NOVELTY - A biofuel cell system (10) comprises a fuel cell (12) with cathode compartment (14) and anode compartment (16); and a bioreactor (26) with a vessel having chemolithotrophic metal ion-oxidizing microorganisms. The bioreactor is connected to the cathode compartment. The cathode compartment contains cathode with an aqueous electrolyte containing a redox couple (e.g. ferric ions/ferrous ions). At anode (20), a member of redox couple in lower oxidation state, such as ferrous ions, is oxidized to a member of redox couple in higher oxidation state, such as ferric ions, by the microorganisms.    USE - As a biofuel cell system useful for generating electricity; and useful for continuous regeneration of microbial cells, which can be used as a single cell protein.    ADVANTAGE - The biofuel cell system is based on microbial regeneration of oxidant (e.g. ferric ions) using the chemolithotrophic microorganisms, which exhibit very high efficiency and consumes carbon dioxide from atmosphere during electricity generation. The consumed carbon dioxide is transformed into cellular biomass. The fuel cell has improved economy and environmental effect, due to increase in the current efficiency, elimination of the use of platinum at cathode, removal of carbon dioxide from atmosphere and production of potentially-useful product i.e. single-cell protein. The biofuel cell system can be operated continuously, without loss of activity.    DETAILED DESCRIPTION - A biofuel cell system (10) comprises a fuel cell (12) including a cathode compartment (14) and an anode compartment (16) separated by a membrane (18); and a bioreactor (26) including a vessel that encloses chemolithotrophic metal ion-oxidizing microorganisms. The cathode compartment contains a cathode with an aqueous electrolyte containing a redox couple (e.g. ferric ions/ferrous ions), where a first member (e.g. ferric ions) of the redox couple is in a higher oxidation state than a second member (e.g. ferrous ions) of the redox couple. The anode compartment contains an anode (20), and a fuel having a hydrogen constituent pumped into the anode compartment. The anode compartment is separated electrically from the cathode compartment. The chemolithotrophic metal ion-oxidizing microorganisms are selected from members of Leptospirillum excluding Leptospirillum ferrooxidans, Ferroplasma and/or Acidithiobacillus excluding Acidithiobacillus ferroxidans. The bioreactor includes a pump for pumping a fluid containing oxygen and carbon dioxide, into the bioreactor. The bioreactor is in flow communication with the cathode compartment, so that the aqueous solution containing the second member of the redox couple is circulated from the cathode compartment to the bioreactor. At the cathode electrode, the first member of redox couple in higher oxidation state is reduced to the second member of redox couple in lower oxidation state. At the anode electrode, electrochemical oxidation of the fuel takes place to produce electrons and protons that cross from the anode compartment into the aqueous electrolyte, and the second member of the redox couple in the lower oxidation state is oxidized back to the first member of the redox couple in the higher oxidation state by the chemolithotrophic metal ion-oxidizing microorganisms in an aerobic oxidation reaction in the presence of oxygen in the vessel. The separating membrane is permeable to protons. Electrical power is obtained by making electrical connection between a load and the anode and cathode electrodes. The bioreactor and the cathode compartment preferably contain dissolved nutrients for facilitating growth of the chemolithotrophic microorganisms.    DESCRIPTION OF DRAWING(S) - The figure shows a diagrammatic representation of a biofuel cell system.    biofuel cell system (10)    fuel cell (12)    cathode compartment (14)    anode compartment (16)    separating membrane (18)    anode (20)    bioreactor. (26)