Abstract
Mineral weathering is a balanced interplay among physical, chemical, and biological processes. Fundamental knowledge gaps exist in characterizing the biogeochemical mechanisms that transform microbe-mineral interfaces at submicron scales, particularly in complex field systems. Our objective was to develop methods targeting the nanoscale by using high-resolution microscopy to assess biological and geochemical drivers of weathering in natural settings. Basalt, granite, and quartz (53–250 µm) were deployed in surface soils (10 cm) of three ecosystems (semiarid, subhumid, humid) for one year. We successfully developed a reference grid method to analyze individual grains using: (1) helium ion microscopy to capture micron to sub-nanometer imagery of mineral-organic interactions; and (2) scanning electron microscopy to quantify elemental distribution on the same surfaces via element mapping and point analyses. We detected locations of biomechanical weathering, secondary mineral precipitation, biofilm formation, and grain coatings across the three contrasting climates. To our knowledge, this is the first time these coupled microscopy techniques were applied in the earth and ecosystem sciences to assess microbe-mineral interfaces and in situ biological contributors to incipient weathering.
Highlights
The weathering of Ca- and Mg-silicate rocks stabilizes global climate over geologic timescales by consuming carbon dioxide (CO2) as carbonic acid and nourishes the biosphere by supplying rock-derived nutrients to microorganisms and plants[1,2,3]
We present imagery and data collected by Helium Ion Microscopy (HIM) and Scanning Electron Microscopy (SEM) that provide potential evidence of (i) biomechanical rock disruption manifested by fungal-enhanced expansion of mineral sheets (Fig. 3) and (ii) biochemical transformation of rock demonstrated www.nature.com/scientificreports for one year. (a) The imaging shows fungal hyphae (Hyp) connecting mineral grains and (b) adhering to mineral surfaces. (c) The fungal-mineral contact area shows its footprint in the biofilm (Bfm) and nanoscale evidence of mineral etching (Ech)
We demonstrate the effectiveness of a bimodal microscopy approach to assess how microbes interact with basalt, granite, and quartz deployed in semiarid, subhumid, and humid climates for one year
Summary
The weathering of Ca- and Mg-silicate rocks stabilizes global climate over geologic timescales by consuming carbon dioxide (CO2) as carbonic acid and nourishes the biosphere by supplying rock-derived nutrients to microorganisms and plants[1,2,3]. Microorganisms actively and selectively interact with mineral surfaces by engaging in element capture and transfer processes from ecosystem to molecular levels This is best demonstrated by the mycorrhiza “underground highway” that facilitates the transfer of carbon and nutrients within and between plants through a common hyphal network[5,6]. Helium ion microscopy offers an opportunity to assess natural nanomaterials produced through biogeochemical processes[16], and provides an avenue for addressing how microorganisms (primarily bacteria and fungi; collectively referred to as microbes) transform minerals at submicron scales. Biochemical processes may have a larger effect on weathering than biomechanical transformations[18] as observed in saturated liquid cultures where fungi accelerated biotite dissolution by acidifying the bulk solution at greater rates than direct fungal-mineral interactions[28], but this may not be the case in unsaturated conditions found in field settings. Mesh bags filled with granular rock substrates showed how mycorrhizal fungi employ “biosensing” mechanisms to preferentially colonize and weather basalt compared to granite and quartz under both laboratory conditions[39] and in forest systems[40]
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