Anaerobia

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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.

Habitats that do not contain oxygen are referred to as anoxic , in earlier language use also as anaerobic (opposite: oxic , containing oxygen, formerly aerobic ).

Anaerobic breathing

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).

Respiratory types according to redox potential
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)

fermentation

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:

Symbioses

Some turbellarians , annelids and enteroparasites such as tapeworms harbor anaerobic bacteria and, thanks to this symbiosis, can also live under anoxic conditions.

ID

Aerobic and anaerobic bacteria can be identified in liquid nutrient solution: (1) Obligatory aerobic bacteria collect at the top where they get enough oxygen. (2) Obligatory anaerobic bacteria collect at the lower end, where there is no oxygen. (3) Optional anaerobic bacteria are found mainly above because oxygen breathing is most effective; since, on the other hand, a lack of O 2 does not prevent them, they also grow in the deeper parts of the test tube. (4) Microaerophiles collect at the top, but not at the very top, as oxygen is only optimal for them in low concentrations. (5) Aerotolerant bacteria are not influenced by oxygen and are therefore distributed evenly in the test tube.

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 .

See also

Individual evidence

  1. ↑ Johannes Ottow: Microbiology of soils. Biodiversity, Ecophysiology and Metagenomics . Springer Verlag, Berlin 2011, ISBN 978-3-642-00823-8 , p. 56.
  2. ↑ J. Zenka, Jan Prokopic: Contribution to the knowledge of aerobic processes in Taenia crassiceps larvae . In: Folia Parasitologica , Vol. 33 (1986), No. 4, pp. 331-336, PMID 3804084 .
  3. ↑ J. Zenka, Jan Prokopic: Malic enzyme, malate dehydrogenase, fumarate reductase and succinate dehydrogenase in the larvae of Taenia crassiceps (Zeder, 1800) . In: Folia Parasitologica , Vol. 34 (1987), No. 2, pp. 131-136, PMID 3596392 .

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