Bacteria in nature: nutrition, helpers and enemies

How do bacteria get their food?

Bacteria feed in many ways. Some make their own food, and some take it from others.

Autotrophic bacteria

Photosynthetic bacteria. The cyanobacteria (blue-green algae) have chlorophyll a, as green plants have. They are photosynthetic autotrophs. They can be single cells, colonies or filaments, in fresh water, in the sea and on land. Many colonies have a gelatinous sheath. Some, such as Nostoc and Anabaena, have special cells called heterocysts, which fix nitrogen from the air. Cyanobacteria often form blooms in polluted water.

Chemosynthetic bacteria. These do not use light. They oxidise inorganic substances such as nitrates, nitrites and ammonia, and use the energy to make their food. They play a great part in recycling nutrients such as nitrogen, phosphorus, iron and sulphur.

Heterotrophic bacteria

This is the largest group. Most heterotrophic bacteria are important decomposers: they break down dead plants and animals and return the nutrients to the soil.

Many of them help us:

• Curd. A little curd, with the bacterium Lactobacillus, turns milk into curd.
• Antibiotics. Some bacteria are used to make medicines.
• Nitrogen. Some bacteria fix nitrogen in the roots of legumes (pulses), and the soil becomes richer.

You should find that $2$ hectares of pulses fixing $100$ kg of nitrogen in each hectare save about $435$ kg of urea, nearly $10$ bags. This is why farmers grow pulses between other crops.

Some bacteria harm us. Pathogenic bacteria cause diseases. In humans: cholera, typhoid, tetanus. In plants: citrus canker.

Mycoplasma

Mycoplasma are the smallest living cells known. They have no cell wall. They can survive without oxygen. Many cause diseases in animals and plants.

One kind of cell, many ways of life

The kingdom Monera shows that one kind of cell can live in many ways: making food in light, making it from chemicals, feeding on the dead, or feeding on the living. This is why bacteria live in every place on Earth.

Try it in Python

This program counts the cells in a bacterial culture after some time. It uses the doubling of binary fission.

Practise with new numbers

Common mistakes

• Thinking all bacteria are harmful. Most are decomposers, and many help us.
• Calling cyanobacteria true algae. They are bacteria: prokaryotes with chlorophyll a.
• Mixing up chemosynthesis and photosynthesis. Chemosynthetic bacteria use energy from chemicals, not light.
• Giving Mycoplasma a cell wall. They have none.

Remember these

• Cyanobacteria make food by photosynthesis, and some fix nitrogen in heterocysts.
• Chemosynthetic bacteria get energy by oxidising inorganic substances, and recycle nutrients.
• Heterotrophic bacteria are the most abundant. They decompose, give us curd and antibiotics, and fix nitrogen in legume roots.
• Some bacteria are pathogens: cholera, typhoid, tetanus (in humans), citrus canker (in plants).
• Mycoplasma are the smallest cells, and have no cell wall.

Check yourself

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

1. Which cells of Nostoc fix nitrogen?

2. Name two diseases of humans that bacteria cause.

3. Which bacteria have no cell wall?

4. Which bacterium turns milk into curd?