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Applied and Environmental Microbiology, May 2007, p. 3019-3027, Vol. 73, No. 9
0099-2240/07/$08.00+0     doi:10.1128/AEM.00121-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Effect of the Mycorrhizosphere on the Genotypic and Metabolic Diversity of the Bacterial Communities Involved in Mineral Weathering in a Forest Soil

INRA/UHP UMR 1136 "Interactions Arbres Micro-organismes," Centre INRA de Nancy, 54280 Champenoux,¹ INRA UR 1138 "Biogéochimie des Ecosystèmes Forestiers," Centre INRA de Nancy, 54280 Champenoux,² INRA-ENGREF, Ecologie Forestière, 54042 Nancy,³ CNRS LIMOS, UHP Nancy 1, Faculté des Sciences, Domaine Victor Grignard, B.P. 239, 54506 Vandoeuvre-les-Nancy, France⁴

Received 17 January 2007/ Accepted 28 February 2007

To date, several bacterial species have been described as mineral-weathering agents which improve plant nutrition and growth. However, the possible relationships between mineral-weathering potential, taxonomic identity, and metabolic ability have not been investigated thus far. In this study, we characterized a collection of 61 bacterial strains isolated from Scleroderma citrinum mycorrhizae, the mycorrhizosphere, and the adjacent bulk soil in an oak forest. The ability of bacteria to weather biotite was assessed with a new microplate bioassay that measures the pH and the quantity of iron released from this mineral. We showed that weathering bacteria occurred more frequently in the vicinity of S. citrinum than in the bulk soil. Moreover, the weathering efficacy of the mycorrhizosphere bacterial isolates was significantly greater than that of the bulk soil isolates. All the bacterial isolates were identified by partial 16S rRNA gene sequence analysis as members of the genera Burkholderia, Collimonas, Pseudomonas, and Sphingomonas, and their carbon metabolism was characterized by the BIOLOG method. The most efficient isolates belonged to the genera Burkholderia and Collimonas. Multivariate analysis resulted in identification of three metabolic groups, one of which contained mainly bacterial isolates associated with S. citrinum and exhibiting high mineral-weathering potential. Therefore, our results support the hypothesis that by its carbon metabolism this fungus selects in the bulk soil reservoir a bacterial community with high weathering potential, and they also address the question of functional complementation between mycorrhizal fungi and bacteria in the ectomycorrhizal complex for the promotion of tree nutrition.

Published ahead of print on 9 March 2007.