Physiological Responses of Methanosarcina barkeri under Ammonia Stress at the Molecular Level: The Unignorable Lipid...

Acetotrophic methanogens' dysfunction in anaerobic digestion under ammonia pressure has been widely concerned. Lipids, the main cytomembrane structural biomolecules, normally play indispensable roles in guaranteeing cell functionality. However, no studies explored the effects of high ammonia on acetotrophic methanogens' lipids. Here, a high-throughput lipidomic interrogation deciphered lipid reprogramming in representative acetoclastic methanogen (Methanosarcina barkeri) upon high ammonia exposure. The results showed that high ammonia conspicuously reduced polyunsaturated lipids and longer-chain lipids, while accumulating lipids with shorter chains and/or more saturation. Also, the correlation network analysis visualized some sphingolipids as the most active participant in lipid-lipid communications, implying that the ammonia-induced enrichment in these sphingolipids triggered other lipid changes. In addition, we discovered the decreased integrity, elevated permeability, depolarization, and diminished fluidity of lipid-supported membranes under ammonia restraint, verifying the noxious ramifications of lipid abnormalities. Additional analysis revealed that high ammonia destabilized the structure of extracellular polymeric substances (EPSs) capable of protecting lipids, e.g., declining α-helix/(β-sheet + random coil) and 3-turn helix ratios. Furthermore, the abiotic impairment of critical EPS bonds, including C-OH, C═O-NH-, and S-S, and the biotic downregulation of functional proteins involved in transcription, translation, and EPS building blocks' supply were unraveled under ammonia stress and implied as the crucial mechanisms for EPS reshaping.