 |
| Red algae |
Red algae, from the phylum Rhodophyta (from the Greek rhodos, meaning “red”), are named for their reddish color because of their pigment phycoerythrin. Despite their name, not all rhodophytes are red.
Some of them have very little phycoerythrin and appear green or bluish. Most are multicellular and vary greatly in shape; platelike, coral-like, crustlike, leathery, and feather like forms are known.
The bodies of some rhodophytes are relatively complex, characterized by a great deal of branching of their leaf like structures as well as the presence of a holdfast that resembles plant roots.
    |
These rhodophytes are commonly known as seaweeds. Red algae are distinctive from other eukaryotic algae in that they lack flagella (ormotile cells of any kind) in their vegetative cells, spores, and gametes.
There are four thousand to six thousand species of red algae, and although some rhodophytes do inhabit freshwater (about fifty species), red algae are most common in tropical marine environments. Many are found at great depths, living 210-260 meters below the surface of the ocean.
The pigment phycoerythrin allows red algae to live and photosynthesize at these depths. Phycoerythrin absorbs blue light, which penetrates water to a greater depth than light of longer wavelengths normally used in photosynthesis. Reds are the deepest-growing photosynthetic eukaryotes.
Habitats
Rhodophytes are important members of many periphyton communities (which typically grow attached to substrata) from tropical to polar seas. Epiphytismis common among red algae. Epiphytic rhodophytes growon the surface of larger, typically brown, algae. Although their hold fasts penetrate the tissue of their hosts, they do not obtain any nutrients from host algae.
Nearly 15 percent of red algae are parasites, often living on closely related species. Parasitic reds transfer nuclei into host cells and transform them. Host reproductive cells may then carry the parasite’s genes.
Other Rhodophyta are extremophiles. For example, the red alga Cyanidium lives in acidic (pH 2-4) hot springs at 55 degrees Celsius. Some calcified rhodophytes are major contributors to the formation of tropical reefs. Reef-building red algae are called coralline algae.
Structure and Properties
 |
| Red algae on a shallow reef |
A red algal cell is surrounded by a cell wall. In many species, the main wall component is cellulose (similar to cell walls of various other algae and plants), but other reds have mannans (polymers of mannose) and xylans (polymers of xylose). Other polymers associated with the cell walls include agar and carrageenan.
The majority of coralline red algae contain calcium carbonate, which forms limestone in the cell walls. Because of their ability to secrete calcium carbonate, red algae do not decay and have a better preserved fossil record than many other algae.
Fossils of red algae have been found in rocks 500 million years old. Production of calcium carbonate is linked to photosynthetic carbon fixation. Apparently, carbon dioxide fixation results in a pH increase (an increase in alkalinity), which facilitates calcium carbonate precipitation.
An unusual feature of red algae compared to other algae is the occurrence of protein plugs (pit connections) in cell walls between the cells, although some red algae lack them.
All plugs consist of protein, and in some species protein polysaccharides are an additional component. Cells of the Rhodophytamay contain several nuclei as a result of either the fusion of nongamete cells ormitosis without cytokinesis.
      |
Cell fusion is a very important feature of parasitic reds. Red algae lack centrioles, but the mitotic spindle radiates from the “nuclear associated organelle,” which often appears as a pair of short, hollow cylinders. Some red algae have large vacuoles in the centers of their cells.
Cells of the Rhodophyta may produce mucilage, which plays an important role in the attachment of their reproductive cells. Mucilages are polymers of D-xylose, D-glucose, D-glucuronic acid, and galactose and are produced within Golgi apparatuses.
Pigments of red algae include chlorophyll a and two classes of accessory pigments: phycobilins and carotenoids. Phycoerythrin, phycocyanin, and allophycocyanin are phycobilins.
They attach to proteins known as phycobiliproteins, which occur in highly organized structures called phycobilisomes. Phycoerythrin occurs in at least five forms in the Rhodophyta (B-phycoerythrin I and II, R-phycoerythrin I, II, and II). Carotenoids are also found in plants, and those in red algae are similar in structure and function.
Some parasitic forms of red algae lack photosynthetic pigments. In red algae that have pigments, all pigments are located in the chloroplasts. Red algae chloroplasts have a highly distinctive ultrastructure. Two membranes surround each chloroplast.
Chloroplasts of red algae probably originated from cyanobacteria that formed an ancient symbiotic relationship with the reds. Both red algal chloroplasts and cyanobacteria share same phycobilin pigments. Inside the chloroplast are thylakoids, which are not stacked.
