The effect of pH and buffer concentration on anode biofilms of Thermincola ferriacetica

We assessed the effects of pH and buffer concentration on current production and growth of biofilms of Thermincola ferriacetica – a thermophilic, Gram-positive, anode-respiring bacterium (ARB) – grown on anodes poised at a potential of − 0.06 V vs. SHE in microbial electrolysis cells (MECs) at 60 °C. T. ferriacetica generated current in the pH range of 5.2 to 8.3 with acetate as the electron donor and 50 mM bicarbonate buffer. Maximum current density was reduced by ~ 80% at pH 5.2 and ~ 14% at 7.0 compared to pH 8.3. Increasing bicarbonate buffer concentrations from 10 mM to 100 mM resulted in an increase in the current density by 40 ± 6%, from 6.8 ± 1.1 to 11.2 ± 2.7 A m^{− 2}, supporting that more buffer alleviated pH depression within T. ferriacetica biofilms. Confocal laser scanning microscopy (CLSM) images indicated that higher bicarbonate buffer concentrations resulted in larger live biofilm thicknesses: from 68 ± 20 μm at 10 mM bicarbonate to > 150 μm at 100 mM, supporting that buffer availability was a strong influence on biofilm thickness. In comparison to mesophilic Geobacter sulfurreducens biofilms, the faster transport rates at higher temperature and the ability to grow at relatively lower pH allowed T. ferriacetica to produce higher current densities with lower buffer concentrations.