Anaerobia (in ancient Greek ἀήρ aer ' air ' and βίος bios ' life '; with alpha privativum α (ν) - a (n) - 'without') denotes life without oxygen (dioxygen O 2 ). Living things that do not need molecular oxygen for their metabolism are called anaerobic . Those anaerobes that are inhibited or even killed by O 2 are named more precisely obligatory anaerobic .
Anaerobic life forms
Anaerobic unicellular organisms are the oldest forms of life on earth, even before the first oxygen photosynthetically active unicellular organisms in the Precambrian to excrete O 2 . With its accumulation in the hydrosphere and the atmosphere , the living conditions changed over a large area (see Great Oxygen Catastrophe ). The anaerobic organisms living today also all do not need oxygen for their metabolism and can be roughly differentiated according to how well they can cope with an oxygen-containing environment:
- Obligatory anaerobes are organisms that carry out an anaerobic metabolism on their own and can only grow in an oxygen-free environment (under anoxic conditions), as O 2 is harmful to them.
- Aerotolerant anaerobes are organisms that carry out anaerobic metabolism on their own, i.e. do not use O 2 , but tolerate the presence of oxygen and can therefore also live under oxic conditions.
- Facultative anaerobes are organisms that can carry out an anaerobic metabolism, but can also use O 2 in the presence of oxygen , i.e. can grow under both anoxic and oxic conditions.
In contrast to aerobic respiration , anaerobic respiration uses other electron acceptors as oxidants instead of O 2 for oxidative energy metabolism . Alternative electron acceptors commonly used are: nitrate , trivalent iron - ions (Fe 3+ ), tetravalent manganese ions (Mn 4+ ), sulfate , sulfur , fumarate and carbon dioxide (CO 2 ). These redox reactions are known as anaerobic breathing.
The table lists types of anaerobic respiration that are widespread in the environment (aerobic respiration is included for comparison). The respiratory processes were ranked, if possible, according to the standard redox potential of the electron acceptor pair in volts at a pH value of 7. The actual pH values may deviate (e.g. in the case of acetogenesis).
|Breathing type||Organisms||Electron acceptor||Reaction product (s)||E o ' [V]||Example organism|
|aerobic breathing||obligatory and optional aerobes||Oxygen O 2||H 2 O + CO 2||+ 0.82||Eukaryotes|
|Nitrate breathing ( denitrification )||optional aerobes: denitrifiers||Nitrate NO 3 -||Nitrite NO 2 -||+ 0.75||Paracoccus denitrificans , E. coli|
|Manganese reduction||facultative or obligatory anaerobes||Manganese Mn (IV)||Mn (II)||+ 0.41||Desulfuromonadales , Desulfovibrio|
|Iron breathing||facultative aerobes, obligatory anaerobes||Iron Fe (III)||Fe (II)||+ 0.15||Geobacter , Geothermobacter , Geopsychrobacter , Pelobacter carbinolicus , P. acetylenicus , P. venetianus , Desulfuromonadales , Desulfovibrio|
|Cobalt reduction||facultative or obligatory anaerobes||Cobalt Co (III)||Co (II)||Geobacter sulfurreducens|
|Technetium reduction||facultative or obligatory anaerobes||Technetium Tc (VII)||Geobacter sulfurreducens , Geobacter metallireducens|
|Uranium reduction||facultative or obligatory anaerobes||Uranium U (VI)||U (IV)||Geobacter metallireducens , Shewanella putrefaciens , ( Desulfovibrio )|
|Fumarate breathing||optional aerobes||Fumarate||Succinate||+ 0.03||Escherichia coli|
|Sulfate respiration ( desulfurication )||obligate anaerobes: sulphate reducers||Sulfate SO 4 2−||Sulfide HS -||- 0.22||Desulfobacter latus , Desulfovibrio|
|Methanogenesis (carbonate respiration)||methanogenic and obligate anaerobes: methane producers||Carbon dioxide CO 2||Methane CH 4||- 0.25||Methanothrix thermophila|
|Sulfur breathing (sulfur reduction)||facultative aerobes and obligatory anaerobes||Sulfur S 0||Sulfide HS -||- 0.27||Desulfuromonadales|
|Acetogenesis (carbonate breathing)||homoacetogenic and obligate anaerobes||Carbon dioxide CO 2||acetate||- 0.30||Acetobacterium woodii|
|TCA reduction||facultative or obligatory anaerobes||TCA trichloroacetic acid||Dichloroacetic acid||Trichlorobacter (Geobacteraceae)|
Processes in which no external substance is used as a terminal electron acceptor are referred to as fermentation rather than anaerobic respiration. Fermentation organisms are mainly:
- Lactic acid bacteria ( lactic acid fermentation )
- Yeasts ( alcoholic fermentation )
- practically all aerobes master lactic acid fermentation under anoxic conditions: anaerobic metabolism (for humans see also anaerobic threshold )
The behavior of microorganisms towards oxygen, their identification as aerobes, anaerobes, aerotolerant or facultative anaerobes, can be determined by culture in an oxygen concentration gradient. They are cultivated in a gel culture medium that is located in a glass tube ( test tube , culture tube) closed on one side and into which oxygen can only penetrate from the upper, open end by diffusion . In this way, an oxygen concentration gradient is formed with a high oxygen concentration at the top and a low oxygen concentration at the bottom. The microorganisms are evenly distributed in very small quantities in the gel nutrient medium, in which they are fixed in place and cannot move. Where the microorganisms are under suitable conditions with regard to the oxygen concentration, they multiply and after a certain time one can see a growth with the naked eye. The zone in which vegetation appears is an indicator of the behavior of the microorganisms towards oxygen, as is clear from the picture.
Culture of anaerobic microorganisms
Anaerobia is important in the cultivation of microorganisms , among other things . If microorganisms sensitive to O 2 are to be cultivated or if facultative anaerobic microorganisms are to be cultivated under anoxic conditions, it is necessary to exclude O 2 from the culture . So-called anaerobic techniques are used here. One example is the culture in an anaerobic chamber: In this, anoxic conditions are achieved with a gas atmosphere of 10% by volume H 2 + 10% by volume CO 2 + 80% by volume N 2 , which make it possible to cultivate anaerobic microorganisms .
- Johannes Ottow: Microbiology of soils. Biodiversity, Ecophysiology and Metagenomics . Springer Verlag, Berlin 2011, ISBN 978-3-642-00823-8 , p. 56.
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