Short-term changes in the anaerobic digestion microbiome and biochemical pathways with changes in organic load.

Fluctuations in organic loading rate are frequently experienced in practical-scale anaerobic digestion systems. These impose shocks to the microbiome leading to process instability and failure. This study elucidated the short-term changes in biochemical pathways and the contributions of microbial groups involved in anaerobic digestion with varying organic load shocks. A mixture of starch and hipolypeptone corresponding to a carbon-to‑nitrogen ratio of 25 was used as substrate. Batch vial reactors were run using acclimatized sludge fed with organic load varying from 0 to 5 g VS/L. Methane yield decreased with increasing organic load. The microbiome alpha diversity represented as the number of operational taxonomic units (OTUs) and the Shannon index both decreased with organic load indicating microbiome specialization. The biochemical pathways predicted using PICRUSt2 were analyzed along with the corresponding contributions of microbial groups leading to a proposed pathway of substrate utilization. Genus Trichococcus (order Lactobacillales) increased in contribution to starch degradation pathways with increase in organic load while genus Macellibacteroides (order Bacteroidales) was prominent in contribution to bacterial anaerobic digestion pathways. Strictly acetoclastic Methanosaeta increased in prominence over hydrogenotrophic Methanolinea with increase in organic load. Results from this study provide better understanding of how anaerobic digesters respond to organic load shocks.