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Description: The kinetics of bacterial Fe(II) oxidation was investigated 297m underground at the Äspö Hard Rock Laboratory (near Oskarshamn, Sweden) under steady state groundwater flow conditions in a flow-through cell containing well-developed flocculent mats of bacteriogenic iron oxides (BIOS). Pseudo first-order rate constants of 0.004 min^-1 and 0.009 min^-1 were obtained for chemical and bacterial Fe(II) oxidation, respectively, based on the 104 min retention time of groundwater in the flow cell, inlet Fe(II) concentration of 21.0± 0.5μm, outlet Fe(II) concentration of 8.5 ± 0.7 μm, as well as constant pH = 7.42 ± 0.01, dissolved O2 concentration of 0.11 ± 0.01mg/L, and groundwater temperature of 12.4± 0.1°C. Redox potential was lower at the BIOS-free inlet (-135.4± 1.16mV) compared to inside BIOS within the flow cell (-112.6± 1.91mV), consistent with the Nernst relationship and oxidation of Fe(II) to Fe(III). Further evaluation of the redox potential time series data using detrended fluctuation analysis (DFA) revealed power law scaling in the amplitude of fluctuations over increasing intervals of time with significantly different (p<0.01) DFA α scaling exponents of 1.89 ± 0.03 for BIOS and 1.67 ± 0.06 at the inlet. These α values not only signal the presence of long-range correlation in the redox potential time series measurements but also distinguish between the slower rate of chemical Fe(II) oxidation at the inlet and faster rate accelerated by FeOB in BIOS.