Definitions

cystocarp

Red algae

The red algae (Rhodophyta, , from Greek: ῥόδον (rhodon) = rose + φυτόν (phyton) = plant, thus red 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 10,000 species. 

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.  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

Fossil record

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.

There are also calcite crusts which have been interpreted as the remains of coralline red algae dating to the terminal Proterozoic. Thallophytes resembling coralline red algae are known from the late Proterozoic Doushantuou formation.

Evolutionary history

The red algae are considered to be one of the three groups forming the Archaeplastida, together with the glaucophytes and the green lineage or Viridiplantae. The archaeplastida originated from the primary endosymbiosis event between a eukaryote and a cyanobacterium ~1,600 million years ago creating the first chloroplast and the first photosynthetic eukaryote.

Taxonomy

The red algae are classified in the Archaeplastida, along with the glaucophytes and Viridiplantae (green algae and land plants).

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 (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; but 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. The two classification systems below place the red algae in the plant kingdom.

According to synthesis in Lee (2008) Classification system according to
Hwan Su Yoon et al. 2006
Classification system according to
Saunders and Hommersand 2004
Domain Eukaryota (Corticata)
  • Kingdom Chromalveolata Cavalier-Smith, 1998
    • Phylum Rhodophyta
      • Class Rhodophytina
        • Orders
          • Cyanidiales
          • Porphyridiales
          • Bangiales
          • Acrochaetiales
          • Batrachospermales
          • Nemaliales
          • Corallinales
          • Gelidiales
          • Gracilariales
          • Ceramiales

Kingdom Plantae Haeckel

Kingdom Plantae Haeckel

Species of red algae

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 of species and genera of red algae are:

Chemistry

The values of red algae reflect their lifestyles. The largest difference results from their photosynthetic pathway: algae which use as a carbon source have far more negative values than those which only use . An additional difference of about 1.71‰ separates groups which are intertidal from those below the lowest tide line, which are never exposed to atmospheric carbon. The latter group use the more negative which is dissolved in sea water, whereas those with access to atmospheric carbon reflect the more positive signature of this reserve.

Morphology

Red algae have a double cell wall. The outer layer are usually composed of "pectic substances", from which agar can be manufactured. The internal wall is mostly cellulose.

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 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.

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 produce a layer of wall material that seals off the plug.

Function

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.

Reproduction

The reproductive cycle of red algae may be triggered by factors such as day length.

Fertilisation

Red algae lack motile sperm. Hence they rely on water currents to transport their gametes to the female organs – although their sperm are capable "gliding" to a carpogonium's trichogyne.

The trichogyne will continue to grow until it encounters a spermatium; once it has been fertilised, the cell wall at its base progressively thickens, separating it from the rest of the carpogonium at its base .

Upone 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.

The polyamine spermine is produced which triggers carpospore production.

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

Alternation of phases

They display alternation of phases; as well as a gametophyte phase, many have two sporophyte phases, 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 tetresporophyte.

Carpospores may also germinate directly into thalloid gemetophytes, 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.

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" 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.

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

Several species are used as food. Dulse (Palmaria palmata) and Porphyra are the best known in the British Isles. 

In East and Southeast Asia, agar is most commonly produced from Gelidium amansii.

See also

References

External links

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