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FIND OUT MORE. Life on Earth 70–71

Algae

Algae are plant-like organisms that make food

by photosynthesis. All algae contain chlorophyll,

which is green, but many are coloured brown or

red by other pigments. Most algae live in water.

ARE ALGAE AND SEAWEED THE SAME THING?

All seaweeds are algae, but not all algae are seaweeds.

Seaweeds are easily visible, made of many cells, and

grow in the sea. There are red, brown, and green

forms. Many algae, such as diatoms, are microscopic

and consist of a single cell.

A lichen is not a single organism, but a

combination of a fungus and a green alga.

Lichens exist in extreme climates, from dry

deserts to the icy Arctic. They grow on

surfaces such as rocks, bark, and soil.

HOW DO FUNGI AND ALGAE LIVE IN LICHENS?

The fungus forms an outer layer that protects the alga

beneath from drying out and from harmful amounts

of light. The alga makes its own food by

photosynthesis, and shares it with the fungus. In

return, the fungus supplies the alga with essential

minerals, such as nitrogen. This beneficial relationship

between two different species is called symbiosis.

1 REINDEER EATING LICHEN
In the cold, harsh conditions of
the Arctic tundra, lichens are an
important source of food for
grazing reindeer. In winter, they
reach lichens by scraping away
snow with their hooves.

1 LICHENS

Two of the three main

types of lichen – fruticose (shrubby)

and foliose (leafy) – are seen here growing on

tree barks. The third type is crustose (flat and crusty).

4 BLADDER WRACK

This seaweed, like many others, grows

close to the seashore. Its leaf-like fronds

are anchored to rocks by a root-like

structure, or holdfast. Slimy

mucus stops the fronds from

drying out when they are

exposed at low tide.

DIATOMS 3

These single-celled

algae float in

the surface waters of

oceans and lakes. Each

species has its own

distinctively patterned,

glass-like outer casing.

FIND OUT MORE. Life on Earth 70–71 • Microorganisms 85

ALGAE CLASSIFICATION

40,000 species of plant-like

organisms. Single-celled types:

• Golden algae • Yellow-green
  algae • Diatoms • Dinoflagellates.

Multicellular types:

• Brown seaweeds • Red

seaweeds and green algae (both

seaweeds and single-celled forms).

1 RED SEAWEED

Smaller than other seaweeds, red

seaweeds contain a pigment that

lets them photosynthesize in the

dim light of deeper waters.

DO ALGAE LIVE

ONLY IN WATER?

Most species of algae live in

the sea, in lakes, or in ponds. Some

single-celled green algae live in moist

conditions on land, such as on tree trunks,

the surface of the soil, or on damp

brickwork. Others live inside lichens.

Air bladders keep fronds

floating near sea’s surface

FRUTICOSE

LICHEN

LICHENS

BLADDER WRACK

FOLIOSE

LICHEN lichens

algae