Gasification of organic wastes coupled to syngas fermentation allows the recovery of carbon in the form of commodity chemicals, such as carboxylates and biofuels. Acetogenic bacteria ferment syngas to mainly two-carbon compounds, although a few strains can also synthesize four-, and six-carbon molecules. In general, longer carbon chain products have a higher biotechnological (and commercial) value due to their higher energy content and their lower water solubility. However, de-novo synthesis of medium-chain products from syngas is quite uncommon in acetogenic bacteria. An alternative to de-novo synthesis is bioproduction of short-chain products (C2 and C4), and their subsequent elongation to C4, C6, or C8 through reversed β-oxidation metabolism. This two-step synergistic approach has been successfully applied for the production of up to C8 compounds, although the accumulation of alcohols in these mixed cultures remained below detection limits. The present work investigates the production of higher alcohols from syngas by open mixed cultures (OMC). A syngas-fermenting community was enriched from sludge of an anaerobic digester for a period of 109 days in a lab-scale reactor. At the end of this period, stable production of ethanol and butanol was obtained. C6 compounds were only transiently produced at the beginning of the enrichment phase, during which Clostridium kluyveri, a bacterium able to carry out carbon chain elongation, was detected in the community. Further experiments showed pH as a critical parameter to maintain chain elongation activity in the co-culture. Production of C6 compounds was recovered by preventing fermentation pH to decrease below pH 4.5–5. Finally, experiments showed maximal production of C6 compounds (0.8 g/L) and alcohols (1.7 g/L of ethanol, 1.1 g/L of butanol, and 0.6 g/L of hexanol) at pH 4.8. In conclusion, low fermentation pH is critical for the production of alcohols, although detrimental to C. kluyveri. Fine control of fermentation pH to final values around 4.8 could allow sustained production of higher alcohols
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机译：结合合成气发酵的有机废物的气化可以以商品化学品（例如羧酸盐和生物燃料）的形式回收碳。产乙酸细菌将合成气发酵成主要由两个碳组成的化合物，尽管少数菌株还可以合成四个和六个碳的分子。通常，较长的碳链产品由于其较高的能量含量和较低的水溶性而具有较高的生物技术（和商业）价值。然而，在产乙酸细菌中，由合成气从头合成新产物的现象很少见。从头合成的替代方法是生物生产短链产物（C2和C4），然后通过反向的β-氧化代谢将它们随后延伸为C4，C6或C8。尽管在这些混合培养物中酒精的积累仍低于检测极限，但这种两步协同方法已成功应用于生产最多C8化合物。本工作研究了通过开放式混合培养（OMC）从合成气生产高级醇的方法。在实验室规模的反应器中，合成气发酵群落从厌氧消化池的污泥中富集了109天。在此阶段结束时，获得了稳定的乙醇和丁醇生产。 C6化合物仅在富集阶段开始时短暂产生，在此期间，在社区中检测到了能够进行碳链延长的细菌克氏梭菌。进一步的实验表明，pH是维持共培养链延长活性的关键参数。通过防止发酵pH降至pH 4.5-5以下，可回收C6化合物的产量。最后，实验表明，在pH 4.8下，C6化合物（0.8 g / L）和醇类（1.7 g / L乙醇，1.1 g / L丁醇和0.6 g / L己醇）的产量最高。总之，低发酵pH对生产醇至关重要，尽管对克鲁维酵母不利。将发酵pH值精确控制到4.8左右的最终值可以持续生产高级醇
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