2.3. Metabolomic samples, quenching, and metabolite extraction

The samples for metabolic profiling analysis with the same amount of biomass were collected at the logarithmic phase (31.5 h), stationary phase (56 h), and VB₁₂ production phase (106 h). The cellular metabolism was quenched by immediately adding the samples into 1 mL of 40% methanol (−20°C) and mixing gently. Then, the cells were pelleted by centrifugation (4,000 g, 4°C, 1 min), resuspended in 0.8 mL methanol (−80°C), and rapidly frozen in liquid nitrogen. For analysis, the cells were harvested by centrifugation of the quenched mixture for 10 min at 0°C, and the supernatant was collected. Then, the cells were resuspended in 50% methanol (−40°C). The mixtures were frozen in liquid nitrogen and thawed three times. After centrifugation (10,000 g, 4°C, 10 min), the supernatant was collected. The pellet was resuspended in 50% acetonitrile (−40°C), sonicated for 10 min in an ice bath, and then centrifuged (10,000 g, 4°C, 10 min). Finally, all the supernatants were combined in a single tube, filtered through a Teflon filter with 0.22 μm pore‐size, lyophilized overnight, and stored at −80°C until LC‐MS/MS analysis.