Influence of microbial interactions and processes on soil mineral weathering processes
Soil microorganisms play vital roles in both the physical and chemical processes involved in mineral weathering in soil. One way by which soil organisms, in general, can influence weathering is by the production of acids, ligands, and extracellular polymers that interact with mineral surfaces. Our research focuses on low molecular weight organic acids produced by ectomycorrhizae and ectomycorrhizosphere bacteria. These organisms have received much attention because of their role in the diagenesis of minerals and soil formation as well as for their potential in reforestation, soil bioremediation, and plant nutrition management. Some of our studies are carried out using individual organisms while some look at the effect of interactions among different organisms on mineral weathering. The multitude and diversity of microorganisms in soil are expected to result in diverse and complex manners of microbe-microbe and microbe-mineral interactions. For this reason, understanding of processes in the rhizosphere and soil that lead to mineral weathering necessitates looking at microbial interactions.
One study investigates relationships between the organic acid exudates of Abies lasiocarpa and Picea glauca roots and their ectomycorrhizal fungi and ectomycorrhizosphere bacteria. Pinus contorta and Piloderma fallax are used in in vitro experiments to determine differences in organic acid production and mineral weathering by ectomycorrhizae, non-colonized roots, and free-living fungi. These in vitro experiments also look at the effect of ectomycorrhizosphere bacteria on organic acid production and mineral weatheri ng by ectomycorrhizae. Mineral dissolution is analyzed by monitoring released cations, physical changes in minerals, and changes in the composition and distribution of elements in minerals.
Another study focuses on one of the most commonly produced organic acid by soil microorganisms – oxalic acid – and the associated production of calcium oxalate crystals by these organisms. Many fungi, including ectomycorrhizal fungi, exhibit calcium oxalate crystals on their hyphae. The ubiquity of these crystals on fungal hyphae suggests that they present a selective advantage to the organism. Several hypotheses on the functions that these crystals play have been proposed including regulation of pH and Ca levels as well as serving as Ca reservoir under Ca-limited conditions.
The formation of calcium oxalate crystals on fungal hyphae may not simply be a result of precipitation of Ca by released oxalic acid as previous studies show that the crystals seem to form from within the hyphae at specific sites of origin. Crystal morphology also varies widely among genera and species. Our study focuses on some factors that may affect calcium oxalate crystal formation by Piloderma fallax. We are interested in finding out how variations in the morphology, spatial distribution, abundance, composition, crystall inity and hydration state of calcium oxalate on fungal hyphae correlate with fungal species, intra- and extracellular Ca2+ concentrations, intra- and extracellular oxalic acid concentrations and external phosphate concentrations. Part of the study is devoted to finding out if fungi can utilize the calcium oxalate crystals on their hyphae for growth under conditions of calcium deficiency. We are also looking at how the presence of oxalate-degrading bacteria will affect the formation of calcium oxalate crystals by the fungus.
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