Review on the intermediate amino acids and their enantiomers during the anaerobic digestion: the distribution, biofunctions and mechanisms

Amino acids (AAs) are important intermediate metabolites during the anaerobic digestion (AD) of protein-rich organic matter. L (levorotatory, left-handed) AAs are the predominant AAs found in proteolysis process, and can be converted to D (dextrorotatory, right-handed)-AAs. Alkaline, heat pretreatment and fermentation of the organic matter may lead to the transformation of AAs configuration, that is, L-AAs and D-AAs co-exist in the AD system. D-Ala, D-Asp, and D-Glu are the most frequently detected in food waste. D-AAs also widely exist in seeds, leaves and the extracellular biomolecules of bacterial cell walls and can be applied in pharmaceutical industry and agrochemical industry. Compared with L-AAs, D-AAs is not easily decomposed by microorganisms, thus D-AAs accumulate in waste/wastewater or natural environments. Many evidences showed that D-AAs have specific biological functions. AAs and their enantiomers have certain biological characteristics which are closely related to their configuration, which will have different effects on the microorganisms in the anaerobic system, and eventually affect the stability of AD and the yield of biogas. Therefore, this paper mainly reviews the distribution of AAs and enantiomeric function in AD system, analyzes the promoting/inhibitory effect of AAs on AD and the mechanisms, and puts forward some suggestions for future studies of AD of organic matter.