Answer: They are poisoned by O₂
Explanation:
Obligate anaerobes cannot survive in normal concentrations of oxygen. Depending on the species, tolerance varies from 0.5% to 8% oxygen.
Under normal cellular conditions, O₂ turns into O₂⁻ and H₂O₂, toxic to the organism. Obligate anaerobes lack enzymes superoxide dismutase and catalase, capable of turning O₂⁻ and H₂O₂ back into breathable O₂.