ena-STUDY-UFZ-02-11-2020-12:03:22:629-86
RECIRCULATION OF H2, CO2, AND ETHYLENE IMPROVES YIELDS AND CARBON FIXATION OF ANAEROBIC FERMENTATION
Anaerobic fermentation with mixed cultures has been gaining momentum as a bioprocess for its promise to produce platform chemicals from low-value biomass feedstocks. Currently, this technology is limited by 1) low selectivity to the most valuable carboxylates and alcohols; 2) amount of electron donors from the substrate; and 3) competing anaerobic pathways. The most common solutions found for these challenges in lab-scale studies are not always upscalable with the process. In a new approach to tackle these challenges we operated two bioreactors fed with acetate and lactate as a feedstock model with continuously recirculating H2/CO2 to stimulate concomitant autotrophic activity. By 42 days of operation, hydrogenotrophic methanogenesis was predominant and ethylene (=1.3 kPa) was added to one of the reactors, inhibiting methanogenesis completely and allowing net carbon fixation. With methanogenesis inhibited, H2 accounted for 17% (e- equivalent) of the total consumed electron donor. Butyrate/lactate selectivity was 100% (88% in control without ethylene) and caproate/lactate selectivity was 17% (2.3% in control). Community analysis revealed that ethylene caused Methanobaterium to be washed out, giving room to acidogenic genera Eubacterium, Clostridium sensu stricto 12, and Colidextribacter. Culture bottle experiments suggest ethylene to be a more specific methanogenesis inhibitor than 2-bromoethanesulfonate, in particular regarding the non-inhibiting property of ethylene on i-butyrate production. Caproate concentration in the reactor experiment was up to 1.1 g/L, still below extractable titers. However, we believe that longer-term microbial community management, operation with CO, and parameter optimization may enable the concept to become an upscalable strategy to improve efficiency, carbon fixation, and resource conversion of anaerobic fermentation.
Anaerobic fermentation with mixed cultures has been gaining momentum as a bioprocess for its promise to produce platform chemicals from low-value biomass feedstocks. Currently, this technology is limited by 1) low selectivity to the most valuable carboxylates and alcohols; 2) amount of electron donors from the substrate; and 3) competing anaerobic pathways. The most common solutions found for these challenges in lab-scale studies are not always upscalable with the process. In a new approach to tackle these challenges we operated two bioreactors fed with acetate and lactate as a feedstock model with continuously recirculating H2/CO2 to stimulate concomitant autotrophic activity. By 42 days of operation, hydrogenotrophic methanogenesis was predominant and ethylene (=1.3 kPa) was added to one of the reactors, inhibiting methanogenesis completely and allowing net carbon fixation. With methanogenesis inhibited, H2 accounted for 17% (e- equivalent) of the total consumed electron donor. Butyrate/lactate selectivity was 100% (88% in control without ethylene) and caproate/lactate selectivity was 17% (2.3% in control). Community analysis revealed that ethylene caused Methanobaterium to be washed out, giving room to acidogenic genera Eubacterium, Clostridium sensu stricto 12, and Colidextribacter. Culture bottle experiments suggest ethylene to be a more specific methanogenesis inhibitor than 2-bromoethanesulfonate, in particular regarding the non-inhibiting property of ethylene on i-butyrate production. Caproate concentration in the reactor experiment was up to 1.1 g/L, still below extractable titers. However, we believe that longer-term microbial community management, operation with CO, and parameter optimization may enable the concept to become an upscalable strategy to improve efficiency, carbon fixation, and resource conversion of anaerobic fermentation.