| Red algae Temporal range: Mesoproterozoic–present[1] |
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| Scientific classification | |
| Domain: | Eukarya |
| (unranked): | Archaeplastida |
| Division: | Rhodophyta Wettstein, 1922 Classification is currently disputed. See Taxonomy. |
The red algae, or Rhodophyta ( /roʊˈdɒfɨtə/ or /ˌroʊdəˈfaɪtə/; from Greek: ῥόδον (rhodon) = rose + φυτόν (phyton) = plant, thus red plant), are one of the oldest groups of eukaryotic algae,[2] and also one of the largest, with about 5,000–6,000 species [3] of mostly multicellular, marine algae, including many notable seaweeds. Other references indicate as many as 10,000 species;[4] more detailed counts indicate ~4,000 in ~600 genera (3,738 marine spp in 546 genera and 10 orders (plus the unclassifiable); 164 freshwater spp in 30 genera in 8 orders).[5]
The red algae form a distinct group characterized by the following attributes: eukaryotic cells without flagella and centrioles, using floridean starch as food reserve, with phycobiliproteins as accessory pigments (giving them their red color), and with chloroplasts lacking external endoplasmic reticulum and containing unstacked thylakoids. [4] Most red algae are also multicellular, macroscopic, marine, and have sexual reproduction.
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 cuisine and are used to make other products like agar, carrageenans and other food additives. [6]
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Most rhodophytes are marine, although there are freshwater species; these generally prefer clean, running water, although there are some exceptions.[7]
One of 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. [1]
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.
There are also calcite crusts, which have been interpreted as the remains of coralline red algae dating to the terminal Proterozoic.[8] Thallophytes resembling coralline red algae are known from the late Proterozoic Doushantuo formation.[9]
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 sub-divisions are given; the authors say "Traditional subgroups are artificial constructs, and no longer valid." [10]
The system reflected the consensus in 2005. Many studies published since this date have provided evidence which is in agreement.[11][12][13][14] However, other studies have suggested that Archaeplastida is paraphyletic.[15][16] 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).[17]
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 system according to Saunders and Hommersand 2004[17] |
Classification system according to Hwan Su Yoon et al. 2006[18] |
Orders | Multicelluar? | Pit plugs? | Example |
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Cyanidiales | No | No | Cyanidioschyzon merolae |
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Rhodellales | No | No | Rhodella |
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Compsopogonales, Rhodochaetales, Erythropeltidales | Yes | No | Compsopogon |
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Rufusiales, Stylonematales | Yes | No | Stylonema | |
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Yes | Yes | Bangia, "Porphyra" | |
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No | No | Porphyridium | ||
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Hildenbrandiales | Yes | Yes | Hildenbrandia |
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Batrachospermales, Balliales, Balbianiales, Nemaliales, Colaconematales, Acrochaetiales, Palmariales, Thoreales | Yes | Yes | Nemalion | |
| Rhodogorgonales, Corallinales | Yes | Yes | Corallina officinalis | ||
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Ahnfeltiales, Pihiellales | Yes | Yes | Ahnfeltia | |
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Bonnemaisoniales, Gigartinales, Gelidiales, Gracilariales, Halymeniales, Rhodymeniales, Nemastomatales, Plocamiales, Ceramiales | Yes | Yes | Gelidium |
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.[19])
There are around 6,500 to 10,000 known species, [4][6] 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%. [4]
Some examples of species and genera of red algae are:
| Algal group | δ13C range[20] |
|---|---|
| HCO3-using red algae | −22.5‰ – −9.6‰ |
| CO2-using red algae | −34.5‰ – −29.9‰ |
| Brown algae | −20.8‰ – −10.5‰ |
| Green algae | −20.3‰ – −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.[20] 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 use the more 13C negative CO2 dissolved in sea water, whereas those with access to atmospheric carbon reflect the more positive signature of this reserve.
Red algae have a double cell wall.[21] The outer layer are usually composed of "pectic substances", from which agar can be manufactured.[21] The internal wall is mostly cellulose.[21]
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.
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.
The reproductive cycle of red algae may be triggered by factors such as day length.[2]
Red algae lack motile sperm. Hence they rely on water currents to transport their gametes to the female organs – although their sperm are capable of "gliding" to a carpogonium's trichogyne.[2]
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.[2]
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.[2]
The polyamine, spermine is produced, which triggers carpospore production.[2]
Spermatangia may have long delicate appendages, which increase their chances of "hooking up".[2]
They display alternation of generations; as well as a 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.[2] The gametophyte is typically (but not always) identical to the tetrasporophyte.[22]
Carpospores may also germinate directly into thalloid gametophytes, or the carposporophytes may produce a tetraspore without going through a (free living) tetrasporophyte phase.[22] Tetrasporangia may be arranged in a row (Zonate), in a cross (cruciate), or in a tetrad.[2]
The carposporophyte may be enclosed within the gametophyte, which may cover it with branches to form a cystocarp.[22]
A couple of 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"[2] and their spores are "liberated through apex of sporangial cell."[2]
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.[2]
The gametophyte may replicate using monospores, but produces sperm in spermatangia, and "eggs"(?) in carpogonium.[2]
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.[2] They can also reproduce via spermatia, produced internally, which are released to meet a prospective carpogonium in its conceptacle.[2]
Several species are used as food. Dulse (Palmaria palmata)[23] and Porphyra are the best known in the British Isles. [24]
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.
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