NOVELTY - Producing a biofuel using genetically modified iron-oxidizing bacteria (lOB), comprises providing an lOB that have been genetically modified to include a particular metabolic pathway to enable them to generate a particular biofuel; feeding a first source of ferrous iron to the lOB; feeding water, carbon dioxide, and oxygen to the lOB; and producing at least the biofuel, ferric iron, and an lOB biomass.    USE - The methods and system are useful for producing a biofuel or a chemical using genetically modified lOB. The biofuel is one of isobutanol, a long chain alcohol, or an alkane (all claimed).    ADVANTAGE - The IOB can naturally utilize mediators that are more attractive and have the natural ability to fix carbon dioxide while oxidizing ferrous iron to ferric iron. lOB can be grown efficiently in a bioreactor using ferrous iron as the mediator. The systems and methods above use only abundant, inexpensive redox mediators. They do not use costly rare earth elements or organic redox shuttles, and thus can be potentially deployed economically at scale. They potentially exceed an overall efficiency greater than one percent and butanol has desirable fuel properties and is compatible with transportation-fuel infrastructure.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are:    (1) a system for producing biofuels using genetically modified IOB, comprising a bioreactor including lOB that have been genetically modified to include a particular metabolic pathway to enable them to generate a particular biofuel; a first source of ferrous iron in fluid communication with the bioreactor; a source of water in fluid communication with the bioreactor; a source of oxygen in fluid communication with the bioreactor; and a source of carbon dioxide in fluid communication with the bioreactor; and    (2) a method for producing a chemical using genetically modified IOB, by providing an lOB that have been genetically modified to include a particular metabolic pathway to enable them to generate a particular chemical; feeding a first source of ferrous iron to the lOB; feeding water, carbon dioxide, and oxygen to the lOB; producing at least the chemical, ferric iron, and an lOB biomass; electrochemically reducing the ferric iron to a second source of ferrous iron; and feeding the second source of ferrous iron to the lOB.