Temporal range: Mesoproterozoic–present
Classification is currently disputed. See Taxonomy.
The red algae, or Rhodophyta, pronounced /ro.'dɔ.fɪ.tə/ ("roh-DOF-fit-tuh") or /ro.də.'fai.tə/ ("roh-duh-FYE-tuh"), from the Ancient Greek ῥόδον (rhodon, rose) and φυτόν (phyton, plant), are one of the oldest groups of eukaryotic algae, and also one of the largest, with about 5,000–6,000 species  of mostly multicellular, marine algae, including many notable seaweeds. Other references indicate as many as 10,000 species; more detailed counts indicate about 4,000 in about 600 genera (3,738 marine species in 546 genera and 10 orders (plus the unclassifiable); 164 freshwater species in 30 genera in eight orders).
The red algae form a distinct group characterized by these attributes: eukaryotic cells without flagella and centrioles, using floridean polysaccharides as food reserves, with phycobiliproteins as accessory pigments (giving them their red color), and with chloroplasts lacking external endoplasmic reticulum and containing unstacked thylakoids.  Most red algae are also multicellular, macroscopic, marine, and have sexual reproduction. They often have alternation of generations and may have three generations rather than two.
Many 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 laver (nori/gim) are a traditional part of European and Asian cuisines and are used to make other products such as agar, carrageenans and other food additives. 
Most rhodophytes are marine, although freshwater species are found; these generally prefer clean, running water, but with some exceptions.
Fossil record 
One of the oldest fossils 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.
Calcite crusts, which have been interpreted as the remains of coralline red algae, date to the terminal Proterozoic. Thallophytes resembling coralline red algae are known from the late Proterozoic Doushantuo formation.
In the system of Adl et al. 2005, the red algae are classified in the Archaeplastida, along with the glaucophytes and green algae plus land plants (Viridiplantae or Chloroplastida). The authors use a hierarchical arrangement where the clade names do not signify rank; the class name Rhodophyceae is used for the red algae. No subdivisions are given; the authors say, "Traditional subgroups are artiﬁcial constructs, and no longer valid." 
The system reflected the consensus in 2005. Many studies published since then have provided evidence which is in agreement. However, other studies have suggested Archaeplastida is paraphyletic. As of January 2011[update], the situation appears unresolved.
Below are other published taxonomies of the red algae, although none necessarily has to be used, as the taxonomy of the algae is still in a state of flux (with classification above the level of order having received little scientific attention for most of the 20th century).
- If one defines the kingdom Plantae to mean the Archaeplastida, the red algae will be part of that kingdom
- If Plantae are defined more narrowly, to be the Viridiplantae, then the red algae might be considered their own kingdom, or part of the kingdom Protista.
A major research initiative to reconstruct the Red Algal Tree of Life (RedToL) using phylogenetic and genomic approaches is funded by the National Science Foundation as part of the Assembling the Tree of Life Program.
Classification comparison 
Some sources (such as Lee) place all red algae into the class "Rhodophyceae". (Lee's organization is not a comprehensive classification, but a selection of orders considered common or important.)
Species of red algae 
Some examples of species and genera of red algae are:
- Cyanidioschyzon merolae, a primitive red algae
- Atractophora hypnoides
- Gelidiella calcicola
- Lemanea, a freshwater genus
- Palmaria palmata, dulse
- Schmitzia hiscockiana
- Chondrus crispus, Irish moss
- Mastocarpus stellatus
- Vanvoorstia bennettiana, became extinct in the early 20th century
- Acrochaetium efflorescens
|Algal group||δ13C range|
|HCO3-using red algae||−22.5‰ to −9.6‰|
|CO2-using red algae||−34.5‰ to −29.9‰|
|Brown algae||−20.8‰ to −10.5‰|
|Green algae||−20.3‰ to −8.8‰|
The δ13C values of red algae reflect their lifestyles. The largest difference results from their photosynthetic metabolic pathway: algae that use HCO3 as a carbon source have far more negative δ13C values than those that only use CO2. An additional difference of about 1.71‰ separates groups intertidal from those below the lowest tide line, which are never exposed to atmospheric carbon. The latter group uses the more 13C-negative CO2 dissolved in sea water, whereas those with access to atmospheric carbon reflect the more positive signature of this reserve.
