LAUSR

Abstract Syngas (CO/H 2 ) fermentation provides an alternative method for organics utilization. The hollow-fiber membrane biofilm reactor (HfMBR) offers a potential way to use CO and H 2 effectively. However, studies on syngas utilization by mixed culture fermentation (MCF) in HfMBR were seldom reported. This study is the first to demonstrate and compare the in-situ production of volatile fatty acids via syngas by mesophilic and thermophilic MCF, in which CO and H 2 utilization exceeded 95% and even reached 100%. Acetate (4.22 g/L), butyrate (1.35 g/L), caproate (0.88 g/L), and caprylate (0.52 g/L) were detected at 35 °C, whereas acetate was the main metabolite at 55 °C. The proportion of acetate ranged from 47.5% to 62.7% at 35 °C and exceeded 90% at 55 °C in the batch mode. Notably, the acetate concentration and the production rate could reach 24.6 g/L and 16.4 g/(L . d), respectively, at pH 6.5 in the continuous mode at 55 °C. Thus, thermophilic conditions are preferable for high-purity and high-concentration acetate production from syngas. As revealed by Illumina high-throughput sequencing, Clostridium (41.6%) and Thermoanaerobacterium (92.8%) were the dominant bacteria under mesophilic and thermophilic conditions, respectively. This study demonstrates a potential technique for syngas (CO/H 2 ) utilization and biochemicals production by MCF in HfMBR.