Archaeplastida (also known as Primoplantae), a major line of eukaryotes that also contains glaucophytes, green algae and all the land plants. Just like all the other members of this line, red algae are equipped with plastids surrounded by two membranes.
A majority of the known red algae species are multicellular and live in the ocean. You can for instance find a lot of well known sea weeds in this group. A lot of the coralline algae, famous for secreting calcium carbonate and therefore being of imperative importance for the building of coral reefs, are also a part of the red algae group. Only about 200 species of red algae occur in freshwater.
Red algae are red because they contain pigments called phycoerythrins which absorbs blue light and reflects red light. This makes it possible for red algae to carry out photosynthesis in comparatively deep waters, because blue light penetrates deeper than lights of longer wavelengths, such as red light. Some red algae only contain a very small amount of phycoerythrins and such species can look green or bluish rather than red.
Certain types of red algae play an important role when tropical reefs are formed. In some atolls of the Pacific Ocean, red algae have actually contributed more to the reef structure than the corals. Reed building red algae are known as coralline algae and will form a protective shell consisting of carbonate around them selves. This makes them somewhat similar to corals, but they are not closely related.
Since coralline red algae forms this type of carbonate shell, a comparatively large number of fossilized red algae have been found. Coralline algae skeletons make up a significant part of limestone deposits of reef origin. The first definite example of calcareous algae is from the late Cambrian Period. (The Cambrian period took place 542- 488 million years ago.) Unicellular rhodophytes might have occurred in the Precambrian, but no Precambrian fossils with preserved pigments have been found and it is therefore impossible to know if the findings are fossilized unicellular red algae or not. What we do know is that multicellular rhodophytes existed in the late Precambrian.
Red algae are an appreciated source of food in several different parts of the world. Red algae are for instance used to make agar agar and carrageenans. In Japan, red algae has been cultivated for at least three centuries to make nori and the species Porphyra yezoensis andPorphyra tenera are especially popular.
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