Rhodophyceae: Description, Characteristics and Classification

In this article we will discuss about:- 1. Description of Rhodophyceae 2. Characteristics of Rhodophyceae 3. Classification.

Description of Rhodophyceae (Red Algae):

It is a large group of algae consisting of about 831 genera and over 5;250 species. They are commonly known as red algae due to the presence of a water soluble red pigment, r- phycoerythrin. The r-phycoerythrin is, however, present sufficiently and completely to mask the chlorophyll a, giving the characteristic red colouration. More than 98% members are marine and the rest grow in fresh water.

The fresh water members grow in stagnant water (e.g., Asterocystis, Compsopogon etc.) as well as in flowing water (e.g., Lamanea, Thorea, Batrachospermum etc.). The marine species have the ability to live at greater depth (even at 30-90 meters) than the other members of different classes. They also exhibit a high degree of para­sitism and epiphytism. The parasitic members show great reduction in their size and pigmenta­tion.

Some parasitic members are Ceramium condicola on Codium fragile, Polysiphonia lanosa on Ascophyllum nodosum etc. The epi­phytic members like Rhodochorton, Ceratocolax etc. grow on other members of Rhodophyceae. Porphyridium, a unicellular member, is terres­trial and grow on damp soil. Marine members commonly grow in sublittorial zone, but a few members like Rhodocorton, Corallina and Bostrychia grow in intertidal zone.

Characteristics of Rhodophyceae (Red Algae):

1. Most of the members (more than 98%) are marine and 20 species (as per report) grow in fresh water. The members may grow either as saprophytes, parasites and also as epiphytes.

2. The plant body may be unicellular (Porphyridium) or multicellular. The multi­cellular form may be filamentous (Gonio- trichum), parenchymatous (Porphyra, Crinellia), pseudoparenchymatous (Helmin- thocladia), feathery (Polysiphonia) or ribbon like (Chondrus) (Fig. 3.130).

3. They do not attain the size like that of the brown algae (Phaeophyceae), but may reach up to 2 meters in Schizymenia (Abbott, 1967).

4. The flagellated motile stages are totally absent.

5. The cell wall consists of outer pectic and inner cellulosic layer. The mucilaginous material of the outer layer mainly consists of agar-agar and carrageenans and constitute major portion of dry weight of the cell wall.

6. In multicellular forms, the cell walls have pits, through which cytoplasmic connec­tions are maintained. These cytoplasmic threads are the so-called plasmodesmata. (E.M. studies have not confirmed it.)

7. The members of Rhodophyceae show much variation in the number of nuclei in a cell. In the subclass Bangioideae, cells are uni­nucleate, but in the subclass Florideae most of the members are multinucleate and the number of nuclei is 3,000-4,000 in Griffithsia.

8. The cells may have one chromatophore with a central pyrenoid (Bangioideae) or many discoid and parietal chromatophores with pyrenoids (Florideae).

9. The photosynthetic pigments are chlorophyll a, chlorophyll b; α- and β-carotene; xantho- phylls like teraxanthin, lutein, violaxanthin, zeaxanthin, flavoxanthin and biliproteins such as r-phycoerythrin and r-phycocyanin. The characteristic red colouration of the algae is due to the sufficient presence of r-phycoerythrin which completely masks the chlorophyll a. [The chief Xanthcphyll is teraxanthin and chlorophyll b is absent.]

10. The reserve food is floridean starch, floridi- side and mannoglycerate.

11. Reproduction takes place by all the three means: vegetative, asexual and sexual.

a. Vegetative reproduction takes place only in unicellular form.

b. Asexual reproduction takes place by monospore, neutral spore, carpospore, bispore, and tetraspore.

c. Sexual reproduction is of advanced oogamous types.

i. The male sex organs are known as spermatangium. Single non- flagellate male gamete is produced in each spermatangium, called spermatium.

ii. The female sex organs are called carpogonia or procarp. Carpogonia are flask-shaped with a long neck, the trichogyne.

12. During fertilisation, the spermatium comes in contact with the trichogyne with the help of water current.

13. In Rhodophyceae the post-fertilization changes are highly elaborate. They develop carposporophyte. Carposporangia are developed from each carposporophyte and each carposporangium produces single carpospore.

14. Most of the Rhodophycean members show biphasic or triphasic life cycle patterns.

Classification of Rhodophyceae (Red Algae):

Fritsch (1935, 45) classified the Class. Rhodophyceae into two sub-classes and seven orders.

The outline of the classification is given:

Class. Rhodophyceae

Sub-class Bangioideae

Order. 1. Bangiales

Sub-class Florideae

Order. 1. Nemalionales

2. Gelidiales

3. Cryptonemiales

4. Gigartinales

5. Rhodymeniales

6. Ceramiales