13C-Tracer Experiment.

We adopted a steady-state labeling strategy (2). The ¹³C-labeling experiment was performed with 20 mM ¹³C-bicarbonate or ¹³C-formate, respectively, supplemented into the CTFUD defined medium in addition to 14.6 mM cellobiose. For cell growth, C. thermocellum strains were inoculated into these media with a starting OD₆₀₀ of 0.05. When late log-phase was reached (OD₆₀₀ ∼ 0.8), 3 mL of cultures was sampled.