Methanogenesis Recovery Mechanism Under Extreme Acidic Stress in Failed Solid-State Anaerobic Digestion System of Maize Straw

Volatile fatty acid accumulation may cause inhibition, or complete cessation of methanogenesis, which was undesirable for large-scale anaerobic digestion (AD) engineering. In this study, it was unexpectedly observed that methanogenesis gradually resumed after a long period of time when gas production had stopped due to acid inhibition in maize straw solid-state (SS)-AD. The results showed SS-AD achieved the cumulative methane production of 9.32 mL/gVS and maintained the acetic acid degradation rate at 70.8%–88.8% within 8 days of methanogenesis recovery. 16S rRNA amplicon sequencing and metagenomic analysis revealed that Thermoclostridium, Defluviitalea, and Hydrogenispora were the key bacteria resisting extreme acidic stress, while Methanosarcina mazei, Methanoculleus thermophilus, and Methanosarcina thermophila were the key archaea promoting methanogenesis recovery. Microorganisms survived under extreme acidic stress mainly by lysine decarboxylation and biosynthesis of cell membranes/walls and flagella. Meanwhile, enhanced tryptophan synthesis and metabolism accelerated carbon supply to TCA cycle, promoting the growth and reproduction of microorganisms under extreme acidic stress. Moreover, the genetic information processing ability and CRISPR-Cas system were enhanced in M. mazei and M. thermophila, which favored their survival and growth in SS-AD. M. thermophilus mainly contributed to the methanogenesis by CO₂ reduction. This study helps in developing SS-AD methodology for overcoming extreme acidic stress.

Graphical Abstract