Bio-integrated carbon capture and utilization of CO2-to-methane under increasing concentrations of capture agent N-methyl diethanolamine (MDEA)

Conversion of CO 2 to methane (synthetic natural gas) via bio-integrated carbon capture and utilization (BICCU) represents an alternative to conventional CO 2 absorption and subsequent biomethanation. Despite its potential for reducing energy costs associated with carbon capture and conversion, BICCU is constrained by the microbial toxicity of capture agents such as N-methyldiethanolamine (MDEA), which can inhibit methanogenic activity and limit process stability. Knowledge of which microbes are resilient to the effects of MDEA and how this compound interacts with key members of mixed methanogenic communities remains scarce. Therefore, the present study employed a combination of reactor operation and omics-based analyses in a BICCU-to-methane system. The functional basis for microbial conversion of MDEA-captured CO 2 was investigated by monitoring reactor performance and microbial community responses through stepwise increase in CO 2 -saturated MDEA feeding. Starting from 30 mmol∙L -1 MDEA, the reactor produced methane up to 150 mmol∙L -1 , reaching the highest average volumetric methane production rate of 964 NmL∙CH 4 L -1 ∙day -1 at 150 mmol∙L -1 MDEA. Methane production ceased when MDEA concentration reached 175 mmol∙L -1 . 16S rRNA gene sequencing and long-read metagenomics identified Methanothermobacter wolfei and a candidate of the genus Methanobacterium as dominant methanogens, performing hydrogenotrophic methanogenesis, the key pathway for methane production throughout the operation. The increase in MDEA concentrations exerted a distinct selective pressure on the bacterial community, allowing Proteus vulgaris to become dominant. Interestingly, the dominant bacterial taxa did not harbor genes associated with acetate production via the CO 2 -to-acetate formation through the Wood-Ljungdahl pathway. These findings show that progressive MDEA exposure can enrich tolerant methanogenic consortia and define a practical concentration threshold for stable BICCU-to-methane operation.