Sludge retention time in anaerobic digestion affects Archaea by a cascade through microeukaryotes

Anaerobic digestion is a crucial process for treating organic waste, such as wastewater sludge, agricultural residues and food waste. While the influence of physicochemical parameters on the prokaryotic community composition in anaerobic digesters has been extensively characterized, the role of biotic interactions in shaping the prokaryotic communities remains poorly understood. This study addresses this knowledge gap by analyzing the complete active microbiome of nine full-scale anaerobic digesters. Our findings reveal that eukaryotes, consisting primarily of protists and fungi, account for approximately 40 % of RNA sequence reads alongside dominant Archaea, indicating their substantial role in the digestion process. Our results suggest that the chosen sludge retention time during anaerobic digestion indirectly affects the archaeal community composition and thus treatment efficacy by cascading through eukaryotes, highlighting their integral role in the system. This study highlights the critical role of eukaryotes in regulating prokaryotic communities and their indirect contribution to the optimization of anaerobic digestion efficiency.