Table 1.
Literature values reported for maximum specific growth rates and nitrite affinity constants for Nitrobacter spp. and Nitrospira spp.
Fig 1.
A) Comparison between measured ratios (qpcr results) of Nitrobacter/Nitrospira in terms of their functional genes (NrxbNb/ NrxBNsp) (◌) and 16SDNA (◊) and corresponding simulation result at a hydraulic retention time of 1.7 days (corresponding to μmax of 0.58d-1; red dashed line). The experiment and the simulation were based on an inoculation ratio of (0.75:0.25). B) Influence of the dilution rate on the ratios of Nitrobacter/Nitrospira Nb: (Nsp+Nb). C) Corresponding substrate concentration to biomass profile at given dilution rate. All graphs were determined for constant parameter values for Nitrospira (μmax = 0.67d-1, Ks = 0.21 mgNO2-N l-1) and Nitrobacter (μmax = 1d-1, Ks = 1.5mgNO2-N l-1).
Fig 2.
Simulation result of the critical parameter J between Nitrospira (x) and Nitrobacter (▲) in terms of the dilution rate.
Red (x) indicates the dilution rate above which washout of the pure Nitrospira culture occurred. The bacterium with the lower J will win the competition. The value of the maximum specific growth rate and nitrite affinity constant of both bacteria are given in each graph (A,B,C). Results in panel A are based on values from the experiment. In panel B) all values were kept the same as in panel A except the Ks value of Nitrobacter, which was set smaller (higher affinity). In panel C all values were kept the same as in panel A but the growth rate of Nitrobacter, which was defined to be larger. In zone I Nitrospira wins the competition. Zone II indicates the range of dilution rates at which Nitrobacter (Nb) wins the competition despite a lower affinity for nitrite, and given that the prevailing dilution rate was lower than the μmax of Nitrospira (Nsp). From the moment at which the dilution rate exceeds the μmax of Nitrospira (bold x), Nitrospira is washout out and Nitrobacter dominates the system (zone III).