CO2-fixing one-carbon metabolism in a cellulose-degrading bacterium Clostridium thermocellum

High carbon yield in the bioengineering of heterotrophic bacteria is hindered by carbon loss to CO₂ production. We provide evidence showing Clostridium thermocellum, a cellulose-degrading bacterium and a model consolidated bioprocessing organism, can fix CO₂ while growing predominantly on cellobiose, a cellulose-derived disaccharide. By adding ¹³C-bicarbonate to the bacterial culture and tracking ¹³C-labeled metabolites, we discovered active reductive one-carbon (C1) metabolism in this bacterium. We further identified critical enzymes responsible for fixing CO₂ and channeling the fixed carbon to the C1 metabolic pathway. Our findings pave the way to future engineering of this bacterium to use cellulose and CO₂ simultaneously as a means to improve microbial carbon efficiency that is constrained by theoretical limitation and to reduce CO₂ in the environment.