This is the same arrangement found in cyanobacteria, but it is different from that of other algae and from plants. On the thylakoid surface there are many phycobilisomes. Some red algae have chloroplasts that contain pyrenoids, which have no known function.
Photoautotrophy is the principal mode of nutrition in red algae; in other words, they are “self-feeders,” using light energy and photosynthetic apparatuses to produce their own food (organic carbon) from carbon dioxide and water.
A few Rhodophyta are heterotrophic, and these organisms are generally obligate parasites (parasites that must live off a host) of other algae. Carbon and nitrogen metabolism in red algae is similar to that in other algae.
Various rhodophytes produce unusual carbohydrates, such as digeneaside,which is used to regulate osmotic status of cells in response to drought stress in shoreline environments. Some red algae are covered by surface-protein cuticle, which is different from that found in higher plants.
The food storage of red algae is a unique polysaccharide floridean starch. This starch differs from that synthesized by green algae and plants. Floridean starch grains are formed in the cytoplasm. Red algae store inorganic nitrogen in the form of phycobilin pigments.
Reproduction
 |
| Red algae on the beach |
Red algae reproduce both asexually and sexually. Methods of asexual reproduction include discharging spores and fragmentation of the algal bodies. Sexual reproduction, as well as alteration of generations, is widespread among the Rhodophyta, but two classes of red algae (floridean and bangean) have particular variations.
In contrast to the two phases in an alteration of generations of other algae and plants (gametophyte and sporophyte, haploid and diploid stages, correspondingly), most species of red floridean algae have three phases: free-living, haploid gametophytes, diploid carposporophytes, and diploid tetrasporophytes. Male and female gametophytes are often separate.
The male gametophytes produce male nonflagellated gametes called spermatia. Female gamethophytes produce a special branch, the carpogonial branch, that produces a terminal carpogonium (oogonium, an egg-bearing structure).
Contact between spermatia and carpogonia is facilitated bywatermovements. The carposporophyte is a diploid stage that develops fromthe zygote (fertilized carpogonium) and produces carpospores.
Diploid tetrasporophytes develop from carpospores. Tetrasporophytes form tetrasporangia, which produce four haploid tetraspores. When released, tetraspores develop into new gametophytes.
The gametophyte and tetrasporophyte may appear nearly identical, and therefore can be said to be isomorphic, as in the Polysiphonia. Alternatively, the tetrasporophyte and gametophytemay be very different in size and appearance (heteromorphic), as in Phyllophora.
Diversity of Red Algae
Red algae are divided into two subclasses or classes: Florideophyceae (florideophyceans or floridean) and Bangiophyceae (bangiophyceans or bangean). Floridean algae have numerous small chloroplasts and a complex life cycle.
Bangean algae have life cycles without carpogonia and carposporophyte development and have a single central chloroplast. Representative species of Florideophyceae are Batrachospermum, Chondrus, Corallina, Gelidium, and Polysiphonia. Representatives of Bangiophyceae include Porphyra, Bangia, and Cyanidium.
Uses
People have used red algae for thousands of years. Most are collected along seashores for use in human food or for the extraction of gelling compounds.
A few red algae, such as Porphyra, Eucheuma, and Gracilaria, are cultivated. More than 60,000 hectares of sea along Japanese coasts are occupied by “red algal culture.” Thousands of people worldwide are engaged in cultivating red sea weeds.
The most valuable of all algae is Porphyra. The annual Porphyra harvest worldwide has been estimated to be worth 2.5 billion dollars. Porphyra (in Japanese, Nori; in Chinese, Zicai) is used as a wrapper for sushi or may be eaten mixed with rice and fish and in salads. It is very rich in vitamins B and C as well as minerals, including iodine.
There are about seventy species of Porphyra, but the most widely used species is Porphyra yezoensis. Two important compounds derived from red algae are agar and carrageenan, both of which are polymers of galactose.
Agar is used as a medium for culturing microorganisms, including algae; as a food gel (for jams and jelly); and in pharmaceutical capsules. In the United States, agar is used in the canning industry as a protective agent against the unwanted effects of metals.
In addition, agar is the source of agarose, which is widely used in recombinant DNA (deoxyribonucleic acid) technology for gel electrophoresis. The first agar was produced in 1670 in Japan, and Japan is still the largest producer of agar.
The red algae Gelidium, Gracelaria, and Pterocladia are harvested for extraction of agar. Carrageenan is used in toothpaste, cosmetics, and food, such as ice cream and chocolate milk.
Eucheuma, Kappaphycus, and Chondus (the so-called Irishmoss) are the sources of carrageenan. The most important producer of carrageenan is Europe, followed by the Philippines and Indonesia.
