Fig. S1. SEM images showing the surface features of rhodochrosite crystals after reaction with (A) no fungi (control sample), (B) Pyrenochaeta sp. DS3sAY3a and (C and D) Stagonospora sp. SRC1lsM3a. In contrast to the control sample, in the presence of fungi, extensive surface etching of the MnCO3 surface is evident both within dissolution pits and on basal surfaces.

Fig. S2. SEM images showing Stagonospora sp. SRC1lsM3a hyphae growing into cracks (A and B) and dissolution pits (C) on the surface of rhodochrosite crystals.

Fig. S3. SEM images showing organic coatings on rhodochrosite crystals after reaction with (A and B) Pithomyces chartarum DS1bioJ1b and (C) Pleosporales sp. AP3s5JAC2b.

Fig. S4. Bulk EXAFS of Mn oxides formed following Mn(II) oxidation of MnCO3 by fungal species A–F (see Table 1 for species name). Three reference compounds needed to reconstruct the mycogenic oxides by linear combination fitting (LCF) are included in gray. Data are shown in solid lines and fits in dotted lines. Fitting results and parameters are listed in Table 1.

Fig. S5. Stereomicroscope images showing the impact of chemical/enzyme inhibitors on Mn oxide formation by Pyrenochaeta sp. DS3sAY3a (A–D) and Stagonospora sp. SRC1lsM3a (E–H). All images depict fungal hyphae and Mn oxides on AY agar plates after approximately 2 weeks of growth.

A and E. AY agar controls amended with 200 μM Mn(II). Brown colour indicates mycogenic Mn oxides.

B and F. Cells grown with 200 μM Mn(II) and 25 μM (B) or 10 μM (F) DPI, an inhibitor of NADPH oxidases.

C and G. Cells grown with 200 μM Mn(II) and 200 μM Cu(II), a scavenger of superoxide.

D and H. Cells grown with 200 μM Mn(II) and 200 μM Zn(II). Zn(II) imparts a similar level of toxicity as Cu(II) but does not react with superoxide.

Fig. S6. Mycogenic Mn oxide samples were analysed with micro-Fourier transform infrared spectroscopy (μ-FTIR). (Above) Mn oxide clusters in cultures of Pyrenochaeta sp. DS3sAY3a. The Mn oxides were not associated with any cellular materials. Clusters were gently removed from the medium with sterile wooden inoculation sticks, washed six times in distilled water and dissolved with 20 mM ascorbic acid to liberate organics from Mn oxides prior to analysis. Samples were analysed on a μ-FTIR at Bruker Optics (Billerica, MA, USA) in reflectance mode. These preliminary μ-FTIR data are promising in that they indicate the presence of organics directly associated with cell-free Mn oxide clusters.

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