What functional group (or groups) of microbes would be most useful in a microbial fuel cell in which ...

 Iron-reducers and iron-oxidizers would be most useful in this type of microbial fuel cell, as they are adapted to transfer electrons to and from solid electron acceptors and donors. Most terminal oxidases and other electron transport proteins are inside the cell wall in the cytoplasmic membrane where they cannot contact solid surfaces. Iron-reducers in particular have electron transport proteins embedded in the outer membrane where they can effectively transfer iron to and from solid redox active surfaces (such as metal electrodes).

I'm still confused, but thanks for answering correctly