The red algae (Rhodophyta, IPA: /ˌroʊdəˈfaɪtə, roʊˈdɒfɨtə/, from Greek: ῥόδον (rhodon) = rose + φυτόν (phyton) = plant, thus red plant) are a large group, about 5000–6000 species  of mostly multicellular, marine algae, including many notable seaweeds. Other references indicate 10,000 species.  Most of the coralline algae, which secrete calcium carbonate and play a major role in building coral reefs, belong here. Red algae such as dulse (Palmaria palmata) and nori are a traditional part of European and Asian cuisine and are used to make other products like agar, carrageenans and other food additives. 
The oldest fossil identified as a red alga is also the oldest fossil eukaryote that belongs to a specific modern taxon. Bangiomorpha pubescens, a multicellular fossil from arctic Canada, strongly resembles the modern red alga Bangia despite occurring in rocks dating to 1200 million years ago. 
Red algae are important builders of limestone reefs. The earliest such coralline algae, the solenopores, are known from the Cambrian Period. Other algae of different origins filled a similar role in the late Paleozoic, and in more recent reefs.
The diverse eukaryotes that constitute the red algae have been the focus of numerous recent molecular surveys and remain a rich source of undescribed and little known species for the traditional taxonomist. Molecular studies place the red algae in the Archaeplastida (Plantae sensu lato); however, supraordinal classification has been largely confined to debate on subclass vs. class level status for the two recognized subgroups, one of which is widely acknowledged as paraphyletic. This narrow focus has generally masked the extent to which red algal classification needs modification.
Below are two valid published taxonomies of the red algae, although neither necessarily has to be used, as the taxonomy of the algae is still in a state of flux. Note also that there is continued scientific debate as to whether the Rhodophyta should be included in the Kingdom Protista or the Kingdom Plantae. These two classification systems, which place the red algae in the plant kingdom, are shown in the table below.
|Classification system according to
Hwan Su Yoon et al. 2006
|Classification system according to|
Saunders and Hommersand 2004
|Kingdom Plantae Haeckel
||Kingdom Plantae Haeckel
There are around 6,500 to 10,000 known species,  nearly all of which are marine, with about 200 that only live in fresh water. However estimates of the number of real species vary by 100%.  Some examples are:
- Atractophora hypnoides
- Gelidiella calcicola
- Palmaria palmata
- Schmitzia hiscockiana
- Chondrus crispus
- Mastocarpus stellatus
Pit connections and pit plugs
Pit connections and pit plugs are unique and distinctive features of red algae that form during the process of cytokinesis following mitosis. In red algae, cytokinesis is incomplete. Typically, a small pore is left in the middle of the newly formed partition. The pit connection is formed where the daughter cells remain in contact.
Shortly after the pit connection is formed cytoplasmic continuity is blocked by the generation of a pit plug, which is deposited in the wall gap that connects the cells.
Connections between cells having a common parent cell are called a primary pit connections. Because apical growth is the norm in red algae, most cells have two primary pit connections, one to each adjacent cell.
Connections that exist between cells not sharing a common parent cells are labeled secondary pit connections. These connections are formed when an unequal cell division produced a nucleated daughter cell that then fuses to an adjacent cell. Patterns of secondary pit connections can be seen in the order Ceramiales.
After a pit connection is formed, tubular membranes appear. A granular protein, called the plug core, then forms around the membranes. The tubular membranes eventually disappear. While some orders of red algae simply have a plug core, others have an associated membrane at each side of the protein mass, called cap membranes. The pit plug continues to exist between the cells until one of the cells dies. When this happens, the living cell produce a layer of wall material that seals off the plug.
It is thought that the pit connections function as structural reinforcement, and as an avenue for cell to cell communication and/or symplastic transport in red algae. While the presence of the cap membrane could inhibit this transport between cells, it has been hypothesized that the tubular plug cores serve as a means of transport.
- Brown algae
- Green algae
- Red tide (red tides are caused by algae from the phylum Dinoflagellata, and not red algae [Rhodophyta])
- Françoise Ardré (namesake of the red alga known as Pterosiphonia ardreana)
- D. Thomas (2002). Seaweeds. Life Series. Natural History Museum, London. ISBN 0-565-09175-1.
- W. J. Woelkerling (1990). "An introduction". In K. M. Cole & R. G. Sheath. Biology of the Red Algae. Cambridge University Press, Cambridge. pp. 1–6. ISBN 0-521-34301-1.
- M. D. Guiry. "Rhodophyta: red algae". Retrieved 2007-06-28. Unknown parameter
- N. J. Butterfield (2000). "Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes". Paleobiology. 26 (3): 386–404. Unknown parameter
- Hwan Su Yoon, K. M. Müller, R. G. Sheath, F. D. Ott & D. Bhattacharya (2006). "Defining the major lineages of red algae (Rhodophyta)" (PDF). Journal of Phycology. 42: 482–492. doi:10.1111/j.1529-8817.2006.00210.x. Unknown parameter
- G. W. Saunders & M. H. Hommersand (2004). "Assessing red algal supraordinal diversity and taxonomy in the context of contemporary systematic data". American Journal of Botany. 91: 1494–1507. Unknown parameter
- "Dulse: Palmaria palmata". Quality Sea Veg. Retrieved 2007-06-28.
- T. F. Mumford & A. Muira (1988). "Porphyra as food: cultivation and economics". In C. A. Lembi & J. Waaland. Algae and Human Affairs. Cambridge University Press, Cambridge. ISBN 0-521-32115-8.
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