niche. Obligate aerobes such as Rhizobium (a chemoheterotroph), Thiobacillus (a chemoautotroph), and many other soil bacteria require oxygen, which acts as a terminal electron acceptor during aerobic respiration. Obligate anaerobes such as Clostridium pasteurianum require the absence of molecular oxygen and, instead of using inorganic electron acceptors, remove hydrogen atoms from organic compounds and dissipate them through reaction with products of carbohydrate breakdown. Facultative anaerobes, such as the denitrifying chemoheterotroph Pseudomonas aeruginosa or, less commonly, the chemoautotroph Thiobacillus denitrificans, can grow in the presence or absence of oxygen. For these denitri-fiers, nitrate substitutes for oxygen as an alternative terminal electron acceptor during anaerobic respiration and is reduced to either nitrous oxide or free N2. The switch from aerobic to anaerobic metabolism is quite rapid (occurring within a few hours in most cases) and is controlled by oxygen availability:

The stepwise reduction of nitrate to N2 by facultatively anaerobic denitrifying bacteria occurs as follows:

(2) 2NO2 + 3H2 ^ N2O + 3H2O; (2) nitrite ^ nitrous oxide;

(3) N2O + H2 ^ N2 + H2O (3) nitrous oxide ^ free N2.

Because of the accumulation of incompletely oxidized products during growth, the energy yields from anaerobic metabolism in soil are generally low (often only a few percent) compared to aerobic metabolism.

It is becoming increasingly clear that while the classification in Fig. 5.7 holds true, many aerobic processes are carried out in soil using sparingly available supplies of oxygen by microaerophiles, obligate aerobes that grow best at low oxygen tensions.

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