Department of Biotechnology, Delft University of Technology, Van Der Maasweg 9, 2629HZ, Delft, The Netherlands.
Flemish Institute for Technological Research (VITO), Boeretang 200, 2400, Mol, Belgium.
Appl Microbiol Biotechnol. 2023 Sep;107(17):5329-5340. doi: 10.1007/s00253-023-12670-6. Epub 2023 Jul 6.
Syngas fermentation is a leading microbial process for the conversion of carbon monoxide, carbon dioxide, and hydrogen to valuable biochemicals. Clostridium autoethanogenum stands as a model organism for this process, showcasing its ability to convert syngas into ethanol industrially with simultaneous fixation of carbon and reduction of greenhouse gas emissions. A deep understanding on the metabolism of this microorganism and the influence of operational conditions on fermentation performance is key to advance the technology and enhancement of production yields. In this work, we studied the individual impact of acetic acid concentration, growth rate, and mass transfer rate on metabolic shifts, product titres, and rates in CO fermentation by C. autoethanogenum. Through continuous fermentations performed at a low mass transfer rate, we measured the production of formate in addition to acetate and ethanol. We hypothesise that low mass transfer results in low CO concentrations, leading to reduced activity of the Wood-Ljungdahl pathway and a bottleneck in formate conversion, thereby resulting in the accumulation of formate. The supplementation of the medium with exogenous acetate revealed that undissociated acetic acid concentration increases and governs ethanol yield and production rates, assumedly to counteract the inhibition by undissociated acetic acid. Since acetic acid concentration is determined by growth rate (via dilution rate), mass transfer rate, and working pH, these variables jointly determine ethanol production rates. These findings have significant implications for process optimisation as targeting an optimal undissociated acetic acid concentration can shift metabolism towards ethanol production. KEY POINTS: • Very low CO mass transfer rate leads to leaking of intermediate metabolite formate. • Undissociated acetic acid concentration governs ethanol yield on CO and productivity. • Impact of growth rate, mass transfer rate, and pH were considered jointly.
合成气发酵是一种将一氧化碳、二氧化碳和氢气转化为有价值的生化物质的主要微生物过程。产乙酸梭菌是该过程的模式生物，能够在工业上通过同时固定碳和减少温室气体排放，将合成气转化为乙醇。深入了解该微生物的代谢以及操作条件对发酵性能的影响是推进该技术和提高生产产量的关键。在这项工作中，我们研究了乙酸浓度、生长速率和传质速率对 C. autoethanogenum 的 CO 发酵代谢转变、产物产率和速率的单独影响。通过在低传质速率下进行的连续发酵，我们除了乙酸和乙醇外，还测量了甲酸盐的生产。我们假设低传质导致 CO 浓度降低，从而降低 Wood-Ljungdahl 途径的活性，并导致甲酸盐转化的瓶颈，从而导致甲酸盐的积累。向培养基中添加外源性乙酸表明未解离乙酸浓度增加，并控制乙醇产率和生产速率，这可能是为了抵消未解离乙酸的抑制作用。由于乙酸浓度由生长速率（通过稀释速率）、传质速率和工作 pH 决定，因此这些变量共同决定了乙醇的生产速率。这些发现对工艺优化具有重要意义，因为目标是最佳的未解离乙酸浓度可以将代谢转向乙醇生产。关键点： • 非常低的 CO 传质速率会导致中间代谢物甲酸盐泄漏。 • 未解离乙酸浓度控制 CO 上的乙醇产率和生产力。 • 综合考虑了生长速率、传质速率和 pH 的影响。
