Cobalt-dependent inhibition of nitrite oxidation in Nitrobacter winogradskyi

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 (NH 3 ) to nitrate (NO 3 − ) via nitrite (NO 2 − ), 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 NO 2 − oxidation activity of N. winogradskyi in a mineral medium, which strongly depended on accompanying Ca 2+ and Mg 2+ concentrations. This inhibition was bacteriostatic, but susceptible to natural chelators. l- Histidine effectively restored cell growth and NO 2 − oxidation activity of N. winogradskyi in mineral media containing Co 2+ with >90% recovery. Our results suggest that Co 2+ competed with alkaline earth metals during uptake and that its toxicity was significantly reduced by complexation.