Biowaste Composting as a Sustainable Platform for Bacterial <scp>CO</scp> Production: Evidence From <scp> <i>cooS</i> </scp> Gene Expression and Microbial Community Profiling

ABSTRACT Current industrial carbon monoxide (CO) production depends mainly on fossil fuels. Composting offers a sustainable alternative, where CO can be produced by compost‐inhabiting bacteria synthesizing the CO dehydrogenase (CODH) enzyme. This study aimed to characterize the microbial community and, for the first time, to investigate the expression of the cooS gene encoding CODH during biowaste composting under laboratory conditions. Composting was performed at mesophilic (45°C) and thermophilic (70°C) temperatures to evaluate CO production under conditions known to stimulate elevated CO emissions. The results confirmed biotechnological potential for microbial CO production within the mesophilic range, potentially reducing costs compared with thermophilic composting. Although the study revealed higher CO concentrations under thermophilic conditions (average 208 ppm; maximum 818 ppm) compared to mesophilic conditions (average 102 ppm; maximum 499 ppm), the compost obtained at 45°C exhibited a more diverse and balanced microbial community than that observed at higher temperatures. In the 45°C compost, bacteria previously reported as CODH or CO producers were identified, including Bacillus licheniformis and representatives of Alphaproteobacteria and Streptomyces . The cooS gene expression analysis showed a significant increase during composting at 45°C, reaching an 11,016‐fold rise within the week. The expression was likely induced by CO and not inhibited by oxygen. This first report demonstrating cooS expression during composting confirmed the biological origin of CO under mesophilic conditions through the activity of anaerobic Ni, Fe‐CODH.