Electrochemical techniques reveal that total ammonium stress increases electron flow to anode respiration in mixed-species bacterial anode biofilms

When anode-respiring bacteria (ARB) respire electrons to an anode in microbial electrochemical cells (MXCs), they harvest only a small amount of free energy. This means that ARB must have a high substrate-oxidation rate coupled with a high ratio of electrons used for respiration compared to total electrons removed by substrate utilization. It also means that they are especially susceptible to inhibition that slows anode respiration or lowers their biomass yield. Using several electrochemical techniques, we show that a relatively high total ammonium-nitrogen (TAN) concentration (2.2 g TAN/L) induced significant stress on the ARB biofilms, lowering their true yield and forcing the ARB to boost the ratio of electrons respired per electrons consumed from the substrate. In particular, a higher respiration rate, measured as current density (j), was associated with slower growth and a lower net yield, compared to an ARB biofilm grown with a lower ammonium concentration (0.2 g TAN/L). Further increases in influent TAN (to 3 and then to 4.4 g TAN/L) caused nearly complete inhibition of anode respiration. However, the ARB could recover from high-TAN inhibition after a shift of the MXC's feed to 0.2 g TAN/L. In summary, ARB biofilms were inhibited by a high TAN concentration, but could divert more electron flow toward anode respiration with modest inhibition and recover when severe inhibition was relieved. Biotechnol. Bioeng. 2017;114: 1151–1159.