PNNL

Abstract

Fungi-mineral interfaces play an integral role in biogeochemical cycling by transforming minerals and releasing nutrients bound in rocks to the ecosystem. Nutrient foraging by fungi weathers rocks by mechanical and biochemical processes. We examined the role of fungal hyphae in the incipient weathering of granular basalt substrate from a three-year field experiment in a mixed hardwood-pine forest (S. Carolina) to identify grain alteration at the nanometer to micron scales based on microscopy-tomography analyses. Investigations of fungal-grain contacts revealed i) a hypha-biofilm-glass interface with titanomagnetite exposed on the grain surface and embedded in the glass matrix and ii) dendritic and subhedral titanomagnetite inclusions in the upper 1-2 µm of the grain surface that spanned the length of the fungal-grain interface. To our knowledge, we provide the first submicron evidence of basaltic glass dissolution occurring directly at a fungal-grain interface in a soil field setting. An example of how fungal-mediated weathering can be distinguished from abiotic mechanisms in the field was demonstrated by observing hyphal selective occupation and hydrolysis of glass-titanomagnetite surfaces. We hypothesize that the fungi were drawn to glass-titanomagnetite boundaries given that titanomagnetite exposed on or near grain surfaces represents a source of iron to microbes. We discovered that fungal growth on grains coincided with regions of the basalt glass matrix that also contain magnetite inclusions that originated during the formation of the rock. We conclude that magnetite enhances the accessibility of the glass matrix and provides a source of iron at or near the surface of basaltic grains, which creates an advantageous environment for fungi to forage and cycle nutrients. Our findings demonstrate that soil fungi interact with and transform basaltic substrates over short, three-year time scales in field environments, which is central to understanding the rates and pathways of biogeochemical reactions related to nuclear waste disposal, geologic carbon storage, nutrient cycling, cultural artifact preservation, and soil-formation processes.