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An anaerobic organism or anaerobe is any organism that does not require oxygen for growth. It may react negatively or even die if free oxygen is present. In contrast, an aerobic organism (aerobe) is an organism that requires an oxygenated environment. Anaerobes may be unicellular (e.g. , bacteria) or multicellular. Most fungi are obligate aerobes, requiring oxygen to survive. However, some species, such as the Chytridiomycota that reside in the rumen of cattle, are obligate anaerobes; for these species, anaerobic respiration is used because oxygen will disrupt their metabolism or kill them. The sea floor is possibly one of the largest accumulation of anaerobic organisms on Earth, where microbes are primarily concentrated around hydrothermal vents. These microbes produce energy in absence of sunlight or oxygen through a process called chemosynthesis, whereby inorganic compounds such as hydrogen gas, hydrogen sulfide or ferrous ions are converted into organic matter.
In 1913, Martinus Beijerinck repeated Van Leeuwenhoek's experiment and identified Clostridium butyricum as a prominent anaerobic bacterium in the sealed pepper infusion tube liquid. Beijerinck commented:
However, this classification has been questioned after recent research showed that human "obligate anaerobes" (such as Finegoldia magna or the methanogenic archaea Methanobrevibacter smithii) can be grown in aerobic atmosphere if the culture medium is supplemented with antioxidants such as ascorbic acid, glutathione and uric acid.
Fermentative anaerobic organisms typically use the lactic acid fermentation pathway:
The energy released in this reaction (without ADP and phosphate) is approximately 150 kJ per mol, which is conserved in generating two ATP from ADP per glucose. This is only 5% of the energy per sugar molecule that the typical aerobic reaction generates.
Plants and fungi (e.g., yeasts) in general use alcohol (ethanol) fermentation when oxygen becomes limiting:
The energy released is about 180 kJ per mol, which is conserved in generating two ATP from ADP per glucose.
Anaerobic bacteria and archaea use these and many other fermentative pathways, e.g., propionic acid fermentation, butyric acid fermentation, solvent fermentation, mixed acid fermentation, butanediol fermentation, Stickland fermentation, acetogenesis, or methanogenesis.
creatine + ATP ⇌ phosphocreatine + ADP + H+
The reaction is reversible as well, allowing cellular ATP levels to be maintained during anoxic conditions. This process in animals is seen to be coupled with metabolic suppression to allow certain fish, such as goldfish, to survive environmental anoxic conditions for a short period.
On May the 6 2018, a French team evidenced a link between redox and gut anaerobes based on clinical studies of severe acute malnutrition.This study was later retracted over ethical oversight concerns. These findings led to the development of aerobic culture of "anaerobes" by the addition of antioxidants in the culture medium.
In 2010 three species of anaerobic loricifera were discovered in the hypersaline Anoxic waters L'Atalante basin at the bottom of the Mediterranean Sea. They lack mitochondria which contain the oxidative phosphorylation pathway, which in all other animals combines oxygen with glucose to produce metabolic energy, and thus they consume no oxygen. Instead, these loricifera derive their energy from hydrogen, using hydrogenosomes. Oxygen-Free Animals Discovered-A First, National Geographic news
Henneguya zschokkei also lack mitochondria, mitochondrial DNA, and oxidative pathways. The microscopic, parasitic cnidarian is observed to have mitochondria-related organelles contained within it. This mitochondria-related organelle within it is observed to have genes encoding for metabolic functions such as amino acid metabolism. However, these mitochondria-related organelles lack the key features of typical mitochondria found in closely related aerobic Myxobolus squamalus. Due to the difficulty of culturing H. zschokkei, there is little understanding of its anaerobic pathway.
Anaerobiosis and symbiosis are found in interactions between and . Anaerobic ciliates interact with prokaryotes in an Endosymbiont relationship. These relationships are mediated in which the ciliate leaves end products that its prokaryotic symbiont utilizes. The ciliate achieves this through the use of fermentative metabolism. The rumen of various animals house this ciliate alongside many other anaerobic bacteria, protozoans, and fungi. In specific, methanogenic archaea found in the rumen acts as a symbiont to anaerobic ciliates. These anaerobes are useful to those with a rumen due to their ability to break down cellulose, making it bioavailable when otherwise indigestible by animals.
utilize anaerobic bacteria to fix and recapture nitrogen. In specific, the hindgut of the termite is full of nitrogen-fixing bacteria, ranging in function depending on the nitrogen concentration of the diet. Acetylene reduction in termites was observed to upregulate in termites with nitrogen-poor diets, meaning that nitrogenase activity rose as the nitrogen content of the termite was reduced. One of the functions of termite microbiota is to recapture nitrogen from the termite's uric acid. This allows the conservation of nitrogen from an otherwise nitrogen-poor diet. The hindgut microbiome of different termites has been analyzed, showing 16 different anaerobic species of bacteria, including Clostridia, Enterobacteriaceae, and Coccus.
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