To confirm the source of carbon in the C2 products (Figure ), we performed isotopic labelling experiments using C-syngas as the sole substrate for Cladapt and Clwt over 12 days. The isotopologues were analyzed by qNMR spectroscopy (Figure for acetate and Figure for ethanol). Cladapt and Clwt showed acetate (38.2 and 5.3 mM) containing mixtures of C and 12 C, including 13 CH3-13 COO -, 13 CH3-12 COO -, 12 CH3-13 COO -, and 12 CH3-12 COO -. However, only Cladapt produced ethanol (0.7 mM) containing mixtures of 13 CH3-13 CH2OH and 12 CH3-13 CH2OH. For further discussion, see Supplementary Note 1. The 13 C/ 12 C ratios were 86:14 for acetate and 95:5 for ethanol in Cladapt, and 70:30 for acetate in Clwt, indicating that the majority of carbon in the products originates from C-syngas, with a smaller portion of C likely coming from the parental culture or cellular carbon reserves. This finding confirms that both Cladapt and Clwt can produce C2 compounds by converting the carbon in syngas. The higher C content and the presence of ethanol as a more reduced product in Cladapt indicates better syngas uptake and conversion activity compared to Clwt 24 production of C-acetate, when cultivated on C-formate (~30 mM) alongside C-syngas (112 mL headspace) over 4 days.