Microbe plants and animals
Ponds are interesting places. You usually find many different plants growing around the water's edge and if you sit quietly you may see some animal life too. The plants have sent out their green leaves and present them to the sunlight. Inside the leaves the green substance traps some of the sunlight's energy and uses it to make food. There are holes on the underside of the leaves and they take carbon dioxide - one of the raw materials for making food. The other materials, water and minerals, are taken up by the plant roots. Once the food is made it may be used to help the plant grow or to make flowers or seeds or it may not be used by the plant at all.

Plants provide food for animals. If you stay by the pond side for some time you may see a water vole swim through the water and climb onto the bank. The water vole looks a little like a brown rat but it does not have a pointed face or long tail. This water vole pulls down a leaf and nibbles it. The animal is taking the food that the plant made for itself. While the water vole is eating it does not see a large bird alight in the reeds then slowly stride out into the water on its long legs. The bird is a heron and it is looking for something to eat. When the water vole has finished eating, it swims out into the pond, fails to see the heron and becomes the heron's first meal of the day.

It can be easy to watch the plants and animals at a pond and see how they behave and relate to each other. Down in the pond water itself, beyond the range of normal vision, similar activities are taking place on a microscopic scale. Floating in the water are tiny plants called algae. They glisten with the green in their transparent bodies. Each body is made from only one cell. On the outside of the body are spines, which help to hold it near the sunlit surface. When the sunlight strikes the green substance in the algae some of the sunlight is trapped just as in the larger plants on the side of the pond. The algae are surrounded by the raw materials they need to make food. They have all the water they need and dissolved in the water are carbon dioxide and minerals - the other vital ingredients in the food-making process. The algae take in the water and the other raw materials and make their food. They use it to grow and breed. As the food is made the algae also make a second substance, just like the plants in the pond. This substance is oxygen. It dissolves in the water and spreads out through the pond. Animal life, such as fish and snails, depends upon oxygen for its survival.

While the algae seem to behave like miniature versions of the plants, there are creatures which behave like miniature versions of the animals. They are called protozoans. Like algae they have bodies made from a single cell but unlike algae none of them possesses the green substance for making food. This means that they must feed on other living things. Many protozoans feed on algae. They behave in a similar way to the water vole that eats plants. Some protozoans feed on other protozoans. They behave like the heron that ate the water vole. However there is a difference between these carnivorous protozoans and the heron. The heron has few predators while the carnivorous protozoans have many. In fact both the algae and the protozoans are eaten by many other kinds of animals. Some of these animals, like insects and worms, are large enough for us to see. These animals in turn are eaten by small fish, which may be eaten by larger fish. Finally a large fish may become a meal for a heron.

Next time you visit a pond and see animals in the water or on the bank, think about their food and where some of it really came from. It formed in the tiny bodies of the algae floating unseen near the surface of the water.

Why are algae put in a different group to other plants?
A plant has a root, stem and leaves. Algae have none of these so they cannot be called plants. That is why they are put in their own group.

What have algae got in common with plants?
They have the same green substance, called chlorophyll, which is used to trap energy from sunlight. Algae also take in carbon dioxide, water and minerals and use them as raw materials to make food - just as plants do.

Are there any algae large enough for us to see?
Yes, there are. Some algae form long threads called filaments. They can be seen growing on stones in rivers, lakes and ponds. The largest algae are the seaweeds that you can find growing on a rocky shore.

Don't seaweeds have roots to hold them onto the rocks?
No. They have a structure which looks like a root. This structure is called a holdfast and it allows the seaweed to grip onto the rock. It does not take in water and minerals like the root of a plant.

Are microscopic algae found in the sea?
Yes, they are. They make all the food for the animals that live in the open oceans. The algae are eaten by protozoans, tiny shrimps and the young stages of crabs, lobsters, starfish and sea snails. These in turn are eaten by larger shrimps and small fish. The collection of algae and the small animals that feed on it is called the plankton. The small fish are eaten by larger fish and the large shrimps are eaten by whales.

Are algae found in all parts of the ocean?
No. They are mainly found near the surface so they can use the sunlight to make food. As sunlight shines down into the sea it is absorbed by the water. This means that as you go deeper and deeper into the water it grows darker and darker. Eventually there is too little light for algae to make food. Algae have spines on their bodies to help keep them near the sunlit surface. The spines give the algae a larger surface area to catch upward moving water currents. This stops them from sinking into the dark depths where they would die.

Are algae found on land?
Yes, they are, but the surface must be damp. If you look at tree trunks you may see that some surfaces remain damp for most of the year. On these surfaces you will see a bright green coating. This is made by the bodies of thousands of algae.

How do protozoans move?
Many protozoans have a rigid surface covered in tiny hairs. The hairs move backwards and forwards like the oars on a Viking boat and move the animal along. Some protozoans do not have a rigid surface but one that can be pushed and pulled into any shape just like the surface of a balloon filled with water. These protozoans make the liquids inside them push on the surface to make them flow along.

How do protozoans feed?
The ones with a rigid surface have a soft spot in it called a mouth. Some of the hairs around the mouth whisk the water in such a way that it flows into the mouth. Algae in their water pass through the soft spot in the surface and are enclosed in a bubble of water inside the animal. The animal then releases chemicals called enzymes into the bubble, which break down the algae. When the algae have been broken down its soluble substances are taken into the animal's body.

The animals with a soft surface make arms, which flow round their food and make a bubble. They then digest their food with enzymes like the hairy protozoans.

Do protozoans live on land?
They will live in damp places with algae.

Do protozoans live in other animals?
Some of them do. They obtain all the nourishment they need from the animals they are living in. When they are full-grown they will breed. If there is a large number of individual protozoans in an animal it will cause disease. Diseases in humans caused by protozoans are malaria, sleeping sickness and amoebic dysentery.