Open Access

Claudia Gallert, Stefan Dyksma

• Anaerobic digestion (AD) is an important technology for the effective conversion of waste and wastewater to methane. Here, syntrophic bacteria transfer molecular hydrogen (H2), formate, or directly supply electrons (direct interspecies electron transfer, DIET) to the methanogens. Evidence is accumulating that the methanation of short-chain fatty acids can be enhanced by the addition of conductive material to the anaerobic digester, which has often been attributed to the stimulation of DIET. Since little is known about the transcriptional response of a complex AD microbial community to the addition of conductive material, we added magnetite to propionate-fed laboratory-scale reactors that were inoculated with wastewater sludge. Compared to the control reactors, the magnetite-amended reactors showed improved methanation of propionate. A genome-centric metatranscriptomics approach identified the active SCFA-oxidizing bacteria that affiliated with Firmicutes, Desulfobacterota and Cloacimonadota. The transcriptional profiles revealed that the syntrophic bacteria transferred acetate, H2 and formate to acetoclastic and hydrogenotrophic methanogens, whereas transcription of potential determinants for DIET such as conductive pili and outer-membrane cytochromes did not significantly change with magnetite addition. Overall, changes in the transcriptional profiles of syntrophic Bacteria and Archaea in propionate-fed lab-scale reactors amended with magnetite refute a major role of DIET in the studied system.