Box 54 How can oxygen be toxic

It seems strange to us to think of oxygen as a toxic substance, however it can be converted by metabolic enzymes into highly reactive derivatives such as the superoxide free radical (O2-), which are very damaging to cells. Aerobes and most facultative anaerobes convert this to hydrogen peroxide, by means of the enzyme superoxide dismutase. This is further broken down by catalase. Obligate anaerobes do not possess either enzyme, and so cannot tolerate oxygen.

An aerobe is an organism that grows in the presence of molecular oxygen, which it uses as a terminal electron acceptor in aerobic respiration.

An anaerobe is an organism that grows in the absence of molecular oxygen m

(a) Obligate aerobes

(b) Facultative anaerobes

(c) Obligate anaerobes

(d) Aerotolerant (e) Microaerophiles anaerobes

Figure 5.7 Microorganisms have different oxygen requirements. In a static culture, microorganisms occupy different regions of the medium, reflecting their pattern of oxygen usage. (a) Obligate aerobes must grow at or near the surface, where oxygen is able to diffuse. (b) Facultative anaerobes are able to adjust their metabolism to the prevailing oxygen conditions. (c) Obligate anaerobes, in contrast, occupy those zones where no oxygen is present at all. (d) Aerotolerant anaerobes do not use oxygen, but neither are they inhibited by it. (e) Microaerophiles have specific oxygen requirements, and can only grow within a narrow range of oxygen tensions

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