Potassium chloride (kcl)

All chemicals were used as received without further purification and all aqueous solutions were prepared using ultrapure water (Siemens Water Technologies, Barsbüttel, Germany). The electrochemical characterization of the SECM tips, microelectrodes, and recessed microelectrodes as sample surfaces were performed by means of cyclic voltammetry in an electrolyte containing 5 mM [Ru(NH₃)₆]Cl₃ (ABCR, Karlsruhe, Germany) and 100 mM KCl (Riedel-de Haën, Seelze, Germany) as background electrolyte. As a model catalyst a commercial nanoporous ORR catalyst from ETEK (Natick, USA) with 20 wt % Pt on Vulcan XC72 was used. An air saturated 0.1 M sodium phosphate buffer (pH 7, VWR International, Darmstadt, Germany) was used for all investigations of the catalytic activity towards ORR.
Platinum wires with diameters of 25 µm, 100 µm and 250 µm and a gold wire with a diameter of 100 µm were from Goodfellow (Bad Nauheim, Germany). Borosilicate glass capillaries (length 100 mm, outside Ø 1.5 mm, inside Ø 0.75 mm) were purchased from Hilgenberg (Malsfeld, Germany) while quartz glass capillaries (length 95 mm, outside Ø 0.9 mm, inside Ø 0.3 mm) were from Quarzschmelze Illmenau (Illmenau, Germany).

Potassium
chloride (KCl, Honeywell, 99.5%)
and 1,1′-ferrocenedimethanol (FcDM, Sigma-Aldrich, 97%) were
used as supplied. All solutions were prepared with deionized water
(ELGA PURELAB systems; 18.2 MΩ cm at 25 °C). Indium tin
oxide (ITO)-coated glass coverslips, 0.17 mm thick, 20 mm × 20
mm, 8–12 Ω/sq resistivity (SPI Supplies, West Chester,
PA), were cleaned following typical protocols of sonicating in isopropanol
followed by deionized water and then dried in an argon stream.^{20 (link),24 (link)}

S. chartreusis NRRL 3882 was cultivated in chemically defined medium [21 mM NaCl (Carl Roth, Karlsruhe, Germany), 15 mM (NH₄)₂SO₄ (Sigma-Aldrich, St. Louis, MO, USA), 8 mM MgSO₄ (VWR International, Darmstadt, Germany), 27 mM KCl (Honeywell International, Morristown, NJ, USA), 50 mM Tris (Sigma-Aldrich, St. Louis, MO, USA), 0.6 mM KH₂PO₄ (VWR International, Darmstadt, Germany), 2 mM CaCl₂ (Avantor, Radnor, PA, USA), 0.01 mM MnSO₄ (Honeywell International, Morristown, NJ, USA), 11 mM D-glucose (Thermo Fisher Scientific, Waltham, MA, USA), pH 7.5]. The pH was adjusted to 7.5 with HCl. Amino acids were supplemented at a concentration of 0.78 mM. Glutamic acid was added as mono sodium salt. Cultures were incubated for two weeks at 30 °C and 180 rpm in an Innova orbital shaker. If not indicated otherwise, all experiments were conducted three times independently.