The Mineralosphere Concept: Mineralogical Control of the Distribution and Function of Mineral-associated Bacterial Communities

© 2015 Elsevier Ltd. 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

Relation entre l'intensité de l'hydrothermalisme ancien ayant affecté les roches mères et l'actuelle défoliation et jaunissement des résineux adultes : exemple du Louschbach (Vosges)

Le faible taux de saturation en cations basiques est un des facteurs qui prédisposent les peuplements forestiers au dépérissement. Ces sols pauvres sont souvent développés sur des roches pauvres qui fournissent peu de cations basiques en s'altérant. Dans le massif des Vosges, d'âge hercynien, l'hydrothermalisme est par endroit très intense et doit être pris en compte. Il modifie la composition chimique et minéralogique des roches et donc leur potentiel nutritif. Au Louschbach, le granite du Valtin (à quartz, feldspath potassique, plagioclase, biotite et muscovite) est recoupé par un filon de quartz + hématite + (mica blanc). Autour de ce filon, se développe une auréole d'altération où le granite est transformé. Il est injecté par des veinules de quartz. Les biotites sont transformées en micas blancs, hématite et quartz. Les plagioclases sont transformés en micas blancs et quartz. Dans la zone très altérée près du filon, la quantité de calcium et de magnésium a diminué de plus de la moitié. Au tour de ce filon, la défoliation et le jaunissement des sapins et des épicéas sont mesurés sur des placettes de 2 500 m2 en fonction du degré d'hydrothermalisme. Le dépérissement augmente nettement lorsque l'hydrothermalisme est intense. Il est plus fort sur les épicéas. Concernant la roche mère, un autre facteur très important pour le dépérissement est la présence de dépôts glaciaires. Ils sont très hétérogènes et influencent le dépérissement dans les 2 sens selon leur nature. Le temps est un facteur important et les mesures montrent que la défoliation a doublé sur une placette témoin entre 1992 et 1994.Relationships between the intensity of past hydrothermalism in the bedrock and present needle loss and yellowing of adult coniferous stands. Example of Louschbach (Vosges mountains). The low basic cation presence in the cationic exchange capacity of soils is one of the factors which favours forest decline. Such poor soils are often developed on poor rock which releases few basic cations during weathering. In the Vosges mountains dating from the Hercynian age, hydrothermal alteration has been very intense in places and must be taken into consideration. This changes the chemical and mineralogical composition of the rocks, and consequently their nutritive potential. In the Louschbach site, the Valtin granite (quartz, K-feldspar, plagioclase, biotite and muscovite) is crosscut by a large quartz/hematite/white mica vein. This vein is surrounded by an alteration halo where the granite is transformed. It is interspersed by some quartz veins (thickest < 500 μm). Biotite has been transformed into white mica, hematite and quartz. Plagioclase has been transformed into white mica and quartz. In the strongly altered zone near the vein, the amount of calcium and magnesium decreases by more than 50%. Around this large vein, the needle loss and yellowing of fir (Abies alba Mill) and spruce (Picea abies Karst) were measured on plots of 2 500 m2 according to the hydrothermal alteration intensity. The decline clearly increases with the intensity of the hydrothermal alteration and is stronger in the spruces. As regards the parent material, another factor that is very important for decline is glacial sediment. This is heterogeneous and influences the decline in both directions depending on their composition. Time is an important factor and the measurements show that needle loss doubled between 1992 and 1994 on the control plot

Répartition des nutriments dans un sol brun acide développé sur tuf (Beaujolais, France). Conséquences pour l'évolution de la fertilité minérale à long terme

