Competition between Nitrospira spp. and Nitrobacter spp. in nitrite oxidizing bioreactors. Biotechnol Bioeng 95

Abstract: In this work the question was addressed if in nitrite-oxidizing activated sludge systems the environ-mental competition between Nitrobacter spp. and Nitros-pira spp., which only recently has been discovered to play a role in these systems, is affected by the nitrite concentrations. Two parallel chemostats were inoculated with nitrifying-activated sludge containingNitrospira and operated under identical conditions. After addition of Nitrobacter to both chemostats, the nitrite concentration in the influent of one of the chemostats was increased such that nitritepeaks in thebulk liquidof this reactorwere detected. The other chemostat served as control reactor, which always had a constant nitrite influent concentra-tion. The relative cellular area (RCA) of Nitrospira and Nitrobacter was determined by quantitative fluorescence in situ hybridization (FISH). The nitrite perturbation stimulated the growth of Nitrobacter while in the undis-turbed control chemostat Nitrospira dominated. Overall, the results of this experimental study support the hypothesis thatNitrobacter is a superior competitorwhen resources are abundant, while Nitrospira thrive under conditions of resource scarcity. Interestingly, the dom-inance of Nitrobacter over Nitrospira, caused by the elevated nitrite concentrations, could not be reverted by lowering the available nitrite concentration to the original level. One possible explanation for this result is that when Nitrobacter is present at a certain cell density it is able to inhibit the growth of Nitrospira. An alternative explana-tion would be that the length of the experimental period was not long enough to observe an increase of th