Image: Intestinal Bacteria
- D. Colgan
- © Australian Museum
There are many forms of bacteria, which gain their energy in a variety of ways.
Some bacteria are autotrophic, making their own food in a similar way to plants by splitting carbon dioxide using energy from the sun, or through the oxidation of elements such as nitrogen and sulphur.
Bacteria involved in the decomposition of animal bodies are heterotrophic, breaking down complex molecules into their constituent elements through respiration or fermentation (depending on whether they are aerobic or anaerobic bacteria). Bacteria are largely responsible for the recycling of carbon, nitrogen and sulphur into forms where they can be taken up by plants.
For example, heterotrophic bacteria like Bacillus decompose proteins, releasing ammonia, which is oxidised by other bacteria into nitrogen dioxide, and eventually into nitrate. Nitrate can be assimilated by plants as a source of nitrogen.