This thesis reports the development of acid-operating microbial fuel cells (MFCs) for the investigation of elevated electrical conductivity and resulting enhanced bioelectricity generation. This project describes the use of extremophile microorganisms as the biological material in MFCs, for the investigation of low internal resistance biological fuel cells. In particular, this thesis focuses on BPV (biological photovoltaic) cells, a type of MFC that utilises autotrophic biological material, which relies on oxygenic photosynthesis and hence simply requires water as the electron donor (unlike traditional MFCs, which are dependent of an organic substrate feed). Novel reactor designs based on acidophilic and metallotolerant microorganisms, studied using electrochemical techniques, are reported for the first time. The novel strategy consists in the adoption of very low pH and elevated heavy metal concentration levels for biological fuel cell operation, which is possible due to the choice of suitable extremophile microorganisms that are able to thrive under such severe physicochemical conditions. In order to support the analysis of the subject MFCs, a series of electrochemical and fluorescence techniques were employed. Chapter 3 reports the study of standard BPV cells, focusing on classic cell configuration and choice of biological material. BPV cells based on the standard prokaryotic and eukaryotic strains Synechococcus elongatus and Chlorella vulgaris, respectively, were built and electrochemically characterised by means of polarisation curves and continuous power output monitorisation. Subsequently, a study on the potential conditioning of BPV cells was conducted using Pulse Amplitude Modulation (PAM) Fluorimetry; it is the first documented observation of short-term electrolytic potential conditioning effects on photosynthetic efficiency and associated parameters. The work in chapters 4 and 5 explores the extent to which acidophiles may be used as the biological material in MFCs. A search to find a set of naturally-occurring, metallotolerant acidophiles is undertaken throughout the Rio Tinto ecosystem, selected for its unique extreme physicochemical nature and reported extremophile presence. Chapter 4 informs about the physicochemical characterisation of the chosen sampling points, describing the evolution of pH, electrical conductivity, heavy metal concentration, ferric/ferrous ion balance and dissolved oxygen throughout a natural year, in order to identify the sites with the hardest physicochemical conditions. Finally, chapter 5 investigates the presence of living microorganisms in the sampled sites, enabling the identification of the best location for the purpose of this study. A tailored sediment cell was built and tested in situ (for the first time in an extremophilic environment), and compared to the electrical performance of a novel BPV cell based on commercially-available photosynthetic acidophile Dunaliella acidophila.