DISSIMILATORY NITRITE-REDUCTASE PROVIDES A COMPETITIVE ADVANTAGE TO PSEUDOMONAS SP RTC01 TO COLONIZE THE CENTER OF SOIL AGGREGATES

The effect of soil aggregation on denitrification has been studied in different ways: artificial or natural aggregates individually analysed or comparison of denitrifying activity of different size classes of a ggregates. However, until now, no work has been conducted to evaluate the role of denitrification in the microbial colonisation of soil aggr egates. Over a one-month period, we examined on remoulded nonsterile s oil aggregates the survival of inoculated wild-type Pseudomonas strain and of its corresponding isogenic Tn5 mutant (Nir(-)) lacking the abi lity to synthesize the dissimilative nitrite-reductase. Simultaneously , the evolution of the O-2 repartition inside the soil aggregates was assessed by the use of microelectrodes. The inner and outer portions w ere roughly anoxic and oxic, respectively, during the entire experimen t while the intermediate portion showed fluctuating aeration condition s. The values of the Nir(-) to wild-type + Nir(-) ratio were found in the following order: inner < intermediate < outer portion, demonstrati ng that the nitrite-reductase may provide a competitive advantage to t he Pseudomonas strain to colonise the centre of soil aggregates. Howev er, a clear differentiation between inner and outer aggregates portion s was not observed with the indigenous microflora (denitrifiers-to-tot al heterotrophs, biomass or physiological abilities).