Table 2 Basic characteristics of the DRPs of A. tumefaciens compared with P. denitrificans when grown in Sistrom's medium at pH 7 and monitored for respiration and accumulation of intermediates (NO2, NO, N2O and N2)

Abbreviation used: nd, not determined.

CharacterTreatmentA. tumefaciensP. denitrificans
[O2] (μM) in liquid at appearance of detectable NO1 mM NO30.460.95
2 mM NO30.380.46
1 mM NO215.21.15
2 mM NO217.22.0
[NO2]max (μM)1 mM NO336nd
2 mM NO3nd1990
[NO]max (nM) in liquidLow cell density*
 1 mM NO3321614.5
 2 mM NO3133219.5
 1 mM NO25410.3
 2 mM NO217614.3
High cell density*
 1 mM NO3960014.4
 2 mM NO3880014.5
 1 mM NO2810017.1
 2 mM NO23800016.4
Ratio between anoxic and oxic growth rate (μanoxicoxic)1 mM NO30.900.61§
2 mM NO30.900.57§
1 mM NO20.780.58
2 mM NO20.780.57
  • * Cell density reached at the time of oxygen depletion, ‘low cell density’ ~108 cells·ml−1, ‘high cell density’ ~5 × 108 cells·ml−1.

  • Oxic growth rates were determined in cultures with initial concentration of O2=7 vol% in headspace. Anoxic growth rates were determined in cultures with initial oxygen near zero.

  • The high NO concentrations apparently inhibited further denitrification, less than 50% of the added NOx was reduced to N2O.

  • § The ratio is calculated based on electron flows derived from the measurement of gaseous intermediates, thus excluding the first reduction step of denitrification. In organisms which accumulate large amounts of nitrite during nitrate reduction (i.e. P. denitrificans), this way of calculating μanoxicoxic will underestimate the ratio when nitrate is the NOx initially available.