Fungal organic acid uptake of mineral-derived K is dependent on distance from carbon hotspot

Abstract

Fungal species are foundational members of soil ecosystems with vital contributions that support interspecies resource translocation. The minute details of these biogeochemical processes are poorly investigated. Here, we addressed this knowledge gap by probing fungal growth in a novel mineral-doped soil micromodel platform using spatially-resolved imaging methodologies. We found that fungi uptake K from K-rich minerals using organic acids exuded in a distance-dependent manner from a carbon-rich hotspot. While identification of specific mechanisms within soil remains challenging, our findings demonstrate the significance of reduced complexity platforms such as the mineral-doped micromodel in probing biogeochemical processes. These findings provide visualization into hyphal uptake and transport of mineral-derived nutrients in a resource-limited environment.

Keywords: fungi; microbe-mineral interactions; micromodels; organic acid; weathering.

Fig 1

Fungal organic acid production increases in the presence of minerals. (A) Gas chromatography mass spectrometry data of fungal biomass grown in presence (+mineral) and absence (−mineral) of minerals on potato dextrose agar surfaces show an increase in relative concentration of organic acid in presence of minerals. Data were collected from three biological replicate samples. ***P < 0.01; NS, no significant difference. (B) Fungal growth occurs over 7 days on OG 603 polymer micromodels in presence of minerals (+minerals) between nutrient-rich potato dextrose agar plugs at two ends of the micromodel (C1 and C2) (top image). Fungal hyphae growth on micromodels without minerals (−minerals) (bottom image). Data were collected from 10 biological replicates.

Fig 2

Heterogeneous distribution of fungal organic acids on mineral-doped micromodel surfaces, as visualized with mass spectrometry. MALDI-MSI shows different organic acid production after 7 days (green box, left) and 30 days (orange box, center) of fungal growth on micromodels surfaces. The corresponding optical images show fungal growth on OG 603 mineral-doped micromodels after 7 days (green box, top right) and 30 days (red box, center right). Fungi were inoculated along with PDA plugs at C1. C2 is the second carbon-rich nutrient hotspot with only PDA plugs. The presence of minerals on the polymer surface decreases visibility of fungal hyphae, but hyphal growth is demonstrated across mineral grains (bottom right).

Fig 3

Fungi uptake K from mineral interfaces at hotspots along the hyphae. Fungal hyphae removed from mineral-doped micromodel and embedded in PDMS (top image). C1 denotes MALDI-MSI image around the inoculation/nutrient plug as shown in Fig. 1. Bottom, from right to left: approximate region of hyphae in XRF image. XRF map of organic K (green) and hotspots of K-citrate (red). Two XANES spectra from within the mapped region on micromodel indicate the presence of two different forms of organic potassium; organic acid-bound K and an unidentified organic K used in XRF map fitting (bottom, center).

Fig 4

Uptake of K depends on the organic acid chelated as a function of distance from carbon hotspot. Left: Two XRF maps show differences in K content within fungal hyphae from closer to the inoculation point (A) versus closer to the smaller pore space (B). Not only is K less abundant in region B (determined by detector counts, showing 20× less K in B than A), but K also appears more as hotspots rather than along individual hyphae. Right: Spectroscopy from regions A and B showing experimental spectra in black and LCF in red. Region A is predominantly unidentified organic K, while those from region B contain contributions of K-citrate and K-tartrate. A fitting of the end-member spectra to the XRF maps of A and B shows that only a small region within B has enough K-citrate that can be spatially mapped.