Red algae
Red algae Temporal range:
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A-D : J.Ag.
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Scientific classification | |
Domain: | Eukaryota |
Clade: | Diaphoretickes |
(unranked): | Archaeplastida |
Division: | Rhodophyta Wettstein, 1922 |
Clades | |
Red algae, or Rhodophyta (
The red algae form a distinct group characterized by having eukaryotic cells without
Evolution
Chloroplasts probably evolved following an
Red algae are divided into the
Taxonomy
In the classification system of Adl et al. 2005, the red algae are classified in the
Below are other published taxonomies of the red algae using molecular and traditional alpha taxonomic data; however, the taxonomy of the red 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).[37]
- If the kingdom Plantae is defined as the Archaeplastida, then red algae will be part of that group.
- If Plantae are defined more narrowly, to be the Viridiplantae, then the red algae might be excluded.
A major research initiative to reconstruct the Red Algal Tree of Life (
Classification comparison
Classification system according to Saunders and Hommersand 2004[37] |
Classification system according to Hwan Su Yoon et al. 2006[38] |
Orders | Multicelluar? | Pit plugs? | Example |
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Cyanidiales
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No | No | Cyanidioschyzon merolae
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Rhodellales | No | No | Rhodella |
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Rhodochaetales, Erythropeltidales
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Yes | No | Compsopogon |
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Stylonematales
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Yes | No | Stylonema | |
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Yes | Yes | Bangia, "Porphyra" | |
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Porphyridiales
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No | No | Porphyridium cruentum | |
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Hildenbrandiales | Yes | Yes | Hildenbrandia |
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Yes | Yes | Nemalion | ||
Corallinales
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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.[39])
A subphylum - Proteorhodophytina - has been proposed to encompass the existing classes Compsopogonophyceae, Porphyridiophyceae, Rhodellophyceae and Stylonematophyceae.[40] This proposal was made on the basis of the analysis of the plastid genomes.
Species of red algae
Over 7,000 species are currently described for the red algae,[4] but the taxonomy is in constant flux with new species described each year.[37][38] The vast majority of these are marine with about 200 that live only in fresh water.
Some examples of species and genera of red algae are:
- Cyanidioschyzon merolae, a primitive red alga
- 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
- Audouinella, with freshwater as well as marine species
- Polysiphonia ceramiaeformis, banded siphon weed
- Vertebrata simulans
Morphology
Red algal morphology is diverse ranging from
Cell structure
Red algae do not have flagella and centrioles during their entire life cycle. The distinguishing characters of red algal cell structure include the presence of normal spindle fibres, microtubules, un-stacked photosynthetic membranes, phycobilin pigment granules,[44] pit connection between cells, filamentous genera, and the absence of chloroplast endoplasmic reticulum.[45]
Chloroplasts
The presence of the water-soluble pigments called
Storage products
The major photosynthetic products include floridoside (major product), D‐isofloridoside, digeneaside, mannitol, sorbitol, dulcitol etc.[49] Floridean starch (similar to amylopectin in land plants), a long-term storage product, is deposited freely (scattered) in the cytoplasm.[50] The concentration of photosynthetic products are altered by the environmental conditions like change in pH, the salinity of medium, change in light intensity, nutrient limitation etc.[51] When the salinity of the medium increases the production of floridoside is increased in order to prevent water from leaving the algal cells.
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.[52][53] 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
Connections that exist between cells not sharing a common parent cell are labelled 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.[53]
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.
