Continuous dark-photo fermentative H2 production from synthetic lignocellulose hydrolysate with different photoheterotrophic cultures: Sequential vs. co-culture processes

Integration of dark and photo-fermentation is a promising strategy to efficiently produce renewable hydrogen from different types of waste biomass. In this study, sequential and co-culture dark-photo fermentations were evaluated and compared during continuous operation using synthetic lignocellulose hydrolysate as a substrate. Mixed dark-fermentative bacterial consortium was combined with two different photo-fermentative bacterial cultures: pure culture of Rhodobacter sphaeroides (RSC) and mixed photoheterotrophic consortium (MPC). The sequential process showed higher total H2 yields compared to co-culture fermentation for both RSC and MPC. The highest yield of 4.15 mol H2/molsugar was obtained for the sequential process with RSC at an HRT of 3.5 days and pH 7.5. For the co-culture process, the yield reached 2.88 mol H2/molsugar at pH 7.0 and an HRT of 3.5 days. Complete sugar utilization was observed for dark fermentation. RSC preferred acetate as a carbon source, while MPC preferred butyrate. Photoheterotrophic bacteria, Rhodopseudomonas pseudopalustris and Rhodoplanes piscinae, present in the MPC inoculum were outcompeted by undesirable microorganisms during the continuous process, resulting in low HPR. On the other hand, in both sequential and co-culture processes, a high relative abundance of R. sphaeroides (>40%) was observed in the RSC, indicating effective cooperation between dark and photo bacteria.