Phenotypic structure of <i>Pseudomonas</i> populations is altered under elevated pCO₂ in the rhizosphere of perennial grasses

The increasing atmospheric CO₂ content (pCO₂) is likely to modify the ecosystem functioning including rhizosphere bacteria that are directly dependent on rhizodeposition. This may include alteration of <i>Pseudomonas</i> populations that display phenotypic traits in relation with plant fitness. In the present study, 1228 <i>Pseudomonas</i> strains were isolated from the non-rhizosphere soil, rhizosphere soil and root fractions of perennial grassland systems: <i>Lolium perenne</i> and </i>Molinia coerulea</i>. Both plants were grown under ambient (36 Pa) and elevated (60 Pa) pCO₂ in the Swiss Free Air CO₂ Enrichment (FACE) system. <i>Pseudomonas</i> spp. were tested for their ability to produce auxin, siderophores and hydrogen cyanide, and to dissimilate nitrate. No effect of root proximity and elevated pCO₂ was observed on the proportions of auxin producers. For <i>L. perenne</i> and <i>M. coerulea</i>, siderophore and hydrogen cyanide <i>Pseudomonas</i> producers were stimulated in the root fraction. In contrast lower frequencies of nitrate reducers were observed in the root fraction compared to non-rhizosphere soil. The frequencies of siderophore producers and nitrate dissimilating strains were higher, and those of hydrogen cyanide producers lower, under elevated pCO₂ for <i>L. perenne</i>. This alteration of the phenotypic structure of <i>Pseudomonas</i> guild in the root fraction is discussed in relation with the physico-biochemical modifications of the rhizosphere condition via rhizodeposition and environmental changes occurring under elevated pCO₂.