Interspecific competition and adaptation of anammox bacteria at different salinities: Experimental validation of the Monod growth model with salinity inhibition

Ecological niche segregation of anaerobic ammonium oxidizing (anammox) bacteria under saline environments remains unresolved despite its ecological and practical importance. In this study, niche segregation by salinity for Ca. Brocadia sinica, Ca. Jettenia caeni, Ca. Kuenenia stuttgartiensis and Ca. Scalindua sp. was systematically studied. The inhibitory effect of salinity on specific anammox activity (SAA) was measured experimentally and model-fitted to obtain the salinity-dependent maximum specific growth rates (µ_max). The resulting µ_max were incorporated into a Monod growth model with nitrite as the limiting substrate to predict which anammox bacterial species would dominate at a given salinity. The model predictions revealed that there were threshold salinity ranges where specific growth rates were comparable and the determining factor for the dominant species was the availability of nitrite. Ca. B sinica, Ca. J. caeni, and Ca. K. stuttgartiensis could compete at 0 - 0.5% salinity, while Ca. K. stuttgartiensis and Ca. Scalindua sp. could coexist at around 2% salinity. The model prediction was validated by conducting interspecific competition experiments among the four anammox species in nitrite-limiting membrane bioreactors (MBRs) under different salinity concentrations. The interspecific competition experiments showed that Ca. K. stuttgartiensis with relatively high affinity for nitrite was dominant at a wide range of salinities from 0.25 to 1.75%. Outside this salinity range, Ca. B. sinica was dominant at salinity 0 %, and Ca. Scalindua sp. outcompeted exclusively the other species due to its high salinity tolerance at salinities above 2.0%. These experimental results are in good agreement with the model predictions, demonstrating the validity of the Monod model in accounting for salinity inhibition and reflecting the salinity-dependent distributions of anammox bacteria reported in a variety of engineered and natural saline environments.