Influence of Starvation on Potential Ammonia-Oxidizing Activity and amoA mRNA Levels of Nitrosospira briensis

The effect of short-term ammonia starvation on Nitrosospira briensis was investigated. The ammonia-oxidizing activity was determined in a concentrated cell suspension with a NO[subscript x] biosensor. The apparent half-saturation constant [K[subscript m(app)]] value of the NH₃ oxidation of N. briensis was 3 [micro]M NH₃ for cultures grown both in continuous and batch cultures as determined by a NO[subscript x] biosensor. Cells grown on the wall of the vessel had a lower K[subscript m(app)] value of 1.8 [micro]M NH₃. Nonstarving cultures of N. briensis showed potential ammonia-oxidizing activities of between 200 to 250 [micro]M N h⁻¹, and this activity decreased only slowly during starvation up to 10 days. Within 10 min after the addition of fresh NH₄⁺, 100% activity was regained. Parallel with activity measurements, amoA mRNA and 16S rRNA were investigated. No changes were observed in the 16S rRNA, but a relative decrease of amoA mRNA was observed during the starvation period. During resuscitation, an increase in amoA mRNA expression was detected simultaneously. The patterns of the soluble protein fraction of a 2-week-starved culture of N. briensis showed only small differences in comparison to a nonstarved control. From these results we conclude that N. briensis cells remain in a state allowing fast recovery of ammonia-oxidizing activity after addition of NH₄⁺ to a starved culture. Maintaining cells in this kind of active state could be the survival strategy of ammonia-oxidizing bacteria in nature under fluctuating NH₄⁺ availability.