Green, brown and red algae

Colour is a clue

Walk along a rocky shore at low tide. Near the top you see green weeds. Lower down they turn olive and brown, and in the deepest pools some are red. The colour is not just for show. It comes from the pigments that each alga uses to trap light, and it marks three classes.

Chlorophyceae: the green algae

• They may be single cells, colonies or filaments.
• They are grass green, from chlorophyll a and b, held in definite chloroplasts.
• The chloroplasts take many shapes: discoid, plate-like, reticulate (a net), cup-shaped, spiral or ribbon-shaped.
• Most have pyrenoids in the chloroplasts. These store protein and starch. Some algae store food as oil droplets.
• The cell wall is rigid: an inner layer of cellulose and an outer layer of pectose.
• Vegetative reproduction is by fragmentation. Asexual reproduction is by flagellated zoospores, made in zoosporangia. Sexual reproduction may be isogamous, anisogamous or oogamous.
• Examples: Chlamydomonas, Volvox, Ulothrix, Spirogyra and Chara.

Phaeophyceae: the brown algae

• They are found mainly in the sea.
• They range from simple branched filaments, such as Ectocarpus, to the huge kelps, which may reach $100\ \text{m}$.
• Their pigments are chlorophyll a and c, carotenoids and xanthophylls. The colour, from olive green to brown, comes from a xanthophyll called fucoxanthin.
• Food is stored as complex carbohydrates: laminarin or mannitol.
• The cell wall is of cellulose, usually covered outside by a gelatinous coat of algin.
• The body has a holdfast that grips the rock, a stalk called the stipe, and a leaf-like frond.
• Asexual reproduction is by pear-shaped zoospores with two unequal flagella attached at the side. Sexual reproduction may be isogamous, anisogamous or oogamous.
• Examples: Ectocarpus, Dictyota, Laminaria, Sargassum and Fucus.

Rhodophyceae: the red algae

• They are red because of the pigment r-phycoerythrin.
• Most are marine, with more of them in warm seas.
• They live near the surface, and also at great depths, where little light reaches.
• The thallus is mostly multicellular, and some have a complex body.
• Food is stored as floridean starch, which is very like amylopectin and glycogen.
• Asexual reproduction is by non-motile spores. Sexual reproduction is oogamous, with complex changes after fertilisation. No cell has flagella.
• Examples: Polysiphonia, Porphyra, Gracilaria and Gelidium.

Why red algae go deep

Water absorbs red light within a few metres, but blue and green light travel much deeper. Phycoerythrin absorbs blue-green light and passes its energy on to chlorophyll. So red algae can photosynthesise where green algae would starve.

You should find that at $10\ \text{m}$ only about $3\%$ of the red light is left, but more than half of the blue-green light.

Try it in Python

The program works like a key. It names the class of an alga from one pigment and its stored food.

Practise with new numbers

Common mistakes

• Giving brown algae chlorophyll b. They have chlorophyll a and c, with fucoxanthin.
• Saying red algae store ordinary starch. They store floridean starch.
• Giving red algae flagella. Their spores and gametes have none.
• Thinking all brown algae are small. Kelps can be about $100\ \text{m}$ long.

Remember these

• Green algae: chlorophyll a and b, starch in pyrenoids, cellulose wall; Chlamydomonas, Spirogyra, Chara.
• Brown algae: chlorophyll a and c, fucoxanthin, laminarin or mannitol, algin; Laminaria, Fucus, Sargassum.
• Red algae: r-phycoerythrin, floridean starch, oogamy, no flagella; Porphyra, Gelidium, Polysiphonia.
• Phycoerythrin lets red algae use the blue-green light of deep water.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which pigment makes brown algae brown?

2. What do pyrenoids store?

3. Name the three parts of the body of a large brown alga.

4. Why can red algae grow deeper in the sea than green algae?

5. Name a red alga that gives agar.