Uncovering the Role of Fungal Hyphae in Basalt Weathering

The Science  

The weathering of basalt, an igneous volcanic rock, plays an important role in global nutrient cycling through the release of life-supporting elements from rock and soils to the environment. Both abiotic and biotic processes are involved in basalt weathering, and the biotic component involves fungi that weather rock through both mechanical and biochemical processes. However, it is difficult to disentangle fungal-driven weathering from abiotic mechanisms. A multi-institutional team of researchers used high-resolution microscopy and tomography to examine the role of fungal hyphae in the weathering of granulated basalt from a forested soil system. The team observed fungi inhabiting grain surfaces and edges, including a fungal hypha embedded in an interwoven mat or sticky biofilm coating. The fungi were observed at or near a part of the basaltic grains that contained a specific type of magnetic mineral known as titanomagnetite. These findings suggest that fungi may exhibit opportunistic growth near iron-bearing parts of the basalt as a source of the essential micronutrient.

The Impact 

The very long-term weathering of basaltic rock results in the sequestration of carbon into stable carbonate rock, but at the same time atmospheric carbon dioxide is consumed. Assessing the biotic drivers of basalt rock weathering in natural Earth systems is important for recognizing and predicting long-term ecosystem responses to changes in the global carbon cycle. This research provides a better understanding of the fungal-driven biogeochemical reaction pathways related to soil-formation processes, geologic carbon storage, nutrient cycling, the long-term stability of nuclear waste in subsurface repositories, and cultural artifact preservation. It also helps scientists better understand how fungi might have inhabited basalt and contributed the carbon needed to establish life on early Earth and possibly on other planetary bodies.

Summary 

A multi-institutional team of researchers examined the role of fungal hyphae in the initial stages of the weathering of basaltic rock from a three-year mineral weathering experiment deployed at the Calhoun Critical Zone Observatory, South Carolina, in a mixed hardwood-pine forest. High-resolution microscopy surveys of the basaltic grains revealed fungal hyphae adhering to grain surfaces and growing along grain edges. The grains comprised a basaltic glass matrix with titanomagnetite inclusions as detected using energy dispersive X-ray spectroscopy point analyses. A subset of select fungal–grain interfacial boundaries were examined with focused ion beam/scanning electron microscopy and transmission electron microscopy available at EMSL. These investigations revealed that fungal hyphae grew on basalt rock surfaces near an iron-rich mineral called titanomagnetite. The atomic level analysis also showed a likely enrichment of nutrients, specifically calcium, at these locations as well. Together, these observations suggest that fungi are attracted to basalt grains containing iron-rich inclusions that may present a more accessible source of nutrients from the rock.

This research provides new perspectives on the initial stages of basalt rock weathering when exposed to humid, wet conditions in a natural soil system. This also broadens scientific understanding of the role of fungi in global carbon cycles and in other Earth system processes.

Contacts 

Odeta Qafoku, Environmental Molecular Sciences Laboratory, odeta.qafoku@pnnl.gov  

Rebecca Lybrand, University of California, Davis, ralybrand@ucdavis.edu  

Funding 

This research was sponsored by the Department of Energy, Office of Science, Biological and Environmental Research program and the National Science Foundation.  

Publications 

Lybrand, R. A., et al. (2022). Fungal hyphae develop where titanomagnetite inclusions reach the surface of basalt grains. Scientific Reports, 12(1), 3407. [DOI: 10.1038/s41598-021-04157-z]