Anaerobic organism

An anaerobic organism or anaerobe is any organism that does not require molecular oxygen for its growth. It may react negatively or even die in the presence of free oxygen. Unlike aerobic organisms, which need oxygen to produce energy, anaerobes use other substances such as nitrate, sulfate, or sulfur as oxidizing agents in their respiration process.

Spinoloricus cinziae, a metazoan that metabolises with hydrogen, lacking mitochondria and instead using hydrogenosomes

Most anaerobes are unicellular organisms, including many types of bacteria and archaea. These tiny life forms have been around since the early days of life on Earth, using a less efficient but very ancient way to create energy. Anaerobic respiration allows them to survive in places where there is no oxygen, such as deep in the seafloor or in the rumen of animals like cows.

Some anaerobic microbes live around hydrothermal vents in the deep ocean. These places spew out hot water and have no sunlight, but the microbes thrive there by using chemicals like sulfur and iron to produce energy through a process called anaerobic respiration. Their ability to survive in such extreme environments helps scientists understand how life might exist in other parts of our solar system.

First recorded observation

In 1680, the scientist Antonie van Leeuwenhoek did an experiment with two glass tubes filled with crushed pepper and rainwater. One tube was sealed with a flame, and the other was left open. After several days, he found tiny living things, called animalcules, in both tubes. He was surprised to see life in the sealed tube, where there was no oxygen, showing that some tiny creatures could live without oxygen.

Later, in 1913, another scientist named Martinus Beijerinck repeated the experiment and found a type of bacteria called Clostridium butyricum. He noted that van Leeuwenhoek had actually observed these oxygen-free bacteria centuries before scientists understood oxygen.

Classifications

Aerobic and anaerobic bacteria can be differentiated by culturing them in test tubes of thioglycolate broth:Obligate aerobes need oxygen because they cannot ferment or respire anaerobically. They gather at the top of the tube where the oxygen concentration is highest.Obligate anaerobes are poisoned by oxygen, so they gather at the bottom of the tube where the oxygen concentration is lowest.Facultative anaerobes can grow with or without oxygen because they can metabolize energy aerobically or anaerobically. They gather mostly at the top because aerobic respiration generates more adenosine triphosphate (ATP) than either fermentation or anaerobic respiration.Microaerophiles need oxygen because they cannot ferment or respire anaerobically. However, they are poisoned by high concentrations of oxygen. They gather in the upper part of the test tube but not the very top.Aerotolerant organisms do not require oxygen as they metabolize energy anaerobically. Unlike obligate anaerobes, however, they are not poisoned by oxygen. They are evenly distributed throughout the test tube.

There are three main types of anaerobic organisms. Obligate anaerobes[/w/0], like Clostridium botulinum, are harmed by oxygen. Aerotolerant organisms[/w/1] can't use oxygen but can live with it around. Facultative anaerobes[/w/2] can live without oxygen but will use it if available. Recent studies show that some organisms once thought to be obligate anaerobes can survive with oxygen if special substances are added to their environment.

Energy metabolism

Some organisms that don't need oxygen, called obligate anaerobes, get their energy by a process called fermentation. Others use a method called anaerobic respiration. Some organisms can switch between using oxygen and not using oxygen, depending on what is available.

Fermentation is a way for these organisms to produce energy without oxygen. For example, they can turn sugar into lactic acid, releasing a small amount of energy. Plants and some fungi, like yeast, can turn sugar into alcohol and carbon dioxide, also getting energy from this process. There are many different types of fermentation used by bacteria and other tiny organisms.

Culturing anaerobes

Example of a workup algorithm of possible bacterial infection in cases with no specifically requested targets (non-bacteria, mycobacteria, etc.), with most common situations and agents seen in a New England community hospital setting. Multiple anaerobic growth media are mentioned among agar plate cultures. Anaerobes may also be identified by MALDI-TOF as shown at bottom right.

Culturing anaerobic organisms requires special methods because normal lab techniques use air, which contains oxygen. Scientists use a special sealed box called a glovebox filled with nitrogen or other sealed containers to keep oxygen away. One common tool is the Gas-pak System, which creates an oxygen-free space by using chemicals that produce hydrogen gas to remove oxygen.

In 2018, researchers in France discovered that adding certain chemicals called antioxidants to the growth mixture allowed some anaerobic organisms to be grown in air. This was an important step in understanding how these tiny living things grow and survive.

Multicellularity

Very few multicellular life forms can live without oxygen, because they need oxygen to get enough energy for their complex processes. However, there are a few exceptions. Three tiny species of Loricifera and a 10-cell creature called Henneguya zschokkei can survive without oxygen.

These Loricifera were found in 2010 in a very salty and oxygen-less part of the Mediterranean Sea, called the L'Atalante basin. They do not have mitochondria, which usually help cells use oxygen to make energy. Instead, they get energy from hydrogen. Henneguya zschokkei also lacks mitochondria and uses different structures to help with its metabolism, but scientists still don’t fully understand how it lives without oxygen.

Symbiosis

Anaerobic respiration helps some organisms work together, or live in symbiosis. This means they help each other, even though they are very different. For example, tiny animals called ciliates and tiny single-celled organisms called prokaryotes can live together. The ciliate makes things that the prokaryote can use, and they both benefit.

Animals like cows have a special stomach area called the rumen, where anaerobic organisms help break down plants that the animal cannot digest on its own. Similarly, termites use anaerobic bacteria in their bodies to help them get nutrients from wood, which is hard to digest. These bacteria help the termite survive on a diet that would otherwise lack important nutrients.