Anaerobic organism

An anaerobic organism or anaerobe is any organism that does not need molecular oxygen to grow. It might get hurt or even die if oxygen is present. Unlike organisms that need oxygen to make energy, anaerobes use other things like nitrate, sulfate, or sulfur to create energy.

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

Most anaerobes are unicellular organisms, such as many kinds of bacteria and archaea. These small life forms have lived on Earth since early times, using an older but special way to make energy. Anaerobic respiration lets them live where there is no oxygen, like deep in the seafloor or in the rumen of animals such as cows.

Some anaerobic microbes live near hydrothermal vents in the deep ocean. These places release hot water and have no sunlight, but the microbes survive there. They use chemicals like sulfur and iron to make energy through a process called anaerobic respiration. Their survival in such tough places helps scientists think about how life could exist elsewhere in 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. This showed that some tiny creatures could live without oxygen.

Later, in 1913, another scientist named Martinus Beijerinck repeated the experiment. He 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.

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Energy metabolism

Some organisms do not need oxygen. These are called obligate anaerobes. They get their energy by a process called fermentation.

Fermentation is a way to make energy without oxygen. For example, they can turn sugar into lactic acid and release a small amount of energy. Plants and some fungi, like yeast, can turn sugar into alcohol and carbon dioxide. They also get energy from this process. Many different types of fermentation are used by bacteria and other tiny organisms.

Culturing anaerobes

Growing anaerobic organisms needs special ways because normal lab tools use air, which has oxygen. Scientists use a special sealed box called a glovebox filled with nitrogen or other sealed containers to keep oxygen out. One common tool is the Gas-pak System, which makes a space without oxygen by using chemicals that create hydrogen gas to take away oxygen.

Researchers in France found that adding some chemicals called antioxidants to the mix helped some anaerobic organisms grow in air. This was an important discovery in learning how these tiny living things grow and stay alive.

Multicellularity

Very few multicellular life forms can live without oxygen, because they need it for energy. But there are a few exceptions.

Three tiny species of Loricifera and a small 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. Instead, they get energy from hydrogen. Henneguya zschokkei also lacks mitochondria, but scientists do not 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.