Cultivation of nitrifying bacteria in the retentostat, a simple fermenter with internal biomass retention

Abstract The retentostat was developed for long-term continuous, axenic cultivation of microorganisms at those low growth rates which prevail in most natural habitats and which cannot be established properly in chemostats. How a microbial population approaches ‘zero-growth’ was studied in axenic cultures of Nitrosomonas europaea with complete biomass retention at 25°C and constant input of a nutrient solution containing ammonium (0.57 mM) as energy source. Since only cell-free filtrate left the reactor, biomass accumulated until a stable maximum of 2.7 × 10 9 cells ml −1 (398 mg l −1 dry matter) was reached after about 5 weeks. In this state, growth rate approached zero, and the ammonium input just met the substrate demand required for maintenance energy (1.43 μmol NH 3 N mg dm −1 h −1 ). The potential of the retentostat for studying interactions between different microorganisms was demonstrated with a cascade of cultures of Nitrosomonas, Nitrobacter , and a denitrifying Pseudomonas . Thereby the ammonia was completely eliminated from artificial wastewater.