Anaerobic organism

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hydrogenosomes
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An anaerobic organism or anaerobe is any organism that does not require molecular 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. protozoans,[1] bacteria[2]) or multicellular.[3] 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. Deep waters of the ocean are a common anoxic environment.[3]

First recorded observation

In his 14 June 1680 letter to The Royal Society, Antonie van Leeuwenhoek described an experiment he carried out by filling two identical glass tubes about halfway with crushed pepper powder, to which some clean rain water was added. Van Leeuwenhoek sealed one of the glass tubes using a flame and left the other glass tube open. Several days later, he discovered in the open glass tube 'a great many very little animalcules, of divers sort having its own particular motion.' Not expecting to see any life in the sealed glass tube, Van Leeuwenhoek saw to his surprise 'a kind of living animalcules that were round and bigger than the biggest sort that I have said were in the other water.' The conditions in the sealed tube had become quite anaerobic due to consumption of oxygen by aerobic microorganisms.[4]

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:

We thus come to the remarkable conclusion that, beyond doubt, Van Leeuwenhoek in his experiment with the fully closed tube had cultivated and seen genuine anaerobic bacteria, which would happen again only after 200 years, namely about 1862 by Pasteur. That Leeuwenhoek, one hundred years before the discovery of oxygen and the composition of air, was not aware of the meaning of his observations is understandable. But the fact that in the closed tube he observed an increased gas pressure caused by fermentative bacteria and in addition saw the bacteria, prove in any case that he not only was a good observer, but also was able to design an experiment from which a conclusion could be drawn.[4]

Classifications

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 can/will be evenly distributed throughout the test tube.

    For practical purposes, there are three categories of anaerobe:

    • Obligate anaerobes, which are harmed by the presence of oxygen.[5][6] Two examples of obligate anaerobes are Clostridium botulinum and the bacteria which live near hydrothermal vents on the deep-sea ocean floor.
    • Aerotolerant organisms, which cannot use oxygen for growth, but tolerate its presence.[7]
    • Facultative anaerobes, which can grow without oxygen but use oxygen if it is present.[7]

    However, this classification has been questioned after recent research showed that human "obligate anaerobes" (such as Finegoldia magna or the methanogenic archaea

    Energy metabolism

    Some obligate anaerobes use

    aerobic respiration.[7] In the absence of oxygen, some facultative anaerobes use fermentation, while others may use anaerobic respiration.[7]

    Fermentation

    There are many anaerobic fermentative reactions.

    Fermentative anaerobic organisms typically use the lactic acid fermentation pathway:

    C6H12O6 + 2 ADP + 2 phosphate → 2 lactic acid + 2 ATP + 2 H2O

    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:

    C6H12O6 (glucose) + 2 ADP + 2 phosphate → 2 C2H5OH + 2 CO2↑ + 2 ATP + 2 H2O

    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,[14] butyric acid fermentation,[15] solvent fermentation, mixed acid fermentation, butanediol fermentation, Stickland fermentation, acetogenesis, or methanogenesis.[citation needed]

    CrP hydrolysis

    Creatine, an organic compound found in animals, provides a way for ATP to be utilized in the muscle. The phosphorylation of creatine allows for the storage of readily available phosphate that can be supplied to the muscles.[16]

    creatine + ATP ⇌ phosphocreatine + ADP + H+

    The reaction is reversible as well, allowing cellular ATP levels to be maintained during anoxic conditions.[17] 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.[18]

    Culturing anaerobes

    MALDI-TOF
    as shown at bottom right.

    Since normal microbial culturing occurs in atmospheric air, which contains molecular oxygen, culturing of anaerobes requires special techniques. A number of techniques are employed by microbiologists when culturing anaerobic organisms, for example, handling the bacteria in a

    thioglycollate medium should be used. The thioglycollate supplies a medium mimicking that of a dicot plant, thus providing not only an anaerobic environment but all the nutrients needed for the bacteria to multiply.[19]

    Recently, a French team evidenced a link between redox and gut anaerobes [20] based on clinical studies of severe acute malnutrition.[21] These findings led to the development of aerobic culture of "anaerobes" by the addition of antioxidants in the culture medium.[22]

    Multicellularity

    Few multicellular life forms are anaerobic, since only aerobic respiration can provide enough energy for a complex metabolism. Exceptions include three species of Loricifera (< 1 mm in size) and the 10-cell Henneguya zschokkei.[23]

    In 2010 three species of anaerobic loricifera were discovered in the

    hydrogenosomes.[24][3]

    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.[25]

    Symbiosis

    Anaerobic respiration and its end products can facilitate symbiosis between anaerobes and aerobes. This occurs across taxa, often in compensation for nutritional needs.[26]

    Anaerobiosis, and symbiosis, is found in interactions between ciliates and prokaryotes. Anaerobic ciliates participate in an endosymbiotic relationship with prokaryotes. 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.[27] In specific, methanogenic archaea found in the rumen acts as a symbiont to anaerobic ciliates.[28] 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.[26]

    Termites 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 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.[29] One of the functions of termite microbiota is to recapture nitrogen from the termite's own uric acid. This allows conservation of nitrogen from an otherwise nitrogen-poor diet.[29][30] The hindgut microbiome of different termites has been analyzed, showing 16 different anaerobic species of bacteria, including Clostridia, Enterobacteriaceae, and Gram-positive cocci.[30]

    References