I want to perform some evolutionary studies on exoelectrogenic bacteria.
It is just a wild guess, but I would assume that the reason for the existence of such systems lies in a better capability to use substrates under anaerobic conditions using a symbiotic mode of action.
If I remember correctly from a presentation by Falk Harnisch, the bacteria giving up the electrons release them without taking advantage of the full redox potential available. This indicates that the system evolved to support some kind of symbiosis. I would therefore assume that by transferring electrons via other bacteria to ones in the oxic zone, both partners have an advantage: In the anoxic zone, the substrate can be used completely without the need of fermentation or use of other electron acceptors like SO4 or NO3. In the oxic zone, the bacteria performing the terminal transfer to oxygen get also some energy, so both sides win.
This system would most likely evolve in areas with a stable layer of sediments (no disturbance by lugworms and the like) where the anoxic zone is poor in alternative electron acceptors. Thus, n case of a fuel cell, the electrode takes the place of the partner in the oxic zone.
In 1986 Lovley published his seminal observation that bacteria can use amorphous iron oxides as electron acceptors at considerable rates (Lovley and Phillips, Appl Environ Microbiol 1986, 51, 683-689 ); the rest is history. One hypothesis would be that bacteria started out just as science did: by using amorphous iron oxides as electron acceptor, and evolving to subsequently also use other extracellular electron acceptors, including electrodes.
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