Anaerobic Organism: Definition, Examples, and Quiz | biology dictionary (2023)

Definition of Anaerobic Organism

Anaerobic organisms are those that live in an anoxic environment, that is, lacking in oxygen. While most living things need oxygen to survive (they are aerobic), oxygen can actually be toxic to anaerobic organisms. The vast majority of organisms produce energy molecules called ATP (adenosine triphosphate) through a process of aerobic cellular respiration. This complex set of chemical interactions takes place in the cytoplasm and cell membrane of prokaryotes and in the mitochondria of eukaryotes. During respiration, oxygen acts as the final electron acceptor at the end of an electron transport chain, which is why aerobic organisms must breathe oxygen-containing air in order to survive. However, anaerobic organisms use fermentation or anaerobic cellular respiration to produce ATP. In this case, an atom other than oxygen is the final electron acceptor. For example, some anaerobic bacteria that live deep in the mud in marshy areas use a sulfate ion instead of oxygen, and hydrogen sulfide is produced as a byproduct instead of water. This explains the smell of sulfur in many swamps and marshes.

two types of anaerobes

There are two main types of anaerobes: facultative and obligate. Facultative anaerobes can live with or without oxygen. When there is oxygen in their environment, they use aerobic cellular respiration to produce energy in the form of ATP. If oxygen runs out, they can switch to anaerobic respiration or fermentation. In contrast, obligate anaerobes must live without oxygen. They are only equipped to undergo anaerobic respiration or fermentation, and the presence of oxygen kills them.

facultative anaerobes

Human muscle cells are facultative anaerobic. During exercise in which a person receives enough oxygen to their muscles, such as running long distances, the cells undergo aerobic respiration. But during intense exercise like sprinting, in which the body's oxygen needs exceed the lungs' ability to provide it, muscle cells will switch to lactic acid fermentation. This process is much less efficient than aerobic respiration and produces lactic acid as a byproduct, which accumulates in the muscles and causes the burning sensation commonly felt during strenuous exercise. Because this is much less efficient, a person can only perform such intense activity for a very short period of time before "hitting the wall" and having to stop.

Another familiar facultative anaerobe is the bacterium.Escherichia coli. WhileE. coliAlthough it has gotten a bad rap in the press due to food poisoning incidents, E. coli are actually very important and beneficial residents of the human gastrointestinal tract. They aid in the digestion of food and the absorption of necessary vitamins, as well as protection against potentially harmful infections. These bacteria can easily function with or without oxygen, making them highly adaptable to different environments. In the anaerobic intestine, they use fermentation to produce energy. If they are in the oxygen-rich environment outside the intestine, they switch to aerobic respiration.

Other examples of facultative anaerobes

  • staphylococcus aureus: Causes staphylococcal infections. methicillin resistantS aureusis responsible for MRSA.
  • Lactococcus lactis: Its lactic acid fermentation is used in the production of many types of cheese.

obligate anaerobes

An infamous example of an obligate anaerobe isbotulinum clostridium. This common bacterium produces a potent neurotoxin that can be fatal even in small amounts. It is found growing in items such as home canned goods, foil-wrapped baked potatoes, and honey. In poor survival conditions,C. botulinumIt produces spores with a hard coating that allows them to survive for years. When conditions improve, the bacteria begin to grow and produce potentially lethal toxins. If a person consumes contaminated food with active growthC. botulinumthey are likely to succumb to a deadly food poisoning called botulism, the first symptoms of which are nausea, vomiting, and weakness. Then come the neurological effects: blurred vision, difficulty speaking and swallowing, and impaired muscle control, followed by difficulty breathing and possibly death from suffocation. Infant botulism occurs after a baby swallowsC. botulinumspores, which can be found in soil, dust, or honey. This is why honey should never be given to young babies; Before they are a year old, their immune systems are not strong enough to handle the spores, so they begin to grow and cause serious illness.

Possibly the largest aggregation of obligate anaerobes on the planet is found at the bottom of the deep sea, where they populate hydrothermal vents. These underwater hot springs that rise from the earth's crust are loaded with minerals that bacteria use to power their chemosynthesis process, thus building organic molecules. First discovered in 1977 by researchers off the Galapagos Islands, their existence rewrote all biology textbooks. Photosynthesis was previously thought to be the only means by which autotrophic organisms could convert energy into food for themselves. Bob Ballard, the deep-sea explorer who discovered the wreckage of the Titanic, was on the Alvin submersible the day she sank to film the vents. He later said that the discovery of chemosynthesis in vent bacteria was one of the greatest biological discoveries of the 20th century, far more important than any historical shipwreck. Evolutionists speculate that life began at the bottom of the deep sea, energized by chemosynthesis.

Other examples of obligate anaerobes

  • Clostridium tetani: Cause tetanus
  • Clorobio, Cloroflexusand several other species contribute to the prismatic colors of Yellowstone National Park's hot springs.

Anaerobes: friends or enemies?

It is clear that our planet is well populated with various anaerobic organisms. Some are pathogenic, causing serious infections such as MRSA, botulism, and tetanus. Others are beneficial, adding beauty to hot springs, flavoring cheeses, and shaping ocean communities. For others, likeE. coli, its status depends on its location: whileE. coliit is a necessary and useful resident of the human intestine, it can become pathogenic if ingested orally or in some other way. In summary, anaerobes are important residents of Earth that brilliantly fill their ecological niches.


1. Which of the following metabolic pathways requires oxygen?
A.Aerobic cellular respiration.
B.Anaerobic cellular respiration.
C.Lactic acid fermentation.
D.Alcoholic fermentation.

Answer to question #1

Ait's right. The term "aerobic" refers to oxygen. The other three processes require the absence of oxygen for them to occur.

2. Which of the following is not a facultative anaerobe?
A.Escherichia coli
B.staphylococcus aureus
C.botulinum clostridium
D.human muscle cell

Answer to question #2

Bit's right.C. botulinumIt is the obligate anaerobe responsible for botulism and food poisoning.E. coli,S aureus, and human muscle cells are all facultative anaerobic, capable of switching between aerobic and anaerobic respiration, depending on their environment.

3. Where is the largest aggregation of obligate anaerobes on the planet?
A.swamps and marshes
B.Tropical and temperate forests
C.the bottom of the deep sea
D.Pastures and agricultural soils.

Answer to question #3

Cit's right. The world's largest population of obligate anaerobes inhabit hydrothermal vents found along submarine ridges, which stretch for 40,000 miles along the edges of Earth's tectonic plates.


  • Arnold, P. (2009). Examples of anaerobic bacteria. Retrieved from
  • Reece, JB y Campbell, NA (2011).campbell biology. Boston: Benjamín Cummings/Pearson.
  • Clostridium botulinum. (2013).United States Department of Agriculture, Food Safety and Inspection Service.Retrieved from
  • Hoecker, J. (2015). How can I protect my baby from infant botulism?The Mayo Clinic.Retrieved from
  • Taylor, A. (2016). What makes Yellowstone's hot springs so colorful? Retrieved from


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