Co-occurrence network analysis reveals loss of microbial interactions in anaerobic digester subjected to repeated organic load shocks

Fluctuations in the anaerobic digestion (AD) organic loading rate (OLR) cause shocks to the AD microbiome, which lead to unstable methane productivity. Managing these fluctuations requires a larger digester, which is impractical for community-scale applications, limiting the potential of AD in advancing a circular economy. To allow operation of small-scale AD while managing OLR fluctuations, we need to tackle the issue through elucidation of the microbial community dynamics via 16S rRNA gene sequencing. This study elucidated the interrelation of the AD performance and the dynamics of the microbial interactions within its microbiome in response to repeated high OLR shocks at different frequencies. The OLR shocks were equivalent to 4 times the baseline OLR of 2 g VS/L/d. We found that less frequent organic load shocks result to deterioration of methane productivity. Co-occurrence network analysis shows that this coincides with the breakdown of the microbiome network structure. This suggests loss of microbial interactions necessary in maintaining stable AD. Identification of species influencing the network structure revealed that a species under the genus Anaerovorax has the greatest influence, while orders Spirochaetales and Synergistales represent the greatest number of the influential species. We inferred that the impact imposed by the OLR shocks shifted the microbiome activity towards biochemical pathways that are not contributing to methane production. Establishing a small-scale AD system that permits OLR fluctuations would require developing an AD microbiome resilient to infrequent organic loading shocks.

Automated summary

Fluctuations in the organic loading rate (OLR) of anaerobic digestion (AD) can lead to unstable methane production. This study investigated the impact of repeated high OLR shocks on the microbial community dynamics within AD. The results showed that less frequent organic load shocks led to a decrease in methane productivity and a breakdown of the microbiome network structure. The identification of influential species in the network structure revealed the importance of Anaerovorax, Spirochaetales, and Synergistales. It was inferred that the OLR shocks shifted the microbiome activity away from methane production pathways. Overall, developing a resilient AD microbiome would be necessary for small-scale AD systems that can withstand infrequent organic loading shocks.