Effects of different amino acids and their configurations on methane yield and biotransformation of intermediate metabolites during anaerobic digestion.

Anaerobic digestion (AD) comprises a series of biochemical reactions, with methane as one of the target products. Amino acids (AAs) are important molecular and primary intermediate products when protein is the main component of organic waste/wastewater. The L (levorotatory, left-handed)-configuration is natural for AAs, while D (dextrorotatory, right-handed) -AAs also widely exist in the natural environment and can be generated by racemization. However, the effects and underlying mechanisms of natural AAs and their enantiomers on the methane yield and the underlying mechanisms remain unclear. In this study, the effects of certain widespread L-AAs and their enantiomers on two-stage AD and the mechanisms therein were investigated. The AAs enantiomers showed variable or even opposite effects on different processes. The methane yield from a model monosaccharide (glucose) decreased by 57% with D-leucine addition. The butyrate generation and the methane yield from propionate were sensitive to the AA configuration and were inhibited by D-leucine by 80% and 61.8%, respectively, with D-leucine addition, while the volatile fatty acids concentration was slightly increased with the addition of L-leucine. The related mechanisms were further investigated in terms of key enzymes and microbial communities. The addition of D-Leucine decreased acetic acid production from homoacetogens by 30.2% due to the inhibition of key enzymes involved in hydrogen generation and consumption. The transform of butyryl CoA to butyryl phosphate was the rate-limiting step, with the related enzyme (phosphotransbutylase) was inhibited by D-leucine. Furthermore, the bacteria related to butyric acid generation and organic matter degradation were inhibited by D-leucine, while the methanogenic archaea remained stable irrespective of leucine addition. The effect of D-AAs on microorganisms is related to the type of sludge. In this study, the methanogenetic seed sludge was granular and did not dissociate after treatment; however, the D-AAs could trigger biofilm disassembly and reduce the stability of the sludge floc. The study provides a novel method for regulating AD by adding specific AAs with L or D configuration.