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When looking through genome annotations of strictly anaerobic organisms I see reactions featuring oxygen. I suspect these are likely an artifact of the annotation process. But I am wondering if it is possible for such organisms to have oxygen associated metabolism. Strict anaerobes are known to die in the presence of oxygen due to the lack oxidative stress pathways. So I cannot think of any reason for oxygen to be part of their metabolism.

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    $\begingroup$ Is it a molecular oxygen? Can you show some examples of reactions in anaerobes involving molecular oxygen? $\endgroup$ – user37894 Dec 14 '17 at 8:46
  • $\begingroup$ Here is one of the reactions I came accross: Ferroxidase: 4 Fe2+ + 4 H+ + O2 = 4 Fe3+ + 2H2O $\endgroup$ – Septimus G Dec 15 '17 at 0:03
  • $\begingroup$ This is an aerobic oxidation of ferrous iron---impossible in strict anaerobes. $\endgroup$ – user37894 Dec 15 '17 at 8:01
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Strict anaerobes do not use molecular oxygen for oxidation reactions.

The oxidation of ferrous iron to ferric iron (viz. the example given by Septimus G) can be done anaerobically by anaerobic purple bacteria [eq. 2 in Ehrenreich & Widdel (1994) Appl Environ Microbiol 60: 4517-4526]:

4 FeCO3 + 10 H20 --> 4 Fe(OH)3 + (CH20) + 3 HCO3- + 3 H+

https://www.ncbi.nlm.nih.gov/pubmed/7811087

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