Biolog analysis and fatty acid methyl ester profiles indicate that pseudomonad inoculants that promote phytoremediation alter the root-associated microbial community of Bromus biebersteinii

Inoculating Dahurian wild rye (Elymus dauricus) or meadow brome (Bromus biebersteinii) with a combination of Pseudomonas aeruginosa strain R75 and P. savastanoi strain CB35 increases degradation of 2-chlorobenzoic acid (2CBA) in soil. In this study the effect of these inoculants on the composition and activity of the root surface microbial community of these plants was investigated. The diversity of substrates utilized by the root-associated microbial community was assessed using Biolog GN and GP plates. The communities were also characterized by extracting fatty acid methyl esters (FAME) from roots of the plant-bacteria associations grown in soil. The capacity of the root surface and rhizosphere soil to degrade 2CBA also was assessed. Inoculating Dahurian wild rye increased the potential of rhizosphere soil to degrade 2CBA by 30%, bur had no effect on Biolog substrate utilization patterns or on root FAME profiles. In contrast, inoculating meadow brome increased the potential of the root surface and associated microorganisms to degrade 2CBA 3.5-fold, and also increased the utilization of amine, amide and polymer Biolog substrates. A cluster analysis of FAME profiles indicated that inoculation had a greater effect on root-associated microbial communities of meadow brome compared to Dahurian wild rye. The combination of strains R75 and CB35 increased the potential of Dahurian wild rye and meadow brome in different manners. Inoculating Dahurian wild rye had little effect on the root surface microbial community but increased the rhizosphere soil's potential to degrade 2CBA. Conversely, inoculating meadow brome altered the root surface microbial community and increased the potential of the root surface and associated microorganisms to degrade 2CBA. This suggests that the mechanism by which bacterial inoculants promote phytoremediation differs between plants. (C) 1998 Elsevier Science Ltd. All rights reserved.