Effects of Nitrate Availability and the Presence of Glyceria maxima on the Composition and Activity of the Dissimilatory Nitrate-Reducing Bacterial Community

Competition between different isogenic mutants of Pseudomonas fluorescens unable to carry out the first steps of the denitrification pathway was compared in soil micro-columns non-planted or planted with maize. A new isogenic mutant of P. fluorescens YT101 affected in both nitrate and nitrite respirations was constructed and used as a model of non-denitrifying strain (FM69MS strain). The outcome of the selection exerted by the plant after co-inoculation of FM69MS at the same ratio either with an isogenic denitrifier unable to reduce nitrate (Nar(-) mutant) or with an isogenic NO2 (-) accumulator (Nir(-) mutant) was investigated in non-limiting NO3 (-) conditions. Regardless of the inoculated mixture, both strains were able to grow in both rhizosphere and non-planted soil. The proportion of Nar(-) or Nir(-) strain in the Nar(-)+FM69MS or Nir(-)+FM69MS total introduced population remained stable in non-planted soil. In the rhizosphere, we observed a higher competitiveness of the Nir(-) mutant compared with FM69MS, whereas the latter showed the same competitiveness as the Nar(-) mutant. These results provide the first demonstration that NO3 (-) reduction is the main nitrogen-dissimilating step controlling the competitiveness of P. fluorescens in the rhizosphere.

Section: Discussionmentioning

confidence: 99%

Relative involvement of nitrate and nitrite reduction in the competitiveness of Pseudomonas fluorescens in the rhizosphere of maize under non-limiting nitrate conditions

Ghiglione¹,

Richaume²,

Philippot³

et al. 2002

“…Nitrogen-dissimilating bacteria which are only able to reduce nitrate (NO 3 2 accumulators) are known to be well represented in soil and rhizosphere [19,20]. For example, Nijburg et al [21] reported that NO 3 2 accumulators accounted for about 40% of the dissimilatory nitrate-reducing community in the rhizosphere of Glyceria maxima at low NO 3 3 concentration. Clays-Josserand et al [8] showed that NO 3 2 accumulators represented 38% of Pseudomonas isolates belonging the £uorescent group in the £ax rhizosphere.…”

Section: Discussionmentioning

confidence: 99%

Relative involvement of nitrate and nitrite reduction in the competitiveness of Pseudomonas fluorescens in the rhizosphere of maize under non-limiting nitrate conditions

Ghiglione¹

2002

Competition between different isogenic mutants of Pseudomonas fluorescens unable to carry out the first steps of the denitrification pathway was compared in soil micro‐columns non‐planted or planted with maize. A new isogenic mutant of P. fluorescens YT101 affected in both nitrate and nitrite respirations was constructed and used as a model of non‐denitrifying strain (FM69MS strain). The outcome of the selection exerted by the plant after co‐inoculation of FM69MS at the same ratio either with an isogenic denitrifier unable to reduce nitrate (Nar− mutant) or with an isogenic NO2− accumulator (Nir− mutant) was investigated in non‐limiting NO3− conditions. Regardless of the inoculated mixture, both strains were able to grow in both rhizosphere and non‐planted soil. The proportion of Nar− or Nir− strain in the Nar−+FM69MS or Nir−+FM69MS total introduced population remained stable in non‐planted soil. In the rhizosphere, we observed a higher competitiveness of the Nir− mutant compared with FM69MS, whereas the latter showed the same competitiveness as the Nar− mutant. These results provide the first demonstration that NO3− reduction is the main nitrogen‐dissimilating step controlling the competitiveness of P. fluorescens in the rhizosphere.

“…This species is a common soil heterotroph capable of denitri¢cation isolated from agricultural soils [21] and was the dominant bacterial species isolated from dune soils (De-Boer, personal communication). P. chlororaphis also occurs in the root zone of G. maxima, where it tends to become dominant after amendment with nitrate [22]. We used a strain only capable of reducing nitrate to nitrous oxide [21] for methodological convenience.…”

Section: Bacteriamentioning

confidence: 99%

Interactions between nitrifying and denitrifying bacteria in gnotobiotic microcosms planted with the emergent macrophyte Glyceria maxima

Bodelier¹

1998

The population dynamics of the chemolithoautotrophic nitrifiers Nitrosomonas europaea and Nitrobacter winogradskyi were studied in gnotobiotic microcosms fed with ammonium in response to the presence or absence of the emergent macrophyte Glyceria maxima and the heterotrophic denitrifying bacterium Pseudomonas chlororaphis. By subjecting the plants to different day lengths, the effect of possibly limiting factors (i.e. oxygen and ammonium) on the interactions between the nitrifiers and denitrifying bacterium could be analysed. The presence of the plant had no effect on the growth of nitrifiers suggesting that, in addition to radial oxygen loss from the roots, other non-plant sources of oxygen (e.g. diffusion from the water layer) were important for nitrification. Potential nitrifying activities were suppressed by G. maxima due to ammonium uptake by the plants. Elongation of the day length in combination with the presence of G. maxima led to an increase in the number of P. chlororaphis. The presence of P. chlororaphis suppressed the growth of N. winogradskyi, but the growth of N. europaea and the potential nitrifying activities were not significantly affected. Potential denitrifying activities were stimulated by the plant, but showed no correlations with nitrifier activities or numbers. Apparently ammonium, and not oxygen, was the limiting factor for nitrification in the root zone of G. maxima. However, when the plant did not deplete the ammonium pool, P. chlororaphis could repress the nitrifiers indicating the latter's poor competitive status with respect to oxygen when the presence of root exudates allows for heterotrophic oxygen consumption. z 1998 Federation of European Microbiological Societies. Published by Elsevier Science B.V.