Microbial β-oxidation of synthetic long-chain fatty acids to improve lipid biomethanation.

β-oxidation is a well-known pathway for fatty acid (FA) degradation. However, the wide range of feedstocks, their intermediates, and complex microbial networks involved in anaerobic digestion (AD) make β-oxidation unclear during lipid digestion having a variety of long-chain fatty acids (LCFAs). Here, we demonstrated the detailed metabolic pathway of major bacteria and enzymes responsible for the β-oxidation of individual saturated FAs (C16:0 and C18:0) and unsaturated FAs (C18:1 and C18:2). C16:0 showed no negative impact on AD. The relative enzyme abundance and production of shorter-chain FAs (<C16) were steady with continuous biomethane production (739 mL g VS). C18:0 and C18:1 demonstrated a significant AD inhibition. By contrast, C18:2 demonstrated effective degradation and biomethanation. These results indicate that β-oxidation is directly initiated from saturated C16:0 FAs. The presence of C18:0, C18:1, and C18:2 significantly reduced microbial diversity, which recovered in the presence of C16:0. The bacterial genera Clostridium sensu stricto 1, Syntrophomonas, and Sedimentibacter were found in high abundance indicating they might be responsible for most LCFA degradation that resulted in biomethane precursors. Acetoclastic and hydrogenotrophic methanogens were inhibited in the presence of C18:0, C18:1, and C18:2 FAs. Conversion of these FAs to C16 and shorter-chain FAs revived the methanogens. The level of the initiator enzyme of β-oxidation "LCFA-CoA ligase" was increased after the conversion of C18:0, C18:1, and C18:2 to C16:0. The production of other FAs (i.e., <C16) leads to a slight improvement in biomethanation.