Pit connections and pit plugs 
Pit connections 
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 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.
Pit plugs 
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 produces a layer of wall material that seals off the plug.
The pit connections are thought to function as structural reinforcement, and as avenues 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, the tubular plug cores may serve as a means of transport.
The reproductive cycle of red algae may be triggered by factors such as day length.
The trichogyne will continue to grow until it encounters a spermatium; once it has been fertilized, the cell wall at its base progressively thickens, separating it from the rest of the carpogonium at its base.
Upon their collision, the walls of the spermatium and carpogonium dissolve. The male nucleus divides and moves into the carpogonium; one half of the nucleus merges with the carpogonium's nucleus.
Life cycle 
They display alternation of generations; in addition to gametophyte generation, many have two sporophyte generations, the carposporophyte-producing carpospores, which germinate into a tetrasporophyte – this produces spore tetrads, which dissociate and germinate into gametophytes. The gametophyte is typically (but not always) identical to the tetrasporophyte.
Carpospores may also germinate directly into thalloid gametophytes, or the carposporophytes may produce a tetraspore without going through a (free-living) tetrasporophyte phase. Tetrasporangia may be arranged in a row (zonate), in a cross (cruciate), or in a tetrad.
The carposporophyte may be enclosed within the gametophyte, which may cover it with branches to form a cystocarp.
These case studies may be helpful to understand some of the life histories algae may display:
In a simple case, such as Rhodochorton investiens:
In the Carposporophyte: a spermatium merges with a trichogyne (a long hair on the female sexual organ), which then divides to form carposporangia – which produce carpospores.
Carpospores germinate into gametophytes, which produce sporophytes. Both of these are very similar; they produce monospores from monosporangia "just below a cross wall in a filament" and their spores are "liberated through apex of sporangial cell."
The spores of a sporophyte produce either tetrasporophytes. Monospores produced by this phase germinate immediately, with no resting phase, to form an identical copy of parent. Tetrasporophytes may also produce a carpospore, which germinates to form another tetrasporophyte.[verification needed]
The gametophyte may replicate using monospores, but produces sperm in spermatangia, and "eggs"(?) in carpogonium.
A rather different example is Porphyra gardneri:
In its diploid phase, a carpospore can germinate to form a filamentous "conchocelis stage", which can also self-replicate using monospores. The conchocelis stage eventually produces conchosporangia. The resulting conchospore germinates to form a tiny prothallus with rhizoids, which develops to a cm-scale leafy thallus. This too can reproduce via monospores, which are produced inside the thallus itself. They can also reproduce via spermatia, produced internally, which are released to meet a prospective carpogonium in its conceptacle.
Human consumption 
In East and Southeast Asia, agar is most commonly produced from Gelidium amansii. In Asia, rhodophytes are important sources of food, such as nori. The high vitamin and protein content of this food makes it attractive, as does the relative simplicity of cultivation, which began in Japan more than 300 years ago.
See also 
- Brown algae
- Green algae
- Red tide (red tides are caused by algae from the phylum Pyrrophyta (dinoflagellates), and not red algae [Rhodophyta])
- Françoise Ardré (namesake of the red alga known as Pterosiphonia ardreana)
- History of phycology
- John Cocks
- 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. doi:10.1666/0094-8373(2000)026<0386:BPNGNS>2.0.CO;2. ISSN 0094-8373.
- Lee, R.E. (2008). Phycology, 4th edition. Cambridge University Press. ISBN 978-0-521-63883-8
- 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.
- Dixon, Peter S. (1977). Biology of the Rhodophyta (Reprint. ed.). Koenigstein: Koeltz. ISBN 0-05-002485-X.
- M. D. Guiry. "Rhodophyta: red algae". National University of Ireland, Galway. Archived from the original on 2007-05-04. Retrieved 2007-06-28.
