Mechanism of oxygen detoxification by the surprisingly oxygen-tolerant hyperthermophilic archaeon, Pyrococcus furiosus

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Abstract
The anaerobic archaeon Pyrococcus furiosus grows by fermenting carbohydrates producing H 2 , CO 2 , and acetate. We show here that it is surprisingly tolerant to oxygen, growing well in the presence of 8% (vol/vol) O 2 . Although cell growth and acetate production were not significantly affected by O 2 , H 2 production was reduced by 50% (using 8% O 2 ). The amount of H 2 produced decreased in a linear manner with increasing concentrations of O 2 over the range 2–12% (vol/vol), and for each mole of O 2 consumed, the amount of H 2 produced decreased by approximately 2 mol. The recycling of H 2 by the two cytoplasmic hydrogenases appeared not to play a role in O 2 resistance because a mutant strain lacking both enzymes was not more sensitive to O 2 than the parent strain. Decreased H 2 production was also not due to inactivation of the H 2 -producing, ferredoxin-dependent membrane-bound hydrogenase because its activity was unaffected by O 2 exposure. Electrons from carbohydrate oxidation must therefore be diverted to relieve O 2 stress at the level of reduced ferredoxin before H 2 production. Deletion strains lacking superoxide reductase (SOR) and putative flavodiiron protein A showed increased sensitivity to O 2 , indicating that these enzymes play primary roles in resisting O 2 . However, a mutant strain lacking the proposed electron donor to SOR, rubredoxin, was unaffected in response to O 2 . Hence, electrons from sugar oxidation normally used to produce H 2 are diverted to O 2 detoxification by SOR and putative flavodiiron protein A, but the electron flow pathway from ferredoxin does not necessarily involve rubredoxin.|

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