If there is a free lunch to be had, someone (or something) will eat it. Inspired by faith in this principle, researchers have analysed the chemical energetics of many combinations of naturally occurring oxidants and reductants. Then, for each combination that is energetically favourable, they have sought an organism that exploits the reaction for its growth. Such searches have not always been successful. For example, over 20 years ago it was asserted that an organism that can use nitrite to oxidize ammonium anaerobically, in the so-called anammox reaction NH_4~+ + NO_2~- → N_2 + 2H_2O, was 'missing' from nature.
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