Red Algae are an ancient group of algae with over 5,000 living species.
Majority of them are marine. A few (example- Batrachospermum, Lemnaea) are freshwater aquatic or subaquatic terrestrial (example- Porphyridium).
Their size varies from unicellular microscopic forms to half a metre in length. Several species of red algae are graceful and lace-like plants. Some red algae secrete CaCO3 on their walls and form coralline algae.
They are red in colour due to phycoerythrin and phycocyanin pigments. They have chlorophyll a molecule to capture the blue-green sunlight.
These are photo-autotrophic, but some others are colourless, lack photosynthetic pigments and live as parasites on other photosynthetic red algae (Example- Harveyella).
Reserve food is in the form of floridean starch and a soluble starch, floridoside.
Flagellate forms and stages are completely absent.
Cell Wall consists of cellulose and pectic compounds. Sulphated geloses or phycocolloids occur in most red algae. The important ones are agar, carrageenin and funori.
Reproduction occurs by both sexual and asexual methods. The spores are non-motile. Sexual reproduction is highly specialised. Male gametes are non-motile and called spermatia. Spermatia are carried by water currents to elongated tip trichogyne of female sex organ carpogonium. Following fertilisation, the zygote may produce carpospores. Example- Poryphyra by division directly or indirectly from zygote as in Batrachospermum and Polysiphonia.
The life cycle of many red algae like Polysiphonia has an alternation of haploid and diploid multicellular generations.
Economic Importance of Red Algae:
The red algae like Porphyra (Laver) and Chondrus (Irish Moss) are cultivated in different countries as a food crop for human consumption.
Rhodymenia and a number of other red algae are regularly collected on the seashore to feed the cattle.
Chondrus species yield an extract, the carrageenin. It is used in the preparation of laxatives, cosmetics and in making chocolate milk.
Agar a non-nitrogenous gel-like substance is obtained from red algae like Gelidium and Gracilaria which is used to make gelatin capsules, as a material for making dental impressions and as a base for cosmetics. It is used as a culture medium for bacteria, fungi and other organisms.
Agar prevents baked foods from drying out, and act as a preservative for meat and fish in warmer regions and used in dairy products such as ice creams, in pharmaceutical, leather and textile industries.
Calcareous red algae play an essential role in the formation of so-called coral reefs.
Funori is adhesive phycocolloid obtained from red algae Gloiopeltis. It is used in sizing textiles, paper and as glue.