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An anaerobic organism or anaerobe is any
organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological ...

organism
that does not require
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
for growth. It may react negatively or even die if free oxygen is present. In contrast, an
aerobic organism 300px, Aerobic and anaerobic bacteria can be identified by growing them in test tubes of thioglycollate broth: 1: Obligate aerobe 300px, Aerobic and anaerobic bacteria can be identified by growing them in test tubes of thioglycollate broth: 1 ...
(aerobe) is an organism that requires an oxygenated environment. Anaerobes may be unicellular (e.g.
protozoa Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of Unicellular organism, single-celled eukaryotes, either free-living or Parasitism, parasitic, that feed on organic matter such as other mi ...

protozoa
ns,
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

bacteria
) or multicellular. Most fungi are obligate
aerobes 300px, Aerobic and anaerobic bacteria can be identified by growing them in test tubes of thioglycollate broth: 1: Obligate aerobe 300px, Aerobic and anaerobic bacteria can be identified by growing them in test tubes of thioglycollate broth: 1 ...
, requiring oxygen to survive, however some species, such as the
Chytridiomycota Chytridiomycota are a division of zoosporic organisms in the kingdom (biology), kingdom Fungi, informally known as chytrids. The name is derived from the Ancient Greek ('), meaning "little pot", describing the structure containing unreleased zoo ...
that reside in the rumen of cattle, are obligate
anaerobes An anaerobic organism or anaerobe is any organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxon ...
; 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.


First observation

In his letter of 14 June 1680 to The
Royal Society The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society A learned society (; also known as a learned academy, scholarly society, or academic association) is an organization that exis ...
,
Antonie van Leeuwenhoek Antonie Philips van Leeuwenhoek ( ; ; 24 October 1632 – 26 August 1723) was a Dutch Dutch commonly refers to: * Something of, from, or related to the Netherlands * Dutch people () * Dutch language () *Dutch language , spoken in Belgium ...
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 by 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 owing to consumption of oxygen by aerobic microorganisms.Gest, Howard. (2004) ''The discovery of microorganisms by Robert Hooke and Antoni van Leeuwenhoek, Fellows of the Royal Society'', in: 'The Royal Society May 2004 Volume: 58 Issue: 2: pp. 12. In 1913
Martinus Beijerinck Martinus Willem Beijerinck (, 16 March 1851 – 1 January 1931) was a Dutch microbiologist A microbiologist (from Greek ) is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge i ...

Martinus Beijerinck
repeated Van Leeuwenhoek's experiment and identified ''
Clostridium butyricum ''Clostridium butyricum'' is a strictly anaerobic organism, anaerobic endospore-forming Gram-positive butyric acid–producing bacillus subsisting by means of fermentation (biochemistry), fermentation using an intracellularly accumulated amylope ...
'' 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.'


Classification

For practical purposes, there are three categories of anaerobe: *
Obligate anaerobe{{wiktionary, obligate As an adjective, obligate means "by necessity" (antonym '' facultative'') and is used mainly in biology in phrases such as: * Obligate aerobe, an organism that cannot survive without oxygen * Obligate anaerobe, an organism th ...
s, which are harmed by the presence of oxygen. 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. *
Facultative anaerobe 300px, Aerobic and anaerobic bacteria can be identified by growing them in test tubes of thioglycolate broth: 1: Obligate aerobes need oxygen because they cannot ferment or respire anaerobically. They gather at the top of the tube where the oxygen ...
s, which can grow without oxygen but use oxygen if it is present. However, this classification has been questioned by the fact that 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.


Energy metabolism

Some obligate anaerobes use
fermentation Fermentation is a metabolism, metabolic process that produces chemical changes in organic Substrate (chemistry), substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in ...
, while others use
anaerobic respiration Anaerobic respiration is respiration Respiration may refer to: Biology * Cellular respiration, the process in which nutrients are converted into useful energy in a cell ** Anaerobic respiration, cellular respiration without oxygen ** Maintenan ...
. Aerotolerant organisms are strictly fermentative. In the presence of oxygen, facultative anaerobes use
aerobic respiration Aerobic means "requiring Earth's atmosphere, air," in which "air" usually means oxygen. Aerobic may also refer to * Aerobic exercise, prolonged exercise of moderate intensity * Aerobics, a form of aerobic exercise * Cellular respiration#Aerobic r ...
; without oxygen, some of them ferment; some use anaerobic respiration.


Fermentation

There are many anaerobic fermentative reactions. Fermentative anaerobic organisms mostly use the lactic acid fermentation pathway: : + 2 + 2 phosphate → 2
lactic acid Lactic acid is an organic acid An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, conta ...

lactic acid
+ 2
ATP ATP may refer to: Companies and organizations * Association of Tennis Professionals * American Technical Publishers * ', a Danish pension * Armenia Tree Project * Association for Transpersonal Psychology * ATP architects engineers office * ATP ...

ATP
The energy released in this equation is approximately 150 kJ per mol, which is conserved in regenerating two ATP from ADP per
glucose Glucose is a simple with the . Glucose is the most abundant , a subcategory of s. Glucose is mainly made by and most during from water and carbon dioxide, using energy from sunlight, where it is used to make in s, the most abundant carbohydr ...

