Gildemyn Sylvia, Molitor Bastian, Usack Joseph G, Nguyen Mytien, Rabaey Korneel, Angenent Largus T
Cornell University, Biological and Environmental Engineering, Riley-Robb Hall, Ithaca, NY 14853 USA.
Ghent University, Center for Microbial Ecology and Technology (CMET), Coupure Links 653, 9000 Ghent, Belgium.
Biotechnol Biofuels. 2017 Mar 29;10:83. doi: 10.1186/s13068-017-0764-6. eCollection 2017.
The product of current syngas fermentation systems is an ethanol/acetic acid mixture and the goal is to maximize ethanol recovery. However, ethanol currently has a relatively low market value and its separation from the fermentation broth is energy intensive. We can circumvent these disadvantages of ethanol production by converting the dilute ethanol/acetic acid mixture into products with longer carbon backbones, which are of higher value and are more easily extracted than ethanol. Chain elongation, which is the bioprocess in which ethanol is used to elongate short-chain carboxylic acids to medium-chain carboxylic acids (MCCAs), has been studied with pure cultures and open cultures of microbial consortia (microbiomes) with several different substrates. While upgrading syngas fermentation effluent has been studied with open cultures, to our knowledge, no study exists that has performed this with pure cultures.
Here, pure cultures of were used in continuous bioreactors to convert ethanol/acetic acid mixtures into MCCAs. Besides changing the operating conditions in regards to substrate loading rates and composition, the effect of in-line product extraction, pH, and the use of real syngas fermentation effluent on production rates were tested. Increasing the organic loading rates resulted in proportionally higher production rates of -caproic acid, which were up to 40 mM day (4.64 g L day) at carbon conversion efficiencies of 90% or higher. The production rates were similar for bioreactors with and without in-line product extraction. Furthermore, a lower ethanol/acetic acid ratio (3:1 instead of 10:1) enabled faster and more efficient -caproic acid production. In addition, -caprylic acid production was observed for the first time with (up to 2.19 ± 0.34 mM in batch). Finally, the use of real effluent from syngas fermentation, without added yeast extract, but with added defined growth factors, did maintain similar production rates. Throughout the operating period, we observed that the metabolism of was inhibited at a mildly acidic pH value of 5.5 compared to a pH value of 7.0, while reactor microbiomes perform successfully at mildly acidic conditions.
can be used as a biocatalyst to upgrade syngas fermentation effluent into MCCAs at pH values above 5.5.
当前合成气发酵系统的产物是乙醇/乙酸混合物，目标是使乙醇回收率最大化。然而，乙醇目前市场价值相对较低，且从发酵液中分离乙醇能耗较高。我们可以通过将稀乙醇/乙酸混合物转化为具有更长碳链骨架的产物来规避乙醇生产的这些缺点，这些产物价值更高且比乙醇更容易提取。链延长是一种生物过程，其中乙醇用于将短链羧酸延长为中链羧酸（MCCAs），已经使用几种不同底物对纯培养物和微生物群落（微生物组）的开放培养物进行了研究。虽然已经使用开放培养物对合成气发酵废水的升级进行了研究，但据我们所知，尚无使用纯培养物进行此项研究的报道。
在此，使用的纯培养物在连续生物反应器中，将乙醇/乙酸混合物转化为MCCAs。除了改变底物装载速率和组成方面的操作条件外，还测试了在线产物提取、pH值以及使用实际合成气发酵废水对生产率的影响。提高有机装载速率会使己酸的生产率成比例提高，在碳转化效率达到90%或更高时，己酸生产率高达40 mM/天（4.64 g/L/天）。有无在线产物提取的生物反应器的生产率相似。此外，较低的乙醇/乙酸比（3:1而非10:1）能实现更快、更高效的己酸生产。此外，首次观察到使用 生产辛酸（分批培养中最高可达2.19±0.34 mM）。最后，使用来自合成气发酵的实际废水，不添加酵母提取物，但添加确定的生长因子，确实能维持相似的生产率。在整个运行期间，我们观察到与pH值7.0相比，在pH值5.5的弱酸性条件下， 的代谢受到抑制，而反应器微生物群落在弱酸性条件下能成功运行。
在pH值高于5.5时， 可作为生物催化剂将合成气发酵废水升级为MCCAs。