- Eloranta, P.; Kwandrans, J. (2004). "Indicator value of freshwater red algae in running waters for water quality assessment". International Journal of Oceanography and Hydrobiology. XXXIII (1): 47–54. ISSN 1730-413X. Unknown parameter
- Grant, S. W. F.; Knoll, A. H.; Germs, G. J. B. (1991). "Probable Calcified Metaphytes in the Latest Proterozoic Nama Group, Namibia: Origin, Diagenesis, and Implications". Journal of Paleontology (JSTOR) 65 (1): 1–18. JSTOR 1305691. PMID 11538648
- Yun, Z.; Xun-lal, Y. (1992). "New data on multicellular thallophytes and fragments of cellular tissues from Late Proterozoic phosphate rocks, South China". Lethaia 25 (1): 1–18. doi:10.1111/j.1502-3931.1992.tb01788.x
- Adl, Sina M.; et al. (2005). "The New Higher Level Classification of Eukaryotes with Emphasis on the Taxonomy of Protists". Journal of Eukaryotic Microbiology 52 (5): 399–451. doi:10.1111/j.1550-7408.2005.00053.x. PMID 16248873
- Fabien Burki, Kamran Shalchian-Tabrizi, Marianne Minge, Åsmund Skjæveland, Sergey I. Nikolaev, Kjetill S. Jakobsen, Jan Pawlowski (2007). "Phylogenomics Reshuffles the Eukaryotic Supergroups". In Butler, Geraldine. PLoS ONE 2 (8): e790. doi:10.1371/journal.pone.0000790. PMC 1949142. PMID 17726520.
- Burki, F. et al.; Inagaki, Y.; Brate, J.; Archibald, J. M.; Keeling, P. J.; Cavalier-Smith, T.; Sakaguchi, M.; Hashimoto, T. et al. (2009). "Large-Scale Phylogenomic Analyses Reveal That Two Enigmatic Protist Lineages, Telonemia and Centroheliozoa, Are Related to Photosynthetic Chromalveolates". Genome Biology and Evolution 1: 231–8. doi:10.1093/gbe/evp022. PMC 2817417. PMID 20333193.
- Cavalier-Smith, Thomas (2009). "Kingdoms Protozoa and Chromista and the eozoan root of the eukaryotic tree". Biology Letters 6 (3): 342–5. doi:10.1098/rsbl.2009.0948. PMC 2880060. PMID 20031978.
- Rogozin, I.B.; Basu, M.K.; Csürös, M. & Koonin, E.V. (2009). "Analysis of Rare Genomic Changes Does Not Support the Unikont–Bikont Phylogeny and Suggests Cyanobacterial Symbiosis as the Point of Primary Radiation of Eukaryotes". Genome Biology and Evolution 1: 99–113. doi:10.1093/gbe/evp011. PMC 2817406. PMID 20333181.
- Kim, E.; Graham, L.E. & Graham, Linda E. (2008). "EEF2 analysis challenges the monophyly of Archaeplastida and Chromalveolata". In Redfield, Rosemary Jeanne. PLoS ONE 3 (7): e2621. doi:10.1371/journal.pone.0002621. PMC 2440802. PMID 18612431
- Nozaki, H.; Maruyama, S.; Matsuzaki, M.; Nakada, T.; Kato, S. & Misawa, K. (2009). "Phylogenetic positions of Glaucophyta, green plants (Archaeplastida) and Haptophyta (Chromalveolata) as deduced from slowly evolving nuclear genes". Molecular Phylogenetics and Evolution 53 (3): 872–880. doi:10.1016/j.ympev.2009.08.015. PMID 19698794
- 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 (10): 1494–1507. doi:10.3732/ajb.91.10.1494.
- 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 (2): 482–492. doi:10.1111/j.1529-8817.2006.00210.x.
- Robert Edward Lee (2008). Phycology. Cambridge University Press. p. 107. ISBN 978-0-521-68277-0. Retrieved 31 January 2011.
- Maberly, S. C.; Raven, J. A.; Johnston, A. M. (1992). "Discrimination between 12C and 13C by marine plants". Oecologia 91 (4): 481. doi:10.1007/BF00650320. JSTOR 4220100.
- Fritsch, F. E. (1945), The structure and reproduction of the algae, Cambridge: Cambridge Univ. Press, ISBN 0521050421, OCLC 223742770
- Kohlmeyer, J. (1975). "New Clues to the Possible Origin of Ascomycetes". BioScience (American Institute of Biological Sciences) 25 (2): 86–93. doi:10.2307/1297108. JSTOR 1297108
- "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.