Red algae

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Rhodophyta
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Red algae
Temporal range: Mesoproterozoic–present[1][2]
A-D :
J.Ag.
Scientific classification Edit this classification
Domain: Eukaryota
Clade: Diaphoretickes
(unranked): Archaeplastida
Division: Rhodophyta
Wettstein, 1922
Clades

Red algae, or Rhodophyta (

multicellular, marine algae, including many notable seaweeds.[4][5] Red algae are abundant in marine habitats but relatively rare in freshwaters.[6] Approximately 5% of red algae species occur in freshwater environments, with greater concentrations found in warmer areas.[7] Except for two coastal cave dwelling species in the asexual class Cyanidiophyceae, there are no terrestrial species, which may be due to an evolutionary bottleneck in which the last common ancestor lost about 25% of its core genes and much of its evolutionary plasticity.[8][9]

The red algae form a distinct group characterized by having eukaryotic cells without

multicellular, macroscopic, marine, and reproduce sexually. The life history of red algae is typically an alternation of generations that may have three generations rather than two.[14]
The
food additives.[15]

Evolution

Botryocladia occidentalis scale bar: 2 cm

Chloroplasts probably evolved following an

Alveolata.[17] In addition to multicellular brown algae, it is estimated that more than half of all known species of microbial eukaryotes harbor red-alga-derived plastids.[18]

Red algae are divided into the

lentic waters.[27] Both marine and freshwater taxa are represented by free-living macroalgal forms and smaller endo/epiphytic/zoic forms, meaning they live in or on other algae, plants, and animals.[10] In addition, some marine species have adopted a parasitic lifestyle and may be found on closely or more distantly related red algal hosts.[28][29]

Taxonomy

Further information: Wikispecies:Rhodophyta

In the classification system of Adl et al. 2005, the red algae are classified in the

paraphyletic.[35][36] As of January 2011, the situation appears unresolved.[clarification needed
]

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 (

genomic approach is funded by the National Science Foundation
as part of the Assembling the Tree of Life Program.

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
  • Subkingdom
    Rhodoplantae
Cyanidiales
 No No
Cyanidioschyzon merolae
Rhodellales No No Rhodella
Rhodochaetales, Erythropeltidales
 Yes No Compsopogon
Stylonematales
 Yes No Stylonema

Bangiales

Yes Yes Bangia, "Porphyra"

Porphyridiales

 No No Porphyridium cruentum
Hildenbrandiales Yes Yes Hildenbrandia
  • Subclass
    Nemaliophycidae
Colaconematales, Acrochaetiales, Palmariales, Thoreales
 Yes Yes Nemalion
Corallinales
 Yes Yes Corallina officinalis
Ahnfeltiales, Pihiellales Yes Yes Ahnfeltia
  • Subclass
    Rhodymeniophycidae
 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.

See also: Eukaryote § Phylogeny

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:

Morphology

Red algal morphology is diverse ranging from

unicellular forms to complex parenchymatous and non- parenchymatous thallus.[41] Red algae have double cell walls.[42] The outer layers contain the polysaccharides agarose and agaropectin that can be extracted from the cell walls as agar by boiling.[42] The internal walls are mostly cellulose.[42] They also have the most gene-rich plastid genomes known.[43]

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

phycobilisomes, gives red algae their distinctive color.[46] Their chloroplasts contain evenly spaced and ungrouped thylakoids[47] and contain the pigments chlorophyll a, α- and β-carotene, lutein and zeaxanthin. Their chloroplasts are enclosed in a double membrane, lack grana and phycobilisomes on the stromal surface of the thylakoid membrane.[48]

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

Main article: Pit connection

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

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 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

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.[3]

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]

Spermatangia may have long, delicate appendages, which increase their chances of "hooking up".[3]

Life cycle

They display

tetrasporophyte – this produces spore tetrads, which dissociate and germinate into gametophytes.[3] The gametophyte is typically (but not always) identical to the tetrasporophyte.[57]

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]
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

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.

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.

Fossil record

One of the oldest fossils identified as a red alga is also the oldest fossil

Bangiomorpha pubescens, a multicellular fossil from arctic Canada, strongly resembles the modern red alga Bangia and occurs in rocks dating to 1.05 billion years ago.[2]

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

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.

Doushantuo formation.[74]

Relationship to other algae

endosymbiotic theory is supported by various structural and genetic similarities.[75]

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

dulse (Palmaria palmata)[79] and members of the genus Porphyra, variously known as nori (Japan), gim (Korea), zicai 紫菜 (China), and laver (British Isles).[80]

Some of the red algal species like

polyunsaturated fatty acids (eicopentaenoic acid, docohexaenoic acid, arachidonic acid)[81] and have protein content up to 47% of total biomass.[76] Where a big portion of world population is getting insufficient daily iodine intake, a 150 ug/day requirement of iodine is obtained from a single gram of red algae.[82] Red algae, like Gracilaria, Gelidium, Euchema, Porphyra, Acanthophora, and Palmaria are primarily known for their industrial use for phycocolloids (agar, algin, furcellaran and carrageenan) as thickening agent, textiles, food, anticoagulants, water-binding agents, etc.[83] Dulse (Palmaria palmata) is one of the most consumed red algae and is a source of iodine, protein, magnesium and calcium.[84] Red algae's nutritional value is used for the dietary supplement of algas calcareas.[85]

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

See also

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External links

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