Although both cooperation and conflict are decisive forces in evolution, some of the most successful microbial strategies for survival have arisen from cooperation. At times, two or more microorganisms can even come together to breathe as one. Breathing, or respiration, accomplishes a most challenging fête: the disposal of electrons generated in metabolic cellular reactions to gain energy for growth. The electron donors and acceptors that microorganisms can use for respiration can be aligned hierarchically on a tower based on their redox or reduction potential (to wit, their affinity for the electrons). The more positive the potential, the easiest it is for the molecule to capture the electrons (i. e., to be reduced) and the more energy that is gained from the reaction. The redox tower provides a visual reference of the energy that microorganisms may generate from the transfer of electrons to each chemical species. Oxygen, for example, has a higher redox potential (it is more electropositive) than nitrate [NO3-)] or iron ([Fe(III], often present in nature as hydrated iron oxide -FeOOH-). As a result, more energy is generated when oxygen is used as the terminal electron acceptor.
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