Interfacial boundaries offer insights to assess (a)biotic mineral weathering

Abstract:

Microorganisms support terrestrial ecosystems by extracting and transferring rock-derived nutrients via direct and indirect biological weathering mechanisms. Mineral weathering at the grain-microbe interface is driven by biological processes that include biophysical degradation and biochemical dissolution of mineral particles. Knowledge gaps exist in differentiating the abiotic and biogenic pathways that transform minerals at submicron scales, particularly in field settings. We used correlative high-resolution microscopy and tomography techniques to assess biogeochemical drivers of mineral weathering in natural soils. A reference locator grid method was applied to analyze individual grains retrieved from a field study that deployed mesh bags filled with granular basalt for 3 years. Our method combined: a) helium ion microscopy to capture micron to sub-nanometer scale interactions, and b) scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy to reveal chemical distribution and to generate elemental maps at the same grain locations. Interfacial fungal-grain boundaries were exposed in vertical cross-sections and imaged using focused ion beam SEM, and further interrogated using Transmission Electron Microscopy and Atom Probe Tomography. These analyses were conducted to identify the mineralogical composition of Fe-rich nanominerals observed adjacent to fungal hyphae on the surface of a pyroxene grain. Ti-magnetites were identified as the nanosized Fe-rich crystallites that appeared on the pyroxene surface that were also observed at a depth of at least 20-microns into the grain. Examination of the interfacial boundaries suggest that the Ti-magnetite nanoparticles may have been exposed on the pyroxene surface through direct weathering by fungi adhered to the mineral grain. The correlative imaging method identified grain-organic matter interactions at the micro- to nanoscale and detected evidence of incipient biogenic weathering in complex field systems. The results elucidate the intricate nanoscale environment of microbe-rock interfaces and assess the biologically driven processes in natural environments, which are critical for understanding incipient biogeochemical cycles in Earth’s critical zone.