Red algae

Red algal morphology is diverse, ranging from unicellular forms to complex parenchymatous and non- parenchymatous thallus. Red algae have double cell walls. 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 (phycocyanobilin, phycoerythrobilin, phycourobilin and phycobiliviolin), 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 (major product), D‐isofloridoside, digeneaside, mannitol, sorbitol, dulcitol etc. Floridean starch (similar to amylopectin in land plants), a long-term storage product, is deposited freely (scattered) 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. Reproduction The reproductive cycle of red algae may be triggered by factors such as day length. 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. 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. Carpospores may also germinate directly into thalloid gametophytes, or the carposporophytes may produce a tetraspore without going through a (free-living) tetrasporophyte phase. 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. 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 (Viridiplantae or Chloroplastida). 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 (including red algae). 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 (with classification above the level of order having received little scientific attention for most of the 20th century). • 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 (RedToL) using phylogenetic and genomic approach is funded by the National Science Foundation as part of the Assembling the Tree of Life Program. Classification comparison 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. This proposal was made on the basis of the analysis of the plastid genomes. Species Over 7,000 species are currently described for the red algae, }} == Evolution ==

Chloroplasts probably evolved following an endosymbiotic event between an ancestral, photosynthetic cyanobacterium and an early eukaryotic phagotroph. This event (termed primary endosymbiosis) is at the origin of the red and green algae (including the land plants or Embryophytes which emerged within them) and the glaucophytes, which together make up the oldest evolutionary lineages of photosynthetic eukaryotes, the Archaeplastida. A secondary endosymbiosis event involving an ancestral red alga and a heterotrophic eukaryote resulted in the evolution and diversification of several other photosynthetic lineages such as Cryptophyta, Haptophyta, Stramenopiles (or Heterokontophyta), and Alveolata. Red algae are divided into the Cyanidiophyceae, a class of unicellular and thermoacidophilic extremophiles found in sulphuric hot springs and other acidic environments, an adaptation partly made possible by horizontal gene transfers from prokaryotes, with about 1% of their genome having this origin, and two sister clades called SCRP (Stylonematophyceae, Compsopogonophyceae, Rhodellophyceae and Porphyridiophyceae) and BF (Bangiophyceae and Florideophyceae), which are found in both marine and freshwater environments. The BF are macroalgae, seaweed that usually do not grow to more than about 50 cm in length, but a few species can reach lengths of . In the SCRP clade the class Compsopogonophyceae is multicellular, with forms varying from microscopic filaments to macroalgae. Stylonematophyceae have both unicellular and small simple filamentous species, while Rhodellophyceae and Porphyridiophyceae are exclusively unicellular. Most rhodophytes are marine with a worldwide distribution, and are often found at greater depths compared to other seaweeds. While this was formerly attributed to the presence of pigments (such as phycoerythrin) that would permit red algae to inhabit greater depths than other macroalgae by chromatic adaption, recent evidence calls this into question (e.g. the discovery of green algae at great depth in the Bahamas). Some marine species are found on sandy shores, while most others can be found attached to rocky substrata. Freshwater species account for 5% of red algal diversity, but they also have a worldwide distribution in various habitats; A few freshwater species are found in black waters with sandy bottoms and even fewer are found in more lentic waters. 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. == Genomes and transcriptomes ==

As enlisted in realDB, 27 complete transcriptomes and 10 complete genomes sequences of red algae are available. Listed below are the 10 complete genomes of red algae. • Cyanidioschyzon merolae, Cyanidiophyceae • Galdieria sulphuraria, Cyanidiophyceae • Pyropia yezoensis, Bangiophyceae • Chondrus crispus, Florideophyceae • Porphyridium purpureum, Porphyridiophyceae • Porphyra umbilicalis, Bangiophyceae • Gracilaria changii, Gracilariales • Galdieria phlegrea, Cyanidiophytina • Gracilariopsis lemaneiformis, Gracilariales • Gracilariopsis chorda, Gracilariales ==Fossil record== One of the oldest fossils 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 and occurs in rocks dating to 1.05 billion 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. Calcite crusts that have been interpreted as the remains of coralline red algae, date to the Ediacaran Period. == Uses ==

Human consumption Red algae have a long history of use as a source of nutritional, functional food ingredients and pharmaceutical substances. They are a source of antioxidants including polyphenols, and phycobiliproteins and contain proteins, minerals, trace elements, vitamins and essential fatty acids. Traditionally, red algae are eaten raw, in salads, soups, meal and condiments. Several species are food crops, in particular dulse (Palmaria palmata) and members of the genus Porphyra, variously known as nori (Japan), gim (Korea), (China), and laver (British Isles). Red algal species such as Gracilaria and Laurencia are rich in polyunsaturated fatty acids (eicopentaenoic acid, docohexaenoic acid, arachidonic acid) and have protein content up to 47% of total biomass. 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. Dulse (Palmaria palmata) is one of the most consumed red algae and is a source of iodine, protein, magnesium and calcium. Red algae's nutritional value is used for the dietary supplement of algas calcareas. China, Japan, Republic of Korea are the top producers of seaweeds. 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. Animal feed Researchers in Australia discovered that limu kohu (Asparagopsis taxiformis) can reduce methane emissions in cattle. In one Hawaii experiment, the reduction reached 77%. The World Bank predicted the industry could be worth ~$1.1 billion by 2030. As of 2024, preparation included three stages of cultivation and drying. Australia's first commercial harvest was in 2022. Agriculture accounts for 37% of the world's anthropogenic methane emissions. One cow produces between 154 and 264 pounds of methane/yr. Other Other algae-based markets include construction materials, fertilizers and other agricultural inputs, bioplastics, biofuels and fabric. Red algae also provides ecosystem services such as filtering water and carbon sequestration. == Gallery ==