Nutrient distribution in an acid brown soil developed from a volcanic tuff bedrock (Beaujolais, France). Consequences for the long-term fertility. The study of soil fertility, often measured only by its short-term component (ie, exchangeable cation reserves), appears insufficient and can mask different situations. This work shows that the middle and long-term fertility characterized by the nutrient content of minerals of three acid brown soils was very different although their exchangeable cation reserves were comparable. The middle and long-term fertility of these soils corresponds to reserves of nutrient elements in the fresh tuff, in the weathered tuff and in the coarse soil fractions originating from the desquamation of the weathered tuff. Differences in the fertility of these three soils were due to the colluviation and to the intense hydrothermalism that affected the very top layer of tuff and provoked a major calcium depletion in the bedrock. Calcium appeared to be the first limiting factor for plant nutrition in this ecosystem. Only the flux of calcium originating from the dissolution of calcic primary minerals in the soil fractions (> 2 μm) was able to maintain the temporary reserve of this element on the adsorbing complex and to meet the nutritional demands of the Douglas-fir stand. The stability of this ecosystem therefore depends mainly on the rate of calcium flux originating from minerals located in the coarse soil fractions, assuming that the organic matter compartment is stable.L'étude de la fertilité d'un sol, souvent mesurée uniquement par sa composante à court terme (stock des éléments échangeables), apparaît insuffisante et peut dissimuler des situations très différentes. Ce travail montre que la fertilité à moyen et à long terme (éléments nutritifs dans les minéraux du sol) de trois sols bruns acides du massif forestier des Aiguillettes est très différente, bien que leurs stocks en éléments nutritifs échangeables soient comparables. La fertilité à moyen et à long terme de ces sols correspond au stock d'éléments nutritifs du tuf sain, de la pellicule d'altération et des fractions granulométriques issues de cette pellicule par desquamation. La différence entre la fertilité des trois profils étudiés est due au colluvionnement et à l'hydrothermalisme intense ayant affecté le toit du tuf, et conduisant notamment à un appauvrissement important en calcium. Cet élément apparaît comme le premier facteur limitant pour la nutrition des peuplements dans cet écosystème. Seul le flux de calcium issu de la dissolution des minéraux primaires calciques des fractions du sol supérieures à 2 μm est capable de maintenir le stock temporaire de cet élément sur le complexe adsorbant et de subvenir aux exigences nutritives du peuplement de douglas : la stabilité de cet écosystème dépend donc en grande partie de l'importance de ce flux dans l'hypothèse de stabilité du compartiment organique

Functional Profiling and Distribution of the Forest Soil Bacterial Communities Along the Soil Mycorrhizosphere Continuum

An ectomycorrhiza is a multitrophic association between a tree root, an ectomycorrhizal fungus, free-living fungi and the associated bacterial communities. Enzymatic activities of ectomycorrhizal root tips are therefore result of the contribution from different partners of the symbiotic organ. However, the functional potential of the fungus-associated bacterial communities remains unknown. In this study, a collection of 80 bacterial strains randomly selected and isolated from a soil-ectomycorrhiza continuum (oak-Scleroderma citrinum ectomycorrhizas, the ectomycorrhizosphere and the surrounding bulk soil) were characterized. All the bacterial isolates were identified by partial 16S rRNA gene sequences as members of the genera Burkholderia, Collimonas, Dyella, Mesorhizobium, Pseudomonas, Rhizobium and Sphingomonas. The bacterial strains were then assayed for beta-xylosidase, beta-glucosidase, N-acetyl-hexosaminidase, beta-glucuronidase, cellobiohydrolase, phosphomonoesterase, leucine-aminopeptidase and laccase activities, chitin solubilization and auxin production. Using these bioassays, we demonstrated significant differences in the functional distribution of the bacterial communities living in the different compartments of the soil-ectomycorrhiza continuum. The surrounding bulk soil was significantly enriched in bacterial isolates capable of hydrolysing cellobiose and N-acetylglucosamine. In contrast, the ectomycorrhizosphere appeared significantly enriched in bacterial isolates capable of hydrolysing glucopyranoside and chitin. Notably, chitinase and laccase activities were found only in bacterial isolates belonging to the Collimonas and Pseudomonas genera. Overall, the results suggest that the ectomycorrhizal fungi favour specific bacterial communities with contrasting functional characteristics from the surrounding soil