The inhibitory effect of free ammonia (NH(3) or FA) and free nitrous acid (HNO(2) or FNA) on the ammonia oxidizing bacteria (AOB) and the dependence of the AOB activity on the concentration of total inorganic carbon (TIC) are well-established. In contrast, less is know about the effect of high FA and FNA concentrations in combination with TIC limitation. Respirometric tests performed with an enriched AOB sludge (81% of AOB as measured with fluorescent in situ hybridization) established that AOB inhibition by FA under TIC limitation was higher than under non-limiting TIC conditions (Haldane inhibition coefficients of 139 and 376 mg NH(3) L(-1)). AOB affinity for FA decreased under TIC limitation conditions (half-saturation coefficient of 0.28 mg NH(3) L(-1) without TIC limitation and 4.3 mg NH(3) L(-1) with TIC limitation). Higher inhibition by FNA was observed when TIC was limited since the non-competitive inhibition coefficient decreased from 1.31 mg HNO(2) L(-1) (without TIC limitation) to 0.21 mg HNO(2) L(-1) (with TIC limitation). This study demonstrates that AOB inhibitions by FA and FNA are amplified with TIC limitation and consequently, AOB dynamics are strongly modified under TIC limitation conditions. (C) 2010 Elsevier Ltd. All rights reserved.