PNNL

Abstract

The chemical structure of organic molecules profoundly impacts their interactions with metal ions and mineral phases in soils. Understanding the sources and cycling of metal-chelating compounds is therefore essential to predicting the bioavailability and transport of metals throughout terrestrial environments. Here we investigate the molecular speciation of organic molecules that solubilize trace metals in calcareous soils from Eastern Washington. Ultra-high mass resolution (21 Tesla) Fourier transform ion cyclotron resonance enables us to quickly and confidently identify a wide variety of metal chelators that are produced by microbes that inhabit these soils based on the detection of chromatographic features that match diagnostic metal isotope patterns. We compared two approaches, one based on direct infusion using the incorporation of a rare isotope to validate true iron-binding features, and another based. While the isotopic exchange method requires significantly shorter analysis time, nearly twice as many features were observed with LCMS, mostly due to the reduction in ion suppression where major features limit the sensitivity of minor features. In addition, LCMS enabled cleaner fragmentation spectra to be collected and facilitated feature identification. Siderophores belonging to four major classes were identified, including ferrioxamines, pyoverdines, enterobactins, and arthrobactins. Each of these siderophores likely derives from a unique member of the microbial community, and each possesses different chemical characteristics and uptake pathways, likely contributing to fierce competition for iron within these soils. Our results provide essential insight into the metabolic pathways by which microbes that co-inhabit calcareous soils compete for this essential micronutrient.