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Natural conversion of ferrihydrite (Fh), a widespread Fe(iii)-oxyhydroxide mineral at the Earth's surface, to thermodynamically more stable iron oxides such as goethite (Gt) and hematite (Hm) is a slow process that spans months to years. Here we examined the effects of synthesis and storage conditions on the hydration, the ratio of tetrahedral to octahedral iron sites, and the transformation of naturally aged 2-line Fh at room temperature and mildly acidic pH over an ageing period of 5 years. Fh samples synthesized and aged in either aerobic or anaerobic conditions were characterized over time by XRD, SEM, thermogravimetric analysis - mass spectroscopy (TGA-MS), and X-ray absorption spectroscopies (XANES and XMCD). The findings show that the ratio of tetrahedral to octahedral Fe(iii) sites in Fh is correlated to its extent of hydration, with fresher Fh samples exhibiting a higher ratio and more bound water. Fresh Fh aged in aerobic conditions has similar bound inorganic carbon, is more hydrated, and has less tetrahedral Fe(iii) than that aged in anaerobic conditions. Hence, for relatively fresh Fh there is a link between Fh properties and storage conditions. However, the long-term ageing characteristics, such as the transformation rate and relative phase fraction of Gt and Hm products, are not noticeably impacted by storage conditions. TGA-MS measurements coupled with O K-edge XANES spectra confirm that Fh tends to lose its hydration as it ages, as expected. Corresponding Fe L2,3-edge XMCD spectra reveal that this dehydration is coupled to a steady decrease in the ratio of tetrahedral to octahedral Fe(iii) sites. In addition to the obvious constraints these findings place on making comparisons across Fh samples of different age and environmental settings, they also highlight that Fh structure, and consequently magnetism, are linked to its bound water content.

Nutrient foraging by fungi weathers rocks by mechanical and biochemical processes. Distinguishing fungal-driven transformation from abiotic mechanisms in soil remains a challenge due to complexities within natural field environments. We examined the role of fungal hyphae in the incipient weathering of granulated basalt from a three-year field experiment in a mixed hardwood-pine forest (S. Carolina) to identify alteration at the nanometer to micron scales based on microscopy-tomography analyses. Investigations of fungal-grain contacts revealed (i) a hypha-biofilm-basaltic glass interface coinciding with titanomagnetite inclusions exposed on the grain surface and embedded in the glass matrix and (ii) native dendritic and subhedral titanomagnetite inclusions in the upper 1-2 µm of the grain surface that spanned the length of the fungal-grain interface. We provide evidence of submicron basaltic glass dissolution occurring at a fungal-grain contact 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 basaltic glass-titanomagnetite boundaries given that titanomagnetite exposed on or very near grain surfaces represents a source of iron to microbes. Furthermore, glass is energetically favorable to weathering in the presence of titanomagnetite. Our observations demonstrate that fungi interact with and transform basaltic substrates over a three-year time scale 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.

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.

The marine cyanobacterium Trichodesmium has the remarkable ability to interact with and utilize air-borne dust as a nutrient source. However, dust may adversely affect Trichodesmium through buoyancy loss and exposure to toxic metals. Our study explored the effect of desert dust on buoyancy and mortality of natural Red Sea puff-shaped Trichodesmium thiebautii. Sinking velocities and ability of individual colonies to stay afloat with increasing dust loads were studied in sedimentation chambers. Low dust loads of up to ~400 ng per colony did not impact initial sinking velocity and colonies remained afloat in the chamber. Above this threshold, sinking velocity increased linearly with the colony dust load at a slope matching prediction based on Stoke's law. The potential toxicity of dust was assessed with regards to metal dissolution kinetics, differentiating between rapidly released metals, which may impact surface blooms, and gradually released metals that may impact dustcentering colonies. Incubations with increasing dust concentrations revealed colony death, but the observed lethal dose far exceeded dust concentrations measured in coastal and open ocean systems. Removal of toxic particles as a mechanism to reduce toxicity was explored using SEM-EDX imaging of colonies incubated with Cu-minerals, yet observations did not support this pathway. Combining our current and former experiments, we suggest that in natural settings the nutritional benefits gained by Trichodesmium via dust collection outweigh the risks of buoyancy loss and toxicity. Our data and concepts feed into the growing recognition of the significance of dust for Trichodesmium's ecology and subsequently to ocean productivity. [ABSTRACT FROM AUTHOR]