Competition for nitrate and glucose between Pseudomonas fluorescens and Bacillus licheniformis under continuous or fluctuating anoxic conditions

Abstract

The dissimilatory nitrate-reducing bacterial community in the rhizosphere of aerenchymatous plant species such as Glyceria maxima, consists of oxidative, denitrifying and fermentative nitrate-ammonifying bacteria. To study the respective ecological niches of both types of nitrate-reducing bacteria, competition for nitrate or glucose between the representative denitrifier Pseudomonas fluorescens and the representative fermentative nitrate-ammonifying Bacillus licheniformis under continuous or fluctuating anoxic conditions were performed in continuous culture. Competition started by mixing the separate, steady-state mono-cultures of the two species at different ratios. All the experiments were performed at a dilution rate of 0.05 h31. The competition was followed by measuring concentrations of nitrogen, glucose and fatty acids and by determining the cell numbers of P. fluorescens and B. licheniformis. Under continuous anoxic nitrate-limited conditions and under certain fluctuating anoxic conditions (8 h 10% and 16 h 0% air saturation), B. licheniformis was able to maintain itself in the chemostat at a low percentage of 4^7%. Under continuous anoxic glucose-limited conditions and under specific fluctuating anoxic (16 h 10% and 8 h 0% air saturation) conditions, B. licheniformis washed out. The outcome of the competition was explained by a higher affinity of P. fluorescens for nitrate and glucose compared to B. licheniformis. B. licheniformis was able to maintain itself in the chemostat under continuous anoxic nitrate-limited conditions and under certain fluctuating anoxic conditions (8 h 10% and 16 h 0% air saturation) due to the fermentation of the remaining glucose.