Red algae

Lore & Background

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. Their chloroplasts contain evenly spaced and ungrouped thylakoids and contain the pigments chlorophyll a, α- and β-carotene, lutein and zeaxanthin. The presence of water-soluble pigments called phycobilins (phycocyanobilin, phycoerythrobilin, phycourobilin and phycobiliviolin), localized into phycobilisomes, gives red algae their distinctive color. Red algae reproduce sexually as well as asexually. The life history of red algae is typically an alternation of generations that may have three generations rather than two. Red algae lack motile sperm, relying on water currents to transport their gametes to the female organs, though 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. Pit connections and pit plugs are unique and distinctive features of red algae that form during the process of cytokinesis following mitosis. The polyamine spermine is produced, which triggers carpospore production. 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. The major photosynthetic products include floridoside (major product), D‐isofloridoside, digeneaside, mannitol, sorbitol, dulcitol etc. The concentration of photosynthetic products are altered by environmental conditions like change in pH, the salinity of medium, change in light intensity, nutrient limitation etc.

Reader's Guide

Coralline algae, which secrete calcium carbonate and play a major role in building coral reefs, belong to the red algae. Red algae such as Palmaria palmata (dulse) and Porphyra species (laver/nori/gim) are a traditional part of European and Asian cuisines and are used to make products such as agar, carrageenans, and other food additives. Their cell structure is distinctive: red algae do not have flagella and centrioles during their entire life cycle, and their distinguishing characters 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. The reproductive cycle may be triggered by factors such as day length. Asexual reproduction can occur through the production of spores and by vegetative means (fragmentation, cell division or propagules production). The life cycle displays 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 (but not always) identical to the tetrasporophyte. The δ13C values of red algae reflect their lifestyles, with the largest difference resulting from their photosynthetic metabolic pathway: algae that use HCO3 as a carbon source have less negative δ13C values than those that only use CO2.

Did You Know?

• Red algae have the most gene-rich plastid genomes known.
• Approximately 5% of red algae species occur in freshwater environments, with greater concentrations in warmer areas.
• The last common ancestor of red algae lost about 25% of its core genes and much of its evolutionary plasticity.
• The isopod Idotea balthica was the first animal discovered to help with the dispersal and fertilization of red algal gametes.
• Red algae store sugars as floridean starch, a type of starch that consists of highly branched amylopectin without amylose.

Taxonomic Breadth and Evolutionary Heritage

Red algae, or Rhodophyta, make up one of the oldest and largest phyla of eukaryotic algae, containing over 7,000 recognized species within over 900 genera amidst ongoing taxonomic revisions. The majority of species (6,793) are in the class Florideophyceae, and mostly consist of multicellular marine algae, including many notable seaweeds. Approximately 5% of red algae species occur in freshwater environments, with greater concentrations in warmer areas. This 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. The name Rhodophyta derives from Ancient Greek ῥόδον (rhódon) 'rose' and φυτόν (phutón) 'plant'.

Distinctive Cell Architecture and Pigmentation

Red algae do not have flagella and centrioles during their entire life cycle. Their chloroplasts are enclosed in a double membrane, lack grana and phycobilisomes on the stromal surface of the thylakoid membrane, and contain evenly spaced and ungrouped thylakoids. The presence of water-soluble pigments called phycobilins (phycocyanobilin, phycoerythrobilin, phycourobilin and phycobiliviolin), which are localized into phycobilisomes, gives red algae their distinctive 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. 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, while the internal walls are mostly cellulose. They also have the most gene-rich plastid genomes known.

Reproductive Strategies and Life-Cycle Complexity

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). The reproductive cycle may be triggered by factors such as day length. Red algae lack motile sperm, relying 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 life history of red algae is typically an alternation of generations that may have three generations rather than two. 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. Pit connections and pit plugs are unique and distinctive features of red algae that form during the process of cytokinesis following mitosis. The polyamine spermine is produced, which triggers carpospore production.

Ecological Roles and Culinary Heritage

Coralline algae, which secrete calcium carbonate and play a major role in building coral reefs, belong to the red algae. Red algae such as Palmaria palmata (dulse) and Porphyra species (laver/nori/gim) are a traditional part of European and Asian cuisines and are used to make products such as agar, carrageenans, and other food additives. 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. The major photosynthetic products include floridoside (major product), D‐isofloridoside, digeneaside, mannitol, sorbitol, dulcitol etc. The concentration of photosynthetic products are altered by 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.

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