Function
The pit connections have been suggested to function as structural reinforcement, or as avenues for cell-to-cell communication and transport in red algae, however little data supports this hypothesis.[54]
Reproduction
The reproductive cycle of red algae may be triggered by factors such as day length.[3] Red algae reproduce sexually as well as asexually. Asexual reproduction can occur through the production of spores and by vegetative means (fragmentation, cell division or propagules production).[55]
Fertilization
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.[3] Animals also help with the dispersal and fertilization of the gametes. The first species discovered to do so is the isopod Idotea balthica.[56]
The trichogyne will continue to grow until it encounters a
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.[3]
The polyamine spermine is produced, which triggers carpospore production.[3]
Life cycle
They display
Carpospores may also germinate directly into thalloid gametophytes, or the carposporophytes may produce a tetraspore without going through a (free-living) tetrasporophyte phase.[57] Tetrasporangia may be arranged in a row (zonate), in a cross (cruciate), or in a tetrad.[3]
The carposporophyte may be enclosed within the gametophyte, which may cover it with branches to form a cystocarp.[57]
The two following 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"[3] and their spores are "liberated through the apex of sporangial cell."[3]
- The spores of a sporophyte produce either tetrasporophytes. Monospores produced by this phase germinates immediately, with no resting phase, to form an identical copy of the parent. Tetrasporophytes may also produce a carpospore, which germinates to form another tetrasporophyte.[verification needed][3]
- The gametophyte may replicate asexually using monospores, but also produces nonmotile sperm in spermatangia, and a lower, nucleus-containing "egg" region of the carpogonium.[3][58]
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.[3] They can also reproduce via spermatia, produced internally, which are released to meet a prospective carpogonium in its conceptacle.[3]
Chemistry
Algal group | δ13C range[59] |
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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
Photosynthetic pigments of Rhodophyta are chlorophylls a and d. Red algae are red due to phycoerythrin. They contain the sulfated polysaccharide carrageenan in the amorphous sections of their cell walls, although red algae from the genus Porphyra contain porphyran. They also produce a specific type of tannin called phlorotannins, but in a lower amount than brown algae do.
Genomes and transcriptomes of red algae
As enlisted in realDB,[60] 27 complete transcriptomes and 10 complete genomes sequences of red algae are available. Listed below are the 10 complete genomes of red algae.
- Galdieria sulphuraria, Cyanidiophyceae[63]
- Pyropia yezoensis, Bangiophyceae[64]
- Chondrus crispus, Florideophyceae[65]
- Porphyridium purpureum, Porphyridiophyceae[66]
- Porphyra umbilicalis, Bangiophyceae[67]
- Gracilaria changii, Gracilariales[68]
- Galdieria phlegrea, Cyanidiophytina[69]
- Gracilariopsis lemaneiformis, Gracilariales[70]
- Gracilariopsis chorda, Gracilariales[71]
Fossil record
One of the oldest fossils identified as a red alga is also the oldest fossil
Two kinds of fossils resembling red algae were found sometime between 2006 and 2011 in well-preserved sedimentary rocks in Chitrakoot, central India. The presumed red algae lie embedded in fossil mats of cyanobacteria, called stromatolites, in 1.6 billion-year-old Indian phosphorite – making them the oldest plant-like fossils ever found by about 400 million years.[72]
Red algae are important builders of
Relationship to other algae
Human consumption
Red algae have a long history of use as a source of nutritional, functional food ingredients and pharmaceutical substances.[76] They are a source of antioxidants including polyphenols, and phycobiliproteins[citation needed] and contain proteins, minerals, trace elements, vitamins and essential fatty acids.[77][78]
Traditionally, red algae are eaten raw, in salads, soups, meal and condiments. Several species are food crops, in particular
Some of the red algal species like
China, Japan, Republic of Korea are the top producers of seaweeds.[86] In East and Southeast Asia, agar is most commonly produced from Gelidium amansii. These rhodophytes are easily grown and, for example, nori cultivation in Japan goes back more than three centuries.[citation needed]
Gallery
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Cyanidium sp. (Cyanidiophyceae)
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Porphyra sp., haploid and diploid (Bangiophyceae)
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Gracilaria sp. (Florideophyceae: Gracilariales)
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Corallinales)
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Laurencia sp. (Florideophyceae: Ceramiales)
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Some red algae are iridescent when not covered with water
See also
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