<P> Obligate anaerobes are microorganisms killed by normal atmospheric concentrations of oxygen (20.95% O). Oxygen tolerance varies between species, some capable of surviving in up to 8% oxygen, others losing viability unless the oxygen concentration is less than 0.5% . An important distinction needs to be made here between the obligate anaerobes and the microaerophiles . Microaerophiles, like the obligate anaerobes, are damaged by normal atmospheric concentrations of oxygen . However, microaerophiles metabolise energy aerobically, and obligate anaerobes metabolise energy anaerobically . Microaerophiles therefore require oxygen (typically 2--10% O) for growth . Obligate anaerobes do not . </P> <P> The oxygen sensitivity of obligate anaerobes has been attributed to a combination of factors: </P> <Ul> <Li> Because molecular oxygen contains two unpaired electrons in its outer orbital, it is readily reduced to superoxide (O) and hydrogen peroxide (H O) within cells . Aerobic organisms produce superoxide dismutase and catalase to detoxify these products, but obligate anaerobes produce these enzymes in very small quantities, or not at all . (The variability in oxygen tolerance of obligate anaerobes (<0.5 to 8% O) is thought to reflect the quantity of superoxide dismutase and catalase being produced .) </Li> <Li> Dissolved oxygen increases the redox potential of a solution, and high redox potential inhibits the growth of some obligate anaerobes . For example, methanogens grow at a redox potential lower than - 0.3 V . </Li> <Li> Sulfide is an essential component of some enzymes, and molecular oxygen oxidizes this to form disulfide, thus inactivating certain enzymes (e.g. nitrogenase). Organisms may not be able to grow with these essential enzymes deactivated . </Li> <Li> Growth may be inhibited due to a lack of reducing equivalents for biosynthesis, because electrons are exhausted in reducing oxygen . </Li> </Ul> <Li> Because molecular oxygen contains two unpaired electrons in its outer orbital, it is readily reduced to superoxide (O) and hydrogen peroxide (H O) within cells . Aerobic organisms produce superoxide dismutase and catalase to detoxify these products, but obligate anaerobes produce these enzymes in very small quantities, or not at all . (The variability in oxygen tolerance of obligate anaerobes (<0.5 to 8% O) is thought to reflect the quantity of superoxide dismutase and catalase being produced .) </Li>

Why is oxygen gas toxic to obligate anaerobes
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