Where do you find lots of bacteria?
Practically all surfaces. Bacteria can live and grow in practically any environment. It is this ability that has made bacteria the most numerous species on the planet.
Prokaryotes have a wide range of metabolisms, and this determines where they live. They live in a particular habitat because they are able to “eat” whatever is around them or have the right conditions to make their own food. For example, there are bacteria and archaea that break down hydrogen sulfide to produce energy (ATP). Hydrogen sulfide is the gas that gives rotten eggs and sewage their distinctive smell. It is poisonous to animals, but some prokaryotes depend on it for life.
Organisms that are obligate aerobes need oxygen to live. Humans are obligate aerobes, and so are Mycobacterium tuberculosis bacteria. M. tuberculosis causes tuberculosis (TB). Obligate aerobes are found only in places with molecular oxygen.
An anaerobic organism is any organism that does not need oxygen for growth and may even die in its presence. Obligate anaerobes will die when exposed to atmospheric levels of oxygen. Clostridium perfringens bacteria, which are commonly found in soil around the world, are obligate anaerobes. Infection of a wound by C. perfringens bacteria causes the disease gas gangrene.
Facultative anaerobic organisms that can live in the presence or absence of oxygen. If oxygen is present, they produce ATP by means of cellular respiration, but in the absence of oxygen they can still produce ATP by the process of fermentation. Fermentation is a type of heterotrophic metabolism, Examples of facultative anaerobic bacteria are the Staphylococci, Escherichia coli, Corynebacterium, and Listeria species. Many bacteria that cause human diseases are facultative anaerobic organisms.
Like most organisms, prokaryotes live and grow best within certain temperature ranges. Prokaryotes can be classified by their temperature preferences, as shown in the Table below. Which type of prokaryote would you expect to find inside the human body?
Thermophiles live at relatively high temperatures, above 45°C (113°F). Thermophiles are found in geothermally heated regions of the Earth, such as hot springs like the Morning Glory pool in Yellowstone National Park (see Figure below), and deep sea hydrothermal vents. Some thermophiles live in decaying plant matter such as peat bogs and compost. Many thermophiles are archaea. Extreme thermophiles (or hyperthermophiles), live in temperatures hotter than 80°C (176°F).
Psychrophiles grow and reproduce in cold temperatures. The optimal growth temperature of some psychrophiles is 15°C or lower; they cannot grow in temperatures above 20°C. The environments that psychrophiles inhabit are found all over Earth. Psychrophiles live in such places as permafrost soils, deep-ocean waters, Arctic and Antarctic glaciers and snowfields.
Mesophiles grow best in moderate temperature, typically between 25°C and 40°C (77°F and 104°F). Mesophiles are often found living in or on the bodies of humans or other animals. The optimal growth temperature of many pathogenic mesophiles is 37°C (98°F), the normal human body temperature. Mesophilic organisms have important uses in food preparation, including cheese, yogurt, beer and wine.
|Type of Prokaryote||Preferred Temperature||Where It Might Be Found|
|Thermophile||above 45°C (113°F)||in compost|
|Mesophile||about 37°C (98°F)||inside animals|
|Psychrophile||below 20°C (68°F)||in the deep ocean|
The Morning Glory pool of Yellowstone National Park in the United States is a geothermal pool whose waters are heated to high temperatures by magma deep underground. Hyperthermophilic organisms, such as members of the archaeal genus Sulfolobus can live at temperatures between 60°C-80°C and a pH of 3.
Just like other organisms bacteria have an optimum pH at which they live. This often depends on the enzymes they produce to carry out life processes. Remember that enzymes have an optimum pH and deviation from that pH can slow or even denature the enzymes.
Neutrophiles: most bacteria thrive at a neutral pH range from pH 5 to pH 8. Preservation of food by increasing the acidity is used to prevent the growth of bacteria. Streptococcus pyogenes, which causes strep throat, is an example of a neutrophilic bacterium.
Acidophiles: thrive in environments that have an pH below 5.5. An example would be Helicobacter pylori which can live in the very low pH of the human stomach and are responsible for most stomach ulcers.
Alkalophile: thrive in environments that have a pH above 8.5. Bacillus alcalophilus is an alkalophile that is found in alkaline lakes and soils.
Like all living things, prokaryotes need energy and carbon. They meet these needs in a variety of ways. In fact, prokaryotes have just about every possible type of metabolism. They may get energy from light (photo) or chemical compounds (chemo). They may get carbon from carbon dioxide (autotroph) or other living things (heterotroph). Most prokaryotes are chemoheterotrophs. They depend on other organisms for both energy and carbon. Many break down organic wastes and the remains of dead organisms. They play vital roles as decomposers and help recycle carbon and nitrogen. Photoautotrophs are important producers. They are especially important in aquatic ecosystems.
Classification of Prokaryotes Based on Metabolism
Two major nutritional needs can be used to group prokaryotes. These are (1) carbon metabolism, their source of carbon for building organic molecules within the cells, and (2) energy metabolism, their source of energy used for growth.
In terms of carbon metabolism, prokaryotes are classified as either heterotrophic or autotrophic:
- Heterotrophic organisms use organic compounds, usually from other organisms, as carbon sources.
- Autotrophic organisms use carbon dioxide (CO2) as their only source or their main source of carbon. Many autotrophic bacteria are photosynthetic, and get their carbon from the carbon dioxide in the atmosphere.
Energy metabolism in prokaryotes is classified as one of the following:
- Photo: gets energy from sunlight
- Chemo: gets energy from breaking down compounds
Metabolic classification starts with the question, "Does the bacterium get its carbon from the atmosphere or must it get carbon from "elsewhere" which means from other organisms! The next question concerns the source of energy. Use the graphic below to see how the answers to those two questions are used to determine metabolic classification.
- Prokaryote variation has allowed bacteria to adapt to many habitates with very different temperatures, pH levels, and oxygen content.
- Living things must have a source of energy and carbon. The type of carbon compound, organic or inorganic, and the source of energy needed is one way to classify bacteria.
Use this resource to answer the questions that follow.
→Biology for AP* →Search: Prokaryotic Lifestyles
- Distinguish between aerobes, facultative anaerobes, and obligate anaerobes.
- Describe the habitats of obligate anaerobes.
- Why would temperature and pH be so important to the functioning of bacteria?
- How does the source of carbon differ between autotrophs and heterotrophs?
- If the source of energy for a phototropic organism is sunlight, what is the source of energy for a chemotrophic organism?
1. Explain why many prokaryotes are adapted for living at the normal internal temperature of the human body.
2. Compare psychrophiles to thermophiles.
3. Explain the reason why a particular bacterium that is able to function in an environment with a pH of 4 probably wouldn't survive in an environment with a pH of 8.
4. Describe the metabolism (carbon source and energy source) you would expect from a bacterium living at the bottom of a deep ocean trench. Explain your answer.