Nitrobacter winogradskyi is an abundant, intensively studied autotrophic nitrite-oxidizing bacterium, which is frequently used as a model strain in the two-step nitrification of ammonia (NH3) to nitrate (NO3) via nitrite (NO2-), either in activated sludge, agricultural field studies or more recently in artificial microbial consortia for organic hydroponics. We observed a hitherto unknown cobalt ion-dependent inhibition of cell growth and NO2- oxidation activity of N. winogradskyi in a mineral medium, which strongly depended on accompanying Ca2+ and Mg2+ concentrations. This inhibition was bacteriostatic, but susceptible to natural chelators. L-Histidine effectively restored cell growth and NO2- oxidation activity of N. winogradskyi in mineral media containing Co2+ with >90% recovery. Our results suggest that Co2+ competed with alkaline earth metals during uptake and that its toxicity was significantly reduced by complexation. (C) 2019, The Society for Biotechnology, Japan. All rights reserved.