SRP185861 PRJNA518900 Potential interactions between syntrophic bacteria and methanogens via type IV pili and quorum sensing systems Interspecies electron transfer plays important roles in syntrophic methanogenesis. Direct interspecies electron transfer via conductive pili between syntrophic oxidizers and methanogens has been only confirmed in some specific co-cultures. Potential syntrophic cooperation via type IV pili between widespread syntrophic bacteria and methanogens was examined by metagenomics analysis of 12 anaerobic sludge samples. The distribution and co-occurrence of methanogens and syntrophic genera were comprehensively investigated. Genes related to type IV pili were assigned to large proportions of syntrophic bacteria and the abundance of these genes was positively correlated with the maximum methane production rate, indicating their contribution to syntrophic methanogenesis. The existences of diverse quorum sensing genes suggested that various quorum sensing systems might participate in the communication of anaerobic microorganisms. Specially, diffusible signal factor (DSF) and 3'-5' cyclic diguanosine monophosphate (c-di-GMP) related genes were mainly assigned to syntrophic partners and the combined regulation of these signals might be responsible for the biosynthesis of type IV pili and thus affecting the syntrophic interaction during methanogenesis.