Direct interspecies electron transfer stimulated in anaerobic structured-bed reactors under microbial stress conditions

This study assessed the role of conductive media as the main stimulant of electroactive organisms development during anaerobic self-regulation stress conditions. Thus, two structured bed anaerobic reactors fed with carbohydrate solution were compared: Reactor 1 (R1), which had a structured bed made of nylon granules (NG), and Reactor 2 (R2), which had a structured bed made of granular activated carbon (GAC). Both reactors were inoculated with extremal low inoculum content (0.05 % v:v) and operated under recirculation stages (batch stages). Thresholds of 80 % for the influent COD consumption and 15 % of COD conversion to CH4 were defined as criteria to stop recirculation mode repetition and start continuous operation with the same input COD (continuous stages). In the three batch recirculation stages, R2 was able to reverse the acidogenic stress aided by GAC presence, reaching higher CH4 production yield 2-fold then R1. R2 provided a methanogenic conversion around 18 % higher than R1 during the change of operation to continuous mode under unstable conditions transition. Analysis of the microbial community composition revealed a greater content of electroactive organisms belonging to the Geobacteraceae and Sphaerochaetaceae families in the GAC-bed. A shift for metabolic routes based on direct interspecies electron transfer was mostly likely a key factor for the reactor R2's greater ability to handle high acetogenic loads. Hence, stress conditions had a selective effect on the organisms, stimulating more electrogenic microbiomes in GAC-bed than in NG-bed.