Facultative anaerobic bacteria enable syntrophic fatty acids degradation under micro-aerobic conditions.

Trace amounts of oxygen stimulate facultative anaerobic bacteria (FAB) within anaerobic bioreactors, which was shown to correlate with enhanced methane production from long-chain fatty acids. The relationship between FAB and fatty acid-degrading syntrophic communities under micro-aerobic conditions is still unclear. In this work, two syntrophic co-cultures, Syntrophomonas wolfei + Methanospirillum hungatei and Syntrophomonas zehnderi + Methanobacterium formicicum, were assembled and incubated with short, medium and long-chain fatty acids, with 0-10 % O₂, in the presence and absence of FAB, here represented by Pseudomonas spp. Without Pseudomonas, the syntrophic activity was inhibited by 79 % at 0.5 % O₂, but with Pseudomonas, the syntrophic co-cultures successfully converted the fatty acids to methane with up to 2 % O₂. These findings underscore the pivotal role of FAB in the protection of syntrophic fatty acid-degrading communities under micro-aerobic conditions and emphasizes its significance in real-scale anaerobic digesters where strictly anaerobic conditions may not consistently be maintained.