Differential ion transport and biomass charges create strong electric fields in anammox granules

Quantifying mass transport of substrates and products within biofilms is crucial for the identification of limiting factors and thereby for the optimization of bioprocess. Current models assume that concentration gradients (i.e., molecular diffusion) control the transport of solutes within biofilms. Here, we document for the first time the presence of electric fields within anammox granules. The measured intensity of these electric potential fields was unprecedented (up to 360 V/m) compared to other biological systems. Mathematical modeling indicates that biomass behaves as a weak ion exchanger towards NH₄⁺, thereby inducing a diffusion potential. These electric fields, in turn, support the migration of ions, and the contribution of ionic migration to the total transport of NO₂⁻, NH₄⁺ and NO₃⁻ matches the magnitude of molecular diffusion. Our data indicates that neglecting ionic migration could result in significant error in estimating mass transport and therefore limiting reactants and reaction rates within anammox granules and, potentially, in a broader range of natural and artificial biofilms.