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

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.|