Fig. S1. XRD analysis showing the precipitation of calcite in the FBox treatment (down panel) compared with the sterile soil in presence of oxalate (SSox treatment; upper panel). Mineralogical composition determinations were performed using a Scintag diffractometer. X-ray diffractograms were analysed using Macdiff software V5.4.1. In the lower panel (FBox) only the signature for calcite is indicated. The other major components (quartz, gibbsite, kaolinite and pumpellyite) are also present but are not indicated.


Fig. S2. Changes in the composition of bacteria in the microcosms experiment with the Australian soil.

A. Melting curves of frc qPCR products obtained at different sampling time points. The melting analysis was carried out at the end of the frc qPCR by increasing the temperature from 72°C to 95°C.

B. DGGE analysis of the 16S rRNA gene for the FBox treatment at different time points. For DGGE the nearly complete 16S rRNA gene was amplified using the general bacterial primers GM3f and GM4r (Muyzer et al., 1995). The products were cleaned using a multiscreen plate (Millipore) and diluted 100 times to be used as template for a nested PCR amplification with the primers P3 (GC-clamped) and P2 (Muyzer et al., 1993). A touchdown temperature programme was used for nested PCR. A DCode System (Bio-Rad) was used for DGGE of the 16S rRNA gene PCR products. Separation was carried out in 7.5% polyacrylamide gels with a gradient of 35–65% of denaturants (100% denaturants contained 420 g l−1 urea and 400 ml l−1 deionized formamide in 0.5× TAE) during 5 h at 150 V and 60°C. Gels were stained with GelRed (BioTium). The ladder (L) consisted of 16S rRNA sequences from Pandoraea sp. NEU 45 (1), Oxalicibacterium flavum DSM 15507 (2), Ancylobacter polymorphus DSM 18745 (3), Streptomyces violaceoruber DSM 40783 (4), Methylobacterium thiocyanatum NEU 1216 (5), Cupriavidus necator DSM 428 (6), and Escherichia coli NEU 1007 (7).


Fig. S3. Dispersion of the bacterial strains used for the microcosm experiment with Australian soil using the fungal highway.

A. Schematic representation of the experimental set-up. An inverted Petri dish containing nutrient agar (NA) as target medium and straw (on the cover) is used for the inoculation of bacteria alone or a combination of bacteria and fungi. To access the target medium bacteria must cross the air space between the inoculation and target medium. In the case of a fungal highway, hyphae provide a surface that can be used by bacteria to carry out the crossing.

B. Formation of fungal cords crossing the medium-air barrier between the straw and the target medium NA.

C. Growth tests carried out using bacteria alone (right panel) or in combination with the three fungal strains (left panel) listed in Table 1. 1. Bacillus subtilis; 2. Pandoraea sp.; 3. Oxalicibacterium flavum; 4. Escherichia coli; 5. Pseudomonas aeruginosa; 6. Ancylobacter polymorphus; 7. Streptomyces violaceoruber; 8. Cupriavidus necator; 9. Alcaligenes paradoxus.


Table S1. Effect of pH on growth of the oxalotrophic bacterial strains used in the microcosm experiments in both presence and absence of Pycnoporus cinnabarinus. The specific medium used previously was prepared with 1.3% agar and HCl was added to set the pH. Growth was tested simultaneously at pH 7 and 5. Strains unable to grow at pH 5 were then tested for their ability to overpass the pH stress if grown in the presence of P. cinnabarinus. For that purpose, the same media as described above were prepared. Prior to inoculation a circular cavity was prepared in the middle of each Petri dish, which was filled with sterile Miscanthus sp. straw. The fungus was cultured previously in malt agar (12 g l−1, Biolife, Milan, Italy) and transferred to the straw. Then, the bacterial strains were inoculated and incubated at 30°C in the dark. Growth was checked every 24 h for 20 days. For those strains that grew in the presence of the fungus the number of days required for observing colony formation is indicated in brackets. N.D., not determined.

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