Kucek Leo A, Nguyen Mytien, Angenent Largus T
Biological and Environmental Engineering, Cornell University, Ithaca, NY 14853, USA.
Water Res. 2016 Apr 15;93:163-171. doi: 10.1016/j.watres.2016.02.018. Epub 2016 Feb 15.
Conversion of lactate to n-caproate had been described for the type strain Megasphaera elsdenii in batch systems. Recently, investigators have also described production of n-caproate from endogenous or exogenous lactate with batch-fed reactor microbiome systems. However, no reports exist of lactate to n-caproate conversion within a continuously fed bioreactor. Since continuously fed systems are advantageous for biotechnology production platforms, our objective was to develop such a system. Here, we demonstrated continuous lactate to n-caproate conversion for more than 165 days. The volumetric n-caproate production rate (productivity) was improved when we decreased the operating pH from 5.5 to 5.0, and was again improved when we utilized in-line product recovery via pertraction (membrane-based liquid-liquid extraction). We observed a maximum n-caproate productivity of 6.9 g COD/L-d for a period of 17 days at an L-lactate loading rate of 9.1 g COD/L-d, representing the highest sustained lactate to n-caproate conversion rate ever reported. We had to manage two competing lactate conversion pathways: 1) the reverse β-oxidation pathway to n-caproate; and 2) the acrylate pathway to propionate. We found that maintaining a low residual lactate concentration in the bioreactor broth was necessary to direct lactate conversion towards n-caproate instead of propionate. These findings provide a foundation for the development of new resource recovery processes to produce higher-value liquid products (e.g., n-caproate) from carbon-rich wastewaters containing lactate or lactate precursors (e.g., carbohydrates).
在分批系统中，已报道过模式菌株埃氏巨球型菌可将乳酸转化为正己酸盐。最近，研究人员也描述了在分批补料反应器微生物群落系统中，利用内源性或外源性乳酸生产正己酸盐的情况。然而，尚无关于在连续进料生物反应器中乳酸转化为正己酸盐的报道。由于连续进料系统对生物技术生产平台具有优势，我们的目标是开发这样一种系统。在此，我们展示了连续165天以上将乳酸转化为正己酸盐的过程。当我们将操作pH从5.5降至5.0时，正己酸盐的体积产率（生产率）得到提高，而当我们通过渗透萃取（基于膜的液-液萃取）进行在线产物回收时，生产率再次提高。在L-乳酸负荷率为9.1 g COD/L-d的情况下，我们在17天内观察到最大正己酸盐生产率为6.9 g COD/L·d，这代表了有史以来报道的最高持续乳酸到正己酸盐转化率。我们必须管理两条相互竞争的乳酸转化途径：1）转化为正己酸盐的反向β-氧化途径；2）转化为丙酸盐的丙烯酸途径。我们发现，保持生物反应器肉汤中的残余乳酸浓度较低，对于将乳酸转化导向正己酸盐而非丙酸盐是必要的。这些发现为开发新的资源回收工艺奠定了基础，该工艺可从含有乳酸或乳酸前体（如碳水化合物）的富碳废水中生产高价值液体产品（如正己酸盐）。
