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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.Minerals are a reservoir of nutrients in the soil. The physical weathering, chemical weathering, and biological weathering of minerals play a major role in forest ecosystems as these processes release numerous nutrients required for tree growth (e.g., phosphorus, potassium, magnesium, calcium, and iron). Evidence which indicates that mineral weathering by soil microorganisms affects ion cycling and plant nutrition is accumulating (11,26,44,51). However, to date, little is known about the taxonomic and metabolic diversity of the soil microorganisms involved in this process.The ability to solubilize poorly soluble calcium phosphates, such as hydroxyapatite, or to weather silicates has been described for a range of bacterial genera belonging to the ␣-proteobacteria (Agrobacterium and Rhizobium), the ␤-proteobacteria (Achromobacter and Burkholderia), the ␥-proteobacteria (Acidithiobacillus, Aerobacter, Citrobacter, Enterobacter, Erwinia, and Pseudomonas), and the gram-positive bacteria (Bacillus and Micrococcus) (38), as well as for a range of mycorrhizal (Pisolithus, Paxillus, Trichoderma, and Suillus) (1, 36, 51) and nonmycorrhizal (Aspergillus and Penicillium) (41,45,50) fungi. Phosphate-solubilizing bacteria have been isolated from various environments, such as eutrophic lakes (20), mangrove tree root...

Soil is composed of a mosaic of different rocks and minerals, usually considered as an inert substrata for microbial colonization. However, recent findings suggest that minerals, in soils and elsewhere, favour the development of specific microbial communities according to their mineralogy, nutritive content, and weatherability. Based upon recent studies, we highlight how bacterial communities are distributed on the surface of, and in close proximity to, minerals. We also consider the potential role of the mineral-associated bacterial communities in mineral weathering and nutrient cycling in soils, with a specific focus on nutrient-poor and acidic forest ecosystems. We propose to define this microbial habitat as the mineralosphere, where key drivers of the microbial communities are the physicochemical properties of the minerals.

The principal nutrient source for forest trees derives from the weathering of soil minerals which results from water circulation and from plant and microbial activity. The main objectives of this work were to quantify the respective effects of plant-and root-associated bacteria on mineral weathering and their consequences on tree seedling growth and nutrition. That is why we carried out two column experiments with a quartz-biotite substrate. The columns were planted with or without pine seedlings and inoculated or not with three ectomycorrhizosphere bacterial strains to quantify biotite weathering and pine growth and to determine how bacteria improve pine growth. We showed that the pine roots significantly increased biotite weathering by a factor of 1.3 for magnesium and 1.7 for potassium. We also demonstrated that the inoculation of Burkholderia glathei PML1(12) significantly increased biotite weathering by a factor of 1.4 for magnesium and 1.5 for potassium in comparison with the pine alone. In addition, we observed a significant positive effect of B. glathei PMB1(7) and PML1(12) on pine growth and on root morphology (number of lateral roots and root hairs). We demonstrated that PML1(12) improved pine growth when the seedlings were supplied with a nutrient solution which did not contain the nutrients present in the biotite. No improvement of pine growth was observed when the seedlings were supplied with all the nutrients necessary for pine growth. We therefore propose that the growth-promoting effect of B. glathei PML1(12) mainly resulted from the improved plant nutrition via increased mineral weathering.In ecosystems with low inputs and without any fertilization or soil amendment by humans, the nutrients available to plants come from atmospheric inputs and weathering of soil minerals. This is mainly the case with forest ecosystems which, in addition, are frequently located on poor and acidic soils (3). Plants developing on acidic soils are subjected to two major constraints: (i) high concentrations of ions like Al 3ϩ and H ϩ which inhibit root growth and (ii) low mineral nutrient availability as a result of low reserves and impaired uptake due to high H ϩ concentrations (40).In temperate and boreal forest ecosystems, the vast majority of trees live in close association with symbiotic fungi, the ectomycorrhizal fungi, which connect tree roots to the soil environment via a broad network of hyphae. These fungi contribute to plant nutrition by carrying far away from the roots and from very small pores, the water and the nutrients they released by weathering the primary minerals (30, 59). In addition, the ectomycorrhizal symbiosis modifies root exudation qualitatively and quantitatively by changing the metabolic functions of the roots (34, 51, 56). It creates a special environment called mycorrhizosphere, where the bacterial equilibrium is different from that of the bulk soil (soil without any influence of roots) (36). These modifications of the bacterial equilibrium in the mycorrhizosphere are likely to contribut...

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ABSTRACTPyrosequencing-based analysis of 16S rRNA gene sequences revealed a significant correlation between apatite dissolution and the abundance of betaproteobacteria on apatite surfaces, suggesting a role for the bacteria belonging to this phylum in mineral weathering. Notably, the cultivation-dependent approach demonstrated that the most efficient mineral-weathering bacteria belonged to the betaproteobacterial genusBurhkolderia.

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