Abstract：
Medium-chain carboxylates (MCCs) are used in various industrial applications. These chemicals are typically extracted from palm oil, which is deemed not sustainable. Recent research has focused on microbial chain elongation using reactors to produce MCCs, such as n-caproate (C6) and n-caprylate (C8), from organic substrates such as wastes. Even though the production of n-caproate is relatively well-characterized, bacteria and metabolic pathways that are responsible for n-caprylate production are not. Here, three 5 L reactors with continuous membrane-based liquid-liquid extraction (i.e., pertraction) were fed ethanol and acetate and operated for an operating period of 234 days with different operating conditions. Metagenomic and metaproteomic analyses were employed. n-Caprylate production rates and reactor microbiomes differed between reactors even when operated similarly due to differences in H2 and O2 between the reactors. The complete reverse β-oxidation (RBOX) pathway was present and expressed by several bacterial species in the Clostridia class. Several Oscillibacter spp., including Oscillibacter valericigenes, were positively correlated with n-caprylate production rates, while Clostridium kluyveri was positively correlated with n-caproate production. Pseudoclavibacter caeni, which is a strictly aerobic bacterium, was abundant across all the operating periods, regardless of n-caprylate production rates. This study provides insight into microbiota that are associated with n-caprylate production in open-culture reactors and provides ideas for further work.IMPORTANCEMicrobial chain elongation pathways in open-culture biotechnology systems can be utilized to convert organic waste and industrial side streams into valuable industrial chemicals. Here, we investigated the microbiota and metabolic pathways that produce medium-chain carboxylates (MCCs), including n-caproate (C6) and n-caprylate (C8), in reactors with in-line product extraction. Although the reactors in this study were operated similarly, different microbial communities dominated and were responsible for chain elongation. We found that different microbiota were responsible for n-caproate or n-caprylate production, and this can inform engineers on how to operate the systems better. We also observed which changes in operating conditions steered the production toward and away from n-caprylate, but more work is necessary to ascertain a mechanistic understanding that could be predictive. This study provides pertinent research questions for future work.
摘要：
中链羧酸盐(MCC)用于各种工业应用中。这些化学物质通常是从棕榈油中提取的，这被认为是不可持续的。最近的研究集中在使用反应器生产MCC的微生物链延长上，如正己酸(C6)和正辛酸(C8)，来自有机底物，如废物。尽管正己酸的生产是相对良好的特征，负责产生正辛酸盐的细菌和代谢途径不是。这里,三个具有连续膜基液-液萃取的5L反应器(即，pertraction）进料乙醇和乙酸盐，并在不同的操作条件下操作234天。采用宏基因组和元蛋白质组学分析。即使在类似操作时，由于反应器之间H2和O2的差异，n-辛酸盐的生产率和反应器微生物群也在反应器之间有所不同。完整的反向β-氧化（RBOX）途径存在并由梭菌属中的几种细菌物种表达。几种Oscillibacterspp。,包括镰刀杆菌属，与正辛酸盐生产率呈正相关，而克鲁维梭状芽胞杆菌与正己酸的产量呈正相关。假杆菌caeni,这是一种严格的需氧细菌，在所有运营期间都很丰富，无论正辛酸盐的生产率。这项研究提供了与开放式培养反应器中正辛酸盐生产相关的微生物群的见解，并为进一步的工作提供了思路。重要信息开放培养生物技术系统中的微生物链延伸途径可用于将有机废物和工业侧流转化为有价值的工业化学品。这里,我们研究了产生中链羧酸盐（MCC）的微生物群和代谢途径，包括正己酸（C6）和正辛酸（C8），在反应器中与在线产品提取。尽管本研究中的反应堆运行相似，不同的微生物群落占主导地位，并负责链伸长。我们发现，不同的微生物群负责n-己酸或n-辛酸盐的产生，这可以告诉工程师如何更好地操作系统。我们还观察到操作条件的变化引导生产朝向和远离正辛酸盐，但是需要做更多的工作来确定可以预测的机械理解。本研究为今后的工作提供了针对性的研究问题。
