What are algae?
Algae are living things that can make their own food. They use sunlight, water and air for this. This is called photosynthesis. They have chlorophyll, the green stuff in leaves.
Algae have no true root, stem or leaf. The whole body is called a thallus. Most live in water: ponds, rivers and the sea. Some live on wet soil, rocks or tree bark.
Algae are simple, but they are not tiny only. Some are one cell. Some are a long chain. Some, like kelp, are many metres long.
Green algae
Green algae look like the plants we know because they have the same chlorophyll. They mostly live in fresh water, like ponds.
- Chlamydomonas: one cell, swims with two tails.
- Spirogyra: a thin thread (filament) with a spiral green ribbon in each cell. It feels slimy.
- Ulva (sea lettuce): a thin flat sheet like a leaf.
Their food is stored as starch, just like in land plants.
Brown and red algae
Brown algae live in cold seas. They have a brown pigment (fucoxanthin) that hides the green. Kelp and Sargassum are examples. Some have small air bladders that keep them floating up towards light. Their food is stored as a sugar called laminarin or as mannitol.
Red algae mostly live in warm seas, even deep ones. A red pigment (phycoerythrin) helps them catch the weak blue light that reaches deep water. Examples are Gracilaria and Porphyra. Agar comes from red algae.
How algae multiply (reproduction)
Algae multiply in three ways.
- Fragmentation: a filament breaks into pieces. Each piece grows into a full new alga. Spirogyra does this a lot.
- Spores: tiny cells (often with tails) leave the parent, swim, and grow into new algae. Chlamydomonas and many others make spores.
- Sexual reproduction: two cells join. In Spirogyra two filaments lie side by side, and the contents of one cell flow into the cell of the other filament. This is called conjugation. It forms a thick-walled zygote that can survive dry times.
Why algae matter (role)
Algae are very useful.
- They make a large part of the world's oxygen.
- They are the first link of food chains in water. Small animals eat them and fish eat those.
- People eat some (nori, kelp) and use them to make agar and algin (thickener in ice cream).
- Some are used as fertiliser and fuel research.
But too many algae (an algal bloom) can use up the oxygen in a pond when they die and rot. Then fish die.
Try it
Put a little pond water in a clear glass jar with a lid. Keep it near a bright window for a week. Watch the water turn greener. Now put another jar in a dark cupboard. Predict first: which jar will turn green? Then check. You will see algae need light to grow.
Key formulas and definitions
- Algae = make food by photosynthesis, no true root/stem/leaf (body = thallus)
- Green: chlorophyll, store starch, mostly fresh water (Spirogyra, Ulva)
- Brown: fucoxanthin, cold seas (Kelp, Sargassum)
- Red: phycoerythrin, warm and deep seas (Gracilaria, Porphyra); gives agar
- Reproduction: fragmentation, spores, conjugation (sexual)
Worked examples
1. A pond turns green in summer. Why?
Warm water and strong sunlight help algae grow fast. Many algae floating together make the water look green.
2. Spirogyra is cut into 4 pieces with a blade. What happens after some days?
Each piece can grow into a new filament. This is fragmentation. So we may get 4 new filaments.
3. Which alga would you find deep in a warm sea, and which pigment helps it?
A red alga. Its pigment phycoerythrin catches the blue light that reaches deep water.
Common mistakes
- Thinking algae are plants with roots and leaves. They have a simple body called a thallus.
- Thinking brown algae have no chlorophyll. They do have it, but fucoxanthin hides it.
- Thinking all algae live in the sea. Many green algae live in fresh water.
- Thinking algae are always good. An algal bloom can kill fish by using up oxygen.