As a greenhouse gas, massive carbon dioxide (CO2) has been generated due to organic matter degradation in wastewater treatment processes. Microbial gas fermentation offers a promising approach to capture CO2 and generate various valuable chemicals. However, limited studies have achieved branched or medium-chain fatty acids production via gas fermentation. This study reported the production of isobutyrate and hexanoate by feeding H2 and CO2 into membrane biofilm reactors (MBfRs). The gas fermentation product in the reactor with neutral pH (pH of 7) was dominated by acetate (accounting for 90 % of the product spectrum), whereas a mildly acidic pH (pH of 6) resulted in isobutyrate and hexanoate as the dominant products, with a selectivity of 57 % and 42 %, respectively. Notably, a remarkably high concentration of isobutyrate (266 mmol C/L) was produced in the reactor with pH of 6. Subsequent batch test results suggest that the isobutyrate production in this study is coupled with acetogenesis and ethanol-driven chain elongation processes, rather than via methanol-driven chain elongation reported previously. High-throughput 16S rRNA gene amplicon sequencing revealed that the microbial community under neutral pH was dominated by acetate-producing homoacetogens Acetobacterium. By contrast, a mildly acidic pH promoted the community shifting towards chain elongation microorganisms, dominated by Clostridium sensu stricto 12, Oscillibacter and Caproiciproducens. Collectively, this study demonstrates the significant role of mildly acidic pH in boosting up bioisomerization and chain elongation in gas fermentation systems, thus triggering isobutyrate and hexanoate production. The findings highlight gas fermentation as a new green alternative route for generating highly valuable isobutyrate and hexanoate.

中文翻译：

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弱酸性 pH 值可加快 H2 驱动气体发酵系统中 CO2 向异丁酸盐的转化


作为一种温室气体，由于废水处理过程中的有机物降解，会产生大量的二氧化碳 （CO2）。微生物气体发酵提供了一种很有前途的方法来捕获 CO2 并产生各种有价值的化学物质。然而，有限的研究已经通过气体发酵实现了支链或中链脂肪酸的生产。本研究报告了通过将 H2 和 CO2 送入膜生物膜反应器 （MBfR） 来生产异丁酸盐和己酸盐。在中性 pH 值 （pH 值为 7） 的反应器中，气体发酵产物以乙酸盐为主（占产品谱的 90%），而弱酸性 pH 值 （pH 值为 6） 导致异丁酸盐和己酸盐成为主要产物，选择性分别为 57 % 和 42 %。值得注意的是，在 pH 值为 6 的反应器中产生了非常高浓度的异丁酸盐 （266 mmol C/L）。随后的批次测试结果表明，本研究中的异丁酸酯生产与乙酸生成和乙醇驱动的链伸长过程相结合，而不是通过先前报道的甲醇驱动的链伸长。高通量 16S rRNA 基因扩增子测序显示，在中性 pH 值下，微生物群落以产生乙酸盐的同源乙酸杆菌为主。相比之下，弱酸性 pH 值促进了群落转向链伸长微生物，其中以 Clostridium sensu stricto 12、Oscillibacter 和 Caproiciproducens 为主。总的来说，本研究证明了弱酸性 pH 值在促进气体发酵系统中的生物异构化和链长化方面的重要作用，从而触发异丁酸盐和己酸盐的产生。 研究结果强调，气体发酵是生产高价值异丁酸盐和己酸盐的新型绿色替代途径。