Shewanella species are isolated from the oxic/anoxic regions of seawater and aquatic sediments where redox conditions fluctuate in time and space. Colonization of these environments is by virtue of flexible respiratory chains, many of which are notable for the ability to reduce extracellular substrates including the Fe(III) and Mn(IV) contained in oxide and phyllosilicate minerals. Shewanella oneidensis MR-1 serves as a model organism to consider the biochemical basis of this flexibility. In the present paper, we summarize the various systems that serve to branch the respiratory chain of S. oneidensis MR-1 in order that electrons from quinol oxidation can be delivered the various terminal electron acceptors able to support aerobic and anaerobic growth. This serves to highlight several unanswered questions relating to the regulation of respiratory electron transport in Shewanella and the central role(s) of the tetrahaem-containing quinol dehydrogenase CymA in that process.

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