Red algae

Red algae
Scientific classification
Domain:	Eukaryota
(unranked):	Archaeplastida
Phylum:	Rhodophyta Wettstein, 1922
Class
Rhodophycaea (There were formerly thought to be two classes, Florideophyceae and Bangiophyceae.)

The red algae (Rhodophyta, /ˌroʊdəˈfaɪtə, roʊˈdɒfɨtə/, 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.  Most 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. 

A red alga in a California tide pool displaying some iridescence when not covered with water

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.

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 Subkingdom Biliphyta Wettstein Phylum Rhodophyta Wettstein Subphylum Cyanidiophytina subphylum novus Class Cyanidiophyceae Merola et al Subphylum Rhodophytina subphylum novus Class Bangiophyceae Wettstein Class Compsopogonophyceae Saunders et Hommersand Class Florideophyceae Cronquist Class Porphyridiophyceae classis nova Class Rhodellophyceae Cavalier-Smith Class Stylonematophyceae classis nova	Kingdom Plantae Haeckel Subkingdom Rhodoplantae Phylum Cyanidiophyta Class Cyanidiophyceae Merola et al Phylum Rhodophyta Wettstein Subphylum Rhodellophytina Class Rhodellophyceae Cavalier-Smith Subphylum Metarhodophytina Class Compsopogonophyceae Saunders et Hommersand Subphylum Eurhodophytina Class Bangiophyceae Wettstein Class Florideophyceae Cronquist Subclass Hildenbrandiophycidae Subclass Nemaliophycidae Subclass Ahnfeltiophycidae Subclass Rhodymeniophycidae

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:

• Atractophora hypnoides
• Gelidiella calcicola
• Lemanea, a freshwater genus
• Palmaria palmata, dulse
• Schmitzia hiscockiana
• Chondrus crispus, Irish moss
• Mastocarpus stellatus

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. 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

Red algae lack motile sperm. Hence they rely on water currents to transport their gametes to the female organs. They display alternation of phases; as well as a gemetophyte phase, many have two sporophyte phases, the carposporophyte producing carpospores, which germinate into a tetrasporophyte - this produces spore tetrads, which dissociate and germinate into gemetophytes.

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

• Brown algae
• Green algae
• Red tide (red tides are caused by algae from the phylum Dinoflagellata, and not red algae [Rhodophyta])
• Françoise Ardré (namesake of the red alga known as Pterosiphonia ardreana)

External links

• Monterey Bay Flora
• Life cycle of red algae