ution (20 mg/ml w/v) for 90 min at 30 °C, while shaking at 1000 rpm. After that, 60 µl of MSTFA was added, and a further incubation step at 37 °C for 30 min at 1,000 rpm was performed. Samples were measured directly after derivatization. For further data normalization, GC-MS samples were spiked with ribitol at a final concentration of 10 µg/ml.

GC-MS conditions and data analysis {#Sec11}
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For GC-MS analysis, 1 µl of each sample was injected. Pool samples of each condition and a whole pool sample of all conditions were generated, and were randomized in the sequence, together with the samples and blanks as described for the LC-MS measurements (see also LC-MS conditions and data analysis). At the beginning and before the end of the batch run, and also after every five samples an alkane mix (decane, undecane, dodecane, tridecane, pentadecane, octadecane, nonadecane, docosane, pentacosane, ocatacosane, dotriacontane, hexatriacontae), and the whole pool sample were injected and analyzed as a quality control. The column was washed after 10 samples by injecting cyclohexane. Samples were analyzed on a Thermo Trace Ultra GC - ITQ900MS GC-MS using a Phenomenex ZB-5MS column (30 m × 0.25 mm i.d., 0.25 µm particle size, plus 5 m integrated guard column) for metabolite separation. The used injection mode was a 1 µl split-less injection with injector ramp conditions of 70 °C to 280 °C at 14 °C/s. Gas chromatography was performed using an initial oven temperature at 70 °C for 1 min that was ramped to 330 °C at 8 °C/min. The helium flow was set to 1.2 ml/min. MS parameters were set for positive electron ionization (EI + ) at 70 eV, and full scan MS mode with an m/z acquisition range of 50-600 m/z. Raw data obtained from GC-MS experiments were exported to netCDF using the Excalibur (Thermo Fisher Scientific) software. Further data analysis and metabolite identification was performed using the latest version of MetaboliteDetector