Deeper insights into microbial mediated syngas biomethanation and process optimization

Syngas biomethanation is a complex process mediated by an intricate microbial consortium, with methane as the final targeted product. In the past decades, syngas biomethanation gained much attention because it can be integrated into the existing biogas facilities and thereby save installation costs.

Moreover, compared to the catalytic methanation, biological syngas methanation has many benefits, such as the requirement for milder operating conditions, reduced sensitivity to gas composition and higher tolerance to trace contaminants. The operational conditions such as pH, temperature, and gas partial pressure play important role in syngas biomethanation, which have been extensively investigated in lab-scale experiments.

Due to the toxicity of CO to microbes, its conversion and metabolism attract much more attention in syngas biomethanation. There are two main conversion pathway for CO metabolism with acetate and H2/CO2 as the main precursors for the following methanogenesis. Due to the poor solubility of H2 and CO, it is necessary to study efficient methods to improve the gas liquid transfer as well as the conversion efficiency of syngas fermentation.

Cultivation of synthetic cocultures can be used to improve overall rates of CO bioconversion, and also the CO-enriched mixed culture presents advantage in CO-conversion rate than that of pure culture. Up to now, there is still needing deeper conclusion on how the parameters affect the syngas biomethanation related to change of functional microbes and involved conversion pathway.

Therefore, the aim of the present thesis is to 1) study the effect of main parameters, including syngas composition, partial pressure of CO, biomass-syngas ration (BGR), and pH-buffer on syngas biomethanation; 2) elucidate the involved functional microbes and metabolic relationship in CO/syngas biomethanation via genome-centric metagenomics analysis; 3) analyze the CO consumption enhancement in co-fermentation of CO/syngas and typical organic substrates, and explain the behind enhancement mechanism.

Firstly, three anaerobic sludges obtained from methanogenic reactors feeding, cattle manure (CS), sewage sludge (SS), and gaseous H2/CO2 (GS) were applied for CO and syngas biomethanation to study the effect from the initial microbial community. The results showed that CS gave the  best performance in CO consumption due to its containing highest relative abundance of CO consuming microbes.

In all the three inocula systems, the CO was mainly converted to acetate before methanogenesis. The syntrophic acetate oxidization (SAO) bacteria played critical role in acetate conversion to H2/CO2 for hydrogenotrophic methanogenesis in CS and GS. However, the generated acetate accumulated in GS due to its lacking SAO bacteria and acetoclastic methanogen.

Subsequently, CO/syngas was conducted under different BGR systems. High BGR showed better performance on CO consumption due to its strong buffer ability. The CO and CO2 in syngas were efficiently converted into puremethane via adding stoichiometric H2. While, adding more H2 declined the consumption rate of CO due to it was competing for endogenic biocarbonate in the media.

Additionally, the microbial community showed that syngas composition could significantly affect the bacteria distribution, while, the Methanothermobacter was the dominant methanogen under all conditions. Furthermore, to deeply investigate the involved functional microbes and metabolic pathway in syngas biomethanation process, CO and syngas with different components ratios were used for enriching the functional microbial community through transfer experiments.

There was a simple functional microbial community left for syngas biomethanation after transfer experiment with feeding CO and syngas. Genome-centric metagenomics analysis revealed that there were different CO consumer metagenome assembled genomes (MAGs) with various CO conversion pathways. Apart from the CO consumer, Coprothermobacter sp. and Methanothermobacter sp. dominated all the enriched systems, with predicted function converting generated acetate to H2/CO2 for methane generation by hydrogenotrophic methanogen.

Lastly, CO and syngas were used for co-fermentation with glucose (GL), protein (PR) and lipid (LI). The results showed that the CO consumption rate was significantly improved via co-fermentation with organic matters, amongwhich PR gave the best performance. According to the genome-centric metagenomic analyses, the CO was mainly consumed in autotrophic type for acetate and H2/CO2 generation in CO solely added system.

The generated acetate was further converted to H2/CO2 via a novel glycine cleavage combining partial Wood-Ljungdahl (WL) pathway for methane generation by hydrogenotrophic methanogens. The added PR promoted the growth of heterotrophic CO consumers for faster CO consumption. Additionally, H2 addition further promoted the CO consumption rate mostly through enhancing the sulfur reduction and auxotrophic growth of CO consumers.

Overall, the present Ph.D thesis studied how the operational parameters affect syngas biomethanation related to change of functional microorganisms and metabolic relationship, which paved pathway for efficient strategy for syngas biomethanation. Additionally, it also elucidated the mechanism of CO consumption enhancement in co-fermentation, which provides a novel strategy for syngas bioconversion to methane.