Distribution of extracellular amino acids and their potential functions in microbial cross-feeding in anaerobic digestion systems

Anaerobic digestion is a prevalent bioenergy production process relying on a complex network of symbiotic interactions, where the nutrient based cross-feeding is an essential microbial mechanism. Here, the cross-feeding function was assessed by analyzing extracellular polymeric substances-associated amino acids in microbial ag-gregates collected from 14 lab-scale anaerobic digesters, as well as deciphering their genetically biosynthetic potential by syntrophic bacteria and methanogens. The total concentration of essential amino acids ranged from 1.2 mg/g VSS to 174.0 mg/g VSS. The percentages of glutamic acid (8.5 -37.6%), lysine (2.7 -22.6%), alanine (5.6 -13.2%), and valine (3.0 -10.4%) to the total amount of detected amino acids were the highest in most samples. Through metagenomics analysis, several investigated syntrophs (i.e., Smithella, Syntrophobacter, Syn-trophomonas, and Mesotoga) and methanogens (i.e., Methanothrix and Methanosarcina) were auxotrophies, but the genetic ability of syntrophs and methanogens to synthesize some essential amino acids could be complementary, implying potential cross-feeding partnership.