Red algae
Red algae, or Rhodophyta, make up one of the oldest groups of eukaryotic algae. The Rhodophyta comprises one of the largest phyla of algae, containing over 7,000 recognized species within over 900 genera amidst ongoing taxonomic revisions. The majority of species are Florideophyceae, and mostly consist of multicellular, marine algae, including many notable seaweeds. Red algae are abundant in marine habitats. Approximately 5% of red algae species occur in freshwater environments, with greater concentrations in warmer areas. Except for two coastal cave dwelling species in the asexual class Cyanidiophyceae, no terrestrial species exist, 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.
Red algae form a distinct group characterized by eukaryotic cells without flagella and centrioles, chloroplasts without external endoplasmic reticulum or unstacked thylakoids, and use phycobiliproteins as accessory pigments, which give them their red color. Despite their name, red algae can vary in color from bright green, soft pink, resembling brown algae, to shades of red and purple, and may be almost black at greater depths. Unlike green algae, red algae store sugars as food reserves outside the chloroplasts as floridean starch, a type of starch that consists of highly branched amylopectin without amylose. Most red algae are multicellular, macroscopic, and reproduce sexually. The life history of red algae is typically an alternation of generations that may have three generations rather than two. Coralline algae, which secrete calcium carbonate and play a major role in building coral reefs, belong there.
Red algae such as Palmaria palmata and Porphyra species are a traditional part of European and Asian cuisines and are used to make products such as agar, carrageenans, and other food additives.
Description
Red algal morphology is diverse, ranging from unicellular forms to complex parenchymatous and non- parenchymatous thallus. Red algae have double cell walls. The outer layers contain the polysaccharides agarose and agaropectin that can be extracted from the cell walls as agar by boiling. The internal walls are mostly cellulose. They also have the most gene-rich plastid genomes known.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, pit connection between cells, filamentous genera, and the absence of chloroplast endoplasmic reticulum.Chloroplasts
The presence of the water-soluble pigments called phycobilins, which are localized into phycobilisomes, gives red algae their distinctive color. Their chloroplasts contain evenly spaced and ungrouped thylakoids 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.Storage products
The major photosynthetic products include floridoside, D‐isofloridoside, digeneaside, mannitol, sorbitol, dulcitol etc. Floridean starch, a long-term storage product, is deposited freely in the cytoplasm. 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. 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. 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.
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.Reproduction
The reproductive cycle of red algae may be triggered by factors such as day length. Red algae reproduce sexually as well as asexually. Asexual reproduction can occur through the production of spores and by vegetative means.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. Animals also help with the dispersal and fertilization of the gametes. The first species discovered to do so is the isopod Idotea balthica.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.
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.
The polyamine spermine is produced, which triggers carpospore production.
Spermatangia may have long, delicate appendages, which increase their chances of "hooking up".
Life cycle
They display alternation of generations. In addition to 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. The gametophyte is typically identical to the tetrasporophyte.Carpospores may also germinate directly into thalloid gametophytes, or the carposporophytes may produce a tetraspore without going through a tetrasporophyte phase. Tetrasporangia may be arranged in a row, in a cross, or in a tetrad.
The carposporophyte may be enclosed within the gametophyte, which may cover it with branches to form a cystocarp.
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:
A rather different example is Porphyra gardneri:
Chemistry
The 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 less negative values than those that only use carbon dioxide|. 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 uses the more 13C-negative 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.
Taxonomy
In the classification system of Adl et al. 2005, the red algae are classified in the Archaeplastida, along with the glaucophytes and the green algae plus land plants. 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 subdivisions are given; the authors say, "Traditional subgroups are artificial constructs, and no longer valid." Many subsequent studies provided evidence that is in agreement for monophyly in the Archaeplastida. However, other studies have suggested Archaeplastida is paraphyletic., the general consensus is that Archaeplastida is paraphyletic.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.
- 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.