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 Glucose is a simple with the . Glucose is the most abundant , a subcategory of s. Glucose is mainly made by and most during from water and carbon dioxide, using energy from sunlight, where it is used to make in s, the most abundant carbohydr ...

glucose
) + 2 ADP + 2 phosphate → 2 C2H5OH + 2 CO2↑ + 2 ATP The energy released is about 180 kJ per mol, which is conserved in regenerating two ATP from ADP per glucose. Anaerobic bacteria and
archaea Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...

archaea
use these and many other fermentative pathways, e.g.,
propionic acid Propionic acid (, from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population ...

propionic acid
fermentation,
butyric acid Butyric acid (from grc, βούτῡρον, meaning "butter"), also known under the systematic name butanoic acid, is a straight-chain alkyl In organic chemistry, an alkyl substituent is an alkane missing one hydrogen. The term alkyl is inten ...

butyric acid
fermentation, solvent fermentation, mixed acid fermentation,
butanediol fermentation 2,3-Butanediol fermentation is anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refe ...
,
Stickland fermentation 150px, Stickland fermentation exempflified with D-alanine and glycine Stickland fermentation or The Stickland Reaction is the name for a chemical reaction that involves the coupled oxidation and Redox, reduction of amino acids to organic acids. Th ...

Stickland fermentation
,
acetogenesisAcetogenesis is a process through which acetate is produced either by the reduction of CO2 or by the reduction of organic acids. The different bacterial species that are capable of acetogenesis are collectively termed acetogens. Reduction of CO2 ...
, or
methanogenesis Methanogenesis or biomethanation is the formation of methane Methane ( or ) is a chemical compound with the chemical formula (one atom of carbon and four atoms of hydrogen). It is a group-14 hydride and the simplest alkane , the simplest ...

methanogenesis
.


Culturing anaerobes

Since normal microbial culturing occurs in atmospheric air, which is an aerobic environment, the culturing of anaerobes poses a problem. Therefore, a number of techniques are employed by microbiologists when culturing anaerobic organisms, for example, handling the bacteria in a
glovebox A glovebox (or glove box) is a sealed container that is designed to allow one to manipulate objects where a separate atmosphere is desired. Built into the sides of the glovebox are gloves arranged in such a way that the user can place their hands ...

glovebox
filled with nitrogen or the use of other specially sealed containers, or techniques such as injection of the bacteria into a
dicot The dicotyledons, also known as dicots (or more rarely dicotyls), are one of the two groups into which all the flowering plant Flowering plants include multiple members of the clade Angiospermae (), commonly called angiosperms. The term "a ...
plant, which is an environment with limited oxygen. The GasPak System is an isolated container that achieves an anaerobic environment by the reaction of water with
sodium borohydride Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, is an inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is ...

sodium borohydride
and
sodium bicarbonate Sodium bicarbonate ( IUPAC name: sodium hydrogen carbonate), commonly known as baking soda or bicarbonate of soda, is a chemical compound with the formula NaHCO3. It is a salt composed of a sodium cation (Na+) and a bicarbonate anion ( HCO3 ...

sodium bicarbonate
tablets to produce hydrogen gas and carbon dioxide.
Hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

Hydrogen
then reacts with oxygen gas on a palladium catalyst to produce more water, thereby removing oxygen gas. The issue with the Gaspak method is that an adverse reaction can take place where the bacteria may die, which is why a thioglycollate medium should be used. The thioglycollate supplies a medium mimicking that of a dicot, thus providing not only an anaerobic environment but all the nutrients needed for the bacteria to thrive. Recently, a French team evidenced a link between redox and gut anaerobes based on clinical studies on severe acute malnutrition. These findings led to the development of aerobic culture of "anaerobes" by the addition of antioxidants in the culture medium.


Multicellularity

Few complex multicellular life forms are anaerobic. Exceptions include three species of anaerobic
Loricifera Loricifera (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Rom ...
and the 10-cell ''
Henneguya zschokkei ''Henneguya zschokkei'' is a species of a myxosporean parasite Parasitism is a symbiotic relationship between species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an o ...

Henneguya zschokkei
''. In 2010 three species of anaerobic loricifera were discovered in the
hypersaline A hypersaline lake is a landlocked body of water ( Lysefjord) in Norway Norway ( nb, ; nn, ; se, Norga; smj, Vuodna; sma, Nöörje), officially the Kingdom of Norway, is a Nordic countries, Nordic country in Northern Europe whos ...
anoxic The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts: * Anoxic waters, sea water, fresh water or groundwater that are depleted of disso ...
L'Atalante basin L'Atalante basin is a hypersaline A hypersaline lake is a landlocked body of water ( Lysefjord) in Norway Norway ( nb, ; nn, ; se, Norga; smj, Vuodna; sma, Nöörje), officially the Kingdom of Norway, is a Nordic countries, Nord ...
at the bottom of the
Mediterranean Sea The Mediterranean Sea is a connected to the , surrounded by the and almost completely enclosed by land: on the north by and and , on the south by , and on the east by the . The Sea has played a central role in the . Although the Mediterrane ...
. They lack
mitochondria A mitochondrion (; ) is a double-membrane Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, i ...

mitochondria
which contain the
oxidative phosphorylation Oxidative phosphorylation (UK , US ) or electron transport-linked phosphorylation or terminal oxidation is the metabolic pathway In biochemistry, a metabolic pathway is a linked series of chemical reactions occurring within a cell (biology), c ...

oxidative phosphorylation
pathway, which in all other animals combines
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
with
glucose Glucose is a simple with the . Glucose is the most abundant , a subcategory of s. Glucose is mainly made by and most during from water and carbon dioxide, using energy from sunlight, where it is used to make in s, the most abundant carbohydr ...

glucose
to produce metabolic energy, and thus they consume no oxygen. Instead these loricifera derive their energy from
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

hydrogen
using hydrogenosomes.Oxygen-Free Animals Discovered-A First, National Geographic news
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References

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