pH对嗜盐混合菌发酵挥发性有机酸混合物合成PHA的影响

嗜盐混合菌发酵生产聚羟基脂肪酸酯（polyhydroxyalkanoate，PHA）具有免灭菌程序、产量高、易提取等优势而被广泛关注。集中考察了pH对嗜盐混合菌（MMCs）发酵混合挥发性有机酸（VFA）生产PHA的影响。研究结果表明，在6.5～8.2范围内pH对PHA合成没有明显影响，过高或者过低的pH都会降低PHA合成速率和VFA的吸收速率。发酵体系的pH影响了嗜盐MMC的PHA产量，但是在酸性和碱性条件下却存在不同的影响机制。碱性环境增大了VFA的解离程度，导致MMC对底物的吸收耗费大量的能量，造成VFA吸收速率的下降。这一作用在pH升高至9.2以上时表现显著。酸性环境下，分子态VFA进入细胞后改变了胞内的pH，降低了相关酶和蛋白质的活性，从而影响了底物的利用。当pH=5.2时微生物代谢及底物利用能耗量降低，导致PHA胞内降解利用率降低，刺激了PHA合成量的提高。PHA组分受pH影响不大，初始pH=5.2～10.2时羟基脂肪酸戊酯（hydroxyvalerate，HV）所占比例维持在34.9%～38.3%范围内。本研究对于进一步利用嗜盐MMC发酵含有混合VFA的废弃物生产PHA具有重要的指导意义。

Influence of pH on PHA production by halophilic sludge fermenting mixed VFAs substrate

Polyhydroxyalkanoates (PHA) production by halophilic mixed microbial cultures (MMCs) draw a lot of attention because of great advantages, such as free sterilization, easily extraction and high production. In this study, halophilic MMC was used to investigate the effect of pH on PHA production in the mixed VFAs fermentation process. The results indicated that there was no significant influence on PHA production in the pH range of 6.5-8.2. PHA production rate and substrate utilization rate would be inhibited in lower or higher pH conditions. PHA yield was influenced by pH in the systems, but different influencing mechanism existed in acidic condition and alkaline condition. Alkaline environment led to higher dissociation degree of VFAs. Then more energy needed as substrates being absorbed, which further led to the decrease of VFAