In this article we will discuss about:- 1. Characters of Red Algae 2. Types of Common Red Algae 3. Economic Importance.
Characters of Red Algae:
Red algae are defined as eukaryotic algae which possess chlorophyll a, phycobilins, floridean starch as food reserve, abundant phycocolloids (like agar, carrageenin, and funori) but lack flagellate cells.
1. Red algae/rhodophytes are an ancient group of algae with 5000 living species.
2. They are marine except for a few fresh water species (e.g., Batrachospermum).
3. Red algae are autotrophic with the exception of a few like Harvey Ella which are colourless and parasitic on other red algae.
4. A motile or flagellate stage in the life cycle is absent.
5. The plant body varies from unicellular (e.g., Porphyridium), filamentous (e.g., Spermothamnion), pseudoparenchymatous (Asterocystis), parenchymatous sheets (e.g., Porphyra), ribbons (e.g., Chondrus) to graceful lace-like complex multicellular sea weeds (e.g., Gelidium).
6. Cell wall possesses cellulose, pectic compounds and certain mucopolysaccharides called phycocolloids (e.g., agar, carrageenin and funori). The latter are usually sulphated.
7. Some red algae have an incrustation of calcium carbonate over their walls. They appear coral-like and are called coralline (e.g., Corallina). Coralline algae produce limestone. They are important component of reef formations along with corals.
8. The photosynthetic organelles called chromatophores, have un-stacked or single thylakoids.
9. Photosynthetic pigments include chlorophyll a, carotenoids and phycobilins. Chlorophyll d has been reported in some cases. Phycobilins are water soluble pigments of two types, red coloured phycoerythrin and blue-coloured phycocyanin as well as allophycocyanin. Similar pigments also occur in cyanobacteria (blue-green algae).
10. The red colour of red algae is due to abundant formation of phycoerythrin. Phycoerythrin is able to absorb blue green wavelengths of light. Being shorter, these wavelengths are able to reach the maximum depth in water.
Therefore, red algae reach the maximum depth in sea where no other types of photosynthetic organisms grow. However, rhodophytes living in shallower waters do not appear reddish due to lesser synthesis of phycoerythrin.
11. Reserve food is floridean starch. In constitution, it is very much similar to glycogen.
12. Asexual reproduction takes place through a variety of spores—neutral spores, monospores, tetraspores, carpospores, gemmae.
13. Sexual reproduction is oogamous. The male sex organ is called spermatangium or antheridium. It produces non-flagellate male gamete known as spermatium. The female sex organ is flask-shaped and is termed carpogonium.
Carpogonium possesses an elongated receptive organ named trichogyne. Spermatia are carried by water currents to trichogyne tips for effecting fertilization. The female sex organ remains attached to the plant and forms a new structure called carposporophyte.
14. Multicellular forms have an alternation of haploid and diploid generations.
Types of Common Red Algae:
1. Gelidium:
It is a stiff cartilaginous pinnately branched agar yielding red alga that is attached to the substratum by a number of rhizoids. Appearance is lace-like. Gelidium is the major source of agar which is obtained from cell wall. Agar has been under manufacture in Japan since 1760.
2. Porphyra:
It is an edible flat thalloid marine red alga. The thallus is made of 1-2 layers of cells covered on the outside by a cuticle of solidified gel. Asexual reproduction occurs by means of neutral spores. Sexual reproduction produces a diploid zygote which on. Meiosis forms haploid carpospores.
Each carpospore grows into a filamentous structure called conchocelis stage. The stage may multiply by means of monospores. Ultimately conchocelis stage produces the typical flat parenchymatous thallus.
3. Batrachospermum:
It is a fresh water filamentous rhodophycean alga, commonly called frog spawn alga. The colour depends upon depth at which the alga occurs— blue-green to purple, violet and pink. The filament gives a branched beaded appearance. The beads occur in the region of nodes where whorls of short branches or glomerules occur.
The alga multiplies asexually by mono-spores. Male sex organs are called spermatangia while female sex organs are called carpogonia. Meiosis occurs immediately after fertilization.
A haploid carposporophyte or cystocarp is formed. It produces carpospores. A carpospore forms a highly branched filamentous chantransia stage. It is juvenile stage which can multiply by mono-spores. The adult alga grows over the chantransia stage.
4. Gracilaria:
It is an agar yielding red alga (agarophyte) which grows in lagoons. The thallus is branched, cartilaginous, cylindrical or compressed cylindrical. Plants are unisexual.
5. Polysiphonia:
It is a small upright bushy marine alga with feathery multi-axial structure. The plants are fixed to the substratum by rhizoids, holdfast or prostrate system. The plant bears two types of branches, dwarf and long. Dwarf branches are trichoblasts. They develop sex organs, antheridia on male plants and carpogonia on female plants.
Cells show pit connections. Fertilization produces diploid cystocarp or carposporophyte. The latter bears diploid carpospores. On germination each carpospore forms a tetra sporophyte which resembles the gametophytic plant body in morphology.
Tetra sporophyte develops haploid tetraspores. A tetraspore germinates to produce gametophytic plant body. Life cycle is diplo-diplohaplontic and triphase with one gametophytic (n) and two sporophytic (2n) phases.
Polysiphonia has been one of the sources of bromine. 
Economic Importance of Red Algae:
1. Food:
A number of red algae are edible, e.g., Porphyra (Laver), Rhodymenia (Dulse), Chondrus (Irish Moss). Rhodymenia (also called sheep’s weed) is also used as fodder. Porphyra is cultivated in Japan for commercial exploitation.
2. Phycocolloids:
A number of phycocolloids are extracted for commercial use. They include agar, carrageenin and funori. Agar is used in solidifying laboratory culture media and is added as stabilizer or thickener in the preparation of jellies, puddings, creams, cheese, bakery, etc. Agar is obtained from cell wall of Gelidium and Gracilaria.
Carrageenin is used as a clearing agent in liquors, leather finishing and as emulsifier in chocolates, ice-creams, and tooth pastes, paints, etc. It is extracted from Chondrus. Funori is glue used as adhesive and in sizing textiles, papers, etc. It is got from Gloiopeltis.
3. Bromine:
Bromine is obtained from some of the red algae, e.g., Rhodomela Polysiphonia.
4. Medicines:
Corallina is capable of curing worm infections. Polysiphonia has antibacterial properties. Agar is laxative. Carrageenin can coagulate blood.