Biomass determination
For the chemostat cultures, biomass growth was monitored by optical density (OD) measurement at a wavelength of 660 nm with a Libra S11 spectrophotometer (Biochrom, Cambridge, United Kingdom). For the fed-batch cultures, biomass growth was monitored by optical density (OD) measurement at a wavelength of 600 nm with a Thermo Genesys spectrophotometer (Thermo Fisher Scientific, MA, USA).
For the chemostat cultures, dry weight was determined by filtering 10 ml culture broth over a pre-weighed nitrocellulose filter with a pore size 0.45 μm, washing the filter with demineralized water and drying the filter for 20 min at 360 W in a microwave oven before weighing again (Postma et al., 1989). Duplicate measurements varied less than 3.5% throughout the cultivation. For the fed-batch cultures, dry weight was determined by centrifuging 2 x 5 mL of culture broth at 6,000 × g for 15 min. The pellet was washed once by resuspending in deionized water and centrifuged again at 6,000 × g for 15 min. After washing, the pellet was dried for 24 hours at 105°C and weighed.

Gas analysis

For the chemostat cultures, the offgas was cooled (2 °C) in a condenser and dried, prior to the analysis of O2 and CO2 concentrations using an NGA 2000 analyzer. For the fed-batch cultures, the offgas was analysed with a Thermo Fischer Prima BT Mass Spectrometer (Thermo Fisher Scientific, MA, USA).

Substrate and metabolite analysis

During the runs, rapidly temperature-quenched samples were taken from the fermenters in order to instantly stop metabolism and obtain representative measurements of extracellular metabolites such as formic acid. Immediately after sampling the samples were cooled in syringes filled with precooled steel beads for fast cooling of the sample (Mashego et al. , 2003). The cells were immediately removed by filtration and samples were stored frozen.
Extracellular concentrations of glucose and formic acid in culture filtrates were analysed by high performance liquid chromatography (HPLC) on an Agilent 1260 HPLC, equipped with a Bio-Rad HPX 87H column, operated at 60°C with 5 mM H2SO4 as mobile phase at a flow rate of 0.600 mL min-1. Detection was performed by means of an Agilent refractive index detector and an Agilent 1260 VWD detector at 210 nm.