Red Algae Rhodophyceae: Structure, Pigments and Uses

Red algae, the class Rhodophyceae, are a group of mostly marine algae coloured red by the pigment phycoerythrin, along with phycocyanin. They contain chlorophyll a and d, store food as floridean starch, and have cell walls containing agar and carrageenan.

Red algae matter because their red pigments absorb blue-green light, letting them photosynthesise in deeper water than most other algae. Economically, they are the source of agar and carrageenan, two of the most widely used natural gelling agents in food and laboratories worldwide.

Key Concepts

Most red algae are multicellular and many are delicate and feathery, though some like Porphyridium are unicellular. Well-known examples include Porphyra (used as nori), Gracilaria and Gelidium (agar sources), and Chondrus crispus (Irish moss, a carrageenan source).

• Phycoerythrin: the red pigment that captures light in deep water where red light cannot reach.
• Floridean starch: the storage carbohydrate of red algae, stored outside the chloroplast.
• Agar / carrageenan: sulphated polysaccharides in the wall, extracted for industry.
• No flagella: uniquely among algae, red algae produce no motile cells at any stage of life; even their male gametes (spermatia) cannot swim.

How It Works

Red algae reproduce both asexually by spores and sexually by oogamy. Their life cycles are often complex, with an alternation of three generations in many species: the gametophyte, a small carposporophyte that grows on the female plant after fertilisation, and the tetrasporophyte. Because their spermatia cannot swim, fertilisation depends on water currents carrying them to the female organ, the carpogonium, which has a long receptive hair called a trichogyne.

Worked Example

Agar from red algae is used to solidify microbiological culture media, typically at 1.5% weight per volume. How much agar powder is needed to prepare 1 litre of nutrient agar medium?

1. 1.5% w/v means 1.5 g of agar per 100 mL of medium.
2. 1 litre = 1,000 mL, which is 10 x 100 mL.
3. Agar needed = 1.5 x 10 = 15 g.

Answer: 15 grams of agar per litre. This single application consumes thousands of tonnes of red algae every year.

Common Misconceptions

• Misconception: Red algae are red because of pollution. Correction: The red colour is the natural pigment phycoerythrin, an adaptation for deep-water photosynthesis.
• Misconception: Agar comes from animal bones like gelatin. Correction: Agar is purely plant-like in origin, extracted from the red algae Gracilaria and Gelidium.
• Misconception: All red algae live in the deep sea. Correction: Many grow in shallow water and tide pools; only some extend into deep zones.

Real-World Applications

• Agar solidifies bacterial culture media, makes vegetarian jelly, and coats pharmaceutical capsules.
• Carrageenan thickens dairy products, ice cream and toothpaste.
• Porphyra (nori) is farmed on a huge scale for sushi wraps in Japan, Korea and China.
• Red algae extracts are used in cosmetics and as laboratory reagents.

Red algae (Rhodophyceae) are marine algae coloured by phycoerythrin, storing floridean starch and building agar-rich walls, with no swimming cells at any life stage. They reproduce by spores and complex oogamous life cycles. They give the world agar, carrageenan and nori, making them among the most economically valuable algae.