When Salt Meddles Between Plant, Soil, and Microorganisms.

In extreme environments, the relationships between species are often exclusive and based on complex mechanisms. This review aims to give an overview of the microbial ecology of saline soils, but in particular of what is known about the interaction between plants and their soil microbiome, and the mechanisms linked to higher resistance of some plants to harsh saline soil conditions. Agricultural soils affected by salinity is a matter of concern in many countries. Soil salinization is caused by readily soluble salts containing anions like chloride, sulphate and nitrate, as well as sodium and potassium cations. Salinity harms plants because it affects their photosynthesis, respiration, distribution of assimilates and causes wilting, drying, and death of entire organs. Despite these life-unfavorable conditions, saline soils are unique ecological niches inhabited by extremophilic microorganisms that have specific adaptation strategies. Important traits related to the resistance to salinity are also associated with the rhizosphere-microbiota and the endophytic compartments of plants. For some years now, there have been studies dedicated to the isolation and characterization of species of plants' endophytes living in extreme environments. The metabolic and biotechnological potential of some of these microorganisms is promising. However, the selection of microorganisms capable of living in association with host plants and promoting their survival under stressful conditions is only just beginning. Understanding the mechanisms of these processes and the specificity of such interactions will allow us to focus our efforts on species that can potentially be used as beneficial bioinoculants for crops.

Manganese translocation and concentration on Quercus cerris decomposing leaf and wood litter by an ascomycetous fungus: an active process with ecosystem consequences?

Decomposing fungi translocate manganese (Mn) as demonstrated by the fact that Mn has been found to accumulate on decomposing leaves associated with individual fungal hyphae forming insoluble Mn(III,IV) oxides that remain concentrated in diffuse patches. Here, we studied Mn translocation and precipitation by the saprophytic fungus Alternaria sp. strain FBL507 both on naturally decomposing oak leaves and in vitro experiments. Manganese was translocated and precipitated in beads and encrustations along the fungal hyphae. The combination of X-ray diffraction and scanning electron microscopy-energy dispersive X-ray spectroscopy chemical data showed that the precipitates found on leaves were rhodochrosite (MnCO3), birnessite ([Na, Ca, K]Mn2O4× 1.5H2O) and possibly Mn oxalate. The precipitates on wood were an amorphous Mn-O compound, probably MnO. Thus, Mn oxidation state in the precipitates spanned from +2 to +4, with +3 and +4 only in the birnessite on the leaves. In vitro experiments showed that Mn precipitates formed in living hyphae, suggesting the possibility that Mn precipitation is actively produced by the fungus. Such a possibility raises interesting questions regarding the role of readily available Mn in the activity of saprophytic fungi and other soil microorganisms, such as would result in a large involvement of Mn in the cycles of the major nutrient elements.

Routes of phlogopite weathering by three fungal strains.

Fungi dissolve soil minerals by acidification and mechanical disruption. Dissolution may occur at the microscale (contact between fungus and mineral) and medium scale (entire mineral grains). Mineral weathering by fungi and other microorganisms is thought to be of significant global contribution, perhaps producing specific weathering signatures. We report fungal dissolution of phlogopite mica in experiments with three fungal strains (Alternaria tenuissima, Cladosporium cladosporioides, Stilbella sp.) on solid medium for 30 days at 21 °C and 96-100% relative humidity. The study used variable-pressure SEM-EDS equipped with charge contrast imaging. Statistical analysis of the results discriminated between the weathering activities of the three fungal species, which increased from Stilbella sp. to C. cladosporioides to A. tenuissima, in agreement with the respective decreasing pH in the media (6.4, 5.8, 5.2 ± 0.03). Phlogopite weathering features were irregular and variable, apparently not caused by direct contact with fungal hyphae. EDS values indicated two or more dissolution mechanisms, one of them suggesting cation rearrangement in the mica towards formation of Al-rich smectite. Intimate fungus-mineral interaction was observed, and the lack of observable dissolution traces from such contact interaction is interpreted as the result of effacing by the more intense acid leaching operating at larger scale.