Novel Long-Chain Fatty Acid (LCFA)-Degrading Bacteria and Pathways in Anaerobic Digestion Promoted by Hydrochar as Revealed by Genome-Centric Metatranscriptomics Analysis.

A large amount of long-chain fatty acids (LCFA) are generated after lipids hydrolysis in anaerobic digestion (AD), and LCFA are difficult to be biodegraded. This study showed that hydrochar (HC), which was produced during the hydrothermal liquefaction of organic wastes, significantly increased the methane production rate (by 56.9%) of oleate, a typical refractory model LCFA. Genomic-centric metatranscriptomics analysis revealed that three novel microbes (Bin138 Spirochaetota sp., Bin35 Smithellaceae sp., and Bin54 Desulfomonilia sp.) that were capable of degrading LCFA were enriched by HC, which played an important role in the degradation of oleate. LCFA was degraded to acetate through the well-known LCFA β-oxidation pathway and the combined β-oxidation and butyrate oxidation pathway. In addition, it was found that HC promoted the direct interspecies electron transfer (DIET) between Methanothrix sp. and Bin54 Desulfomonilia sp. The enriched new types of LCFA-degrading bacteria and the promotion of DIET contributed to the improved methane production rate of oleate by HC. IMPORTANCE Long-chain fatty acids (LCFA) are difficult to be degraded in anaerobic digestion (AD), and the known LCFA degrading bacteria are only limited to the families Syntrophomonadaceae and Syntrophaceae. Here, we found that hydrochar effectively promoted AD of LCFA, and the new LCFA-degrading bacteria and a new metabolic pathway were also revealed based on genomic-centric metatranscriptomic analysis. This study provided a new method for enhancing the AD of organic wastes with high content of LCFA and increased the understanding of the microbes and their metabolic pathways involved in AD of LCFA.