The commonest living things are bacteria. They are too small to see without a microscope. Bacteria are all around us in their billions. They are found in every habitat on Earth: soil, rock, oceans and snow. Vast numbers of bacteria live in the human body, especially on the skin and in the intestines. Most are made harmless by the immune system. In fact, we could not survive without bacteria, as we could not digest most foods or produce vitamins. However, a few kinds of bacteria can cause infectious diseases, such as tetanus and cholera.

Kinds of bacteria

There are around 10,000 known kinds of bacteria, but probably many more yet to be identified. Most measure about one to five microns (0.001 to 0.005 millimetres) across. They vary in form but there are three main shapes. These are: spheres or balls known as cocci, cylinders or rods, called bacilli, and corkscrew-like spirilli.

Most bacteria reproduce simply by splitting in two. Some bacteria get their energy from light, like plants. Others absorb nutrients through their cell membranes. Some kinds have a long, whip-like “tail” or flagellum to help them move. This they can do extremely quickly: a bacterium can swim at about 50–60 body lengths per second.

Bacteria in the soil

Many kinds live in the soil and play a vital role in nature because they cause the decay or rotting of dead plants and animals, allowing their nutrients to be recycled. These nutrients are taken up by plants, which provide food for animals. Soil-living bacteria, for example, carry out the vital task called nitrogen fixation. Nitrobacter, rhizobium and others convert nitrogen, a vital element which plants cannot absorb in its gaseous form, into nitrates, which plants are able to easily absorb.

Cyanobacteria, also known as blue-green algae, are bacteria that produce their own food using photosynthesis. The name comes from their colour (cyan is a greenish-blue). Cyanobacteria can be found in almost every habitat: oceans, freshwater, soil and bare rock. They can survive extreme conditions such as hot springs or high concentrations of salt. Aquatic cyanobacteria may form blooms (rapid accumulations) in warm, still ponds or calm seas, appearing as a blue-green scum. A single species of marine cyanobacteria, Prochlorococcus, accounts for more than half of the photosynthesis of the open ocean.

​​​​​​​Cyanobacteria were one of the earliest forms of life on Earth, probably in existence 2.7 billion years ago. By producing and releasing oxygen—a product of photosynthesis—cyanobacteria may have changed the composition of the Earth's atmosphere. Before 2.4 billion years ago, the atmosphere had only around 1% of today's oxygen levels. Scientists call this the Great Oxygenation Event. It had a dramatic effect on the evolution of life on Earth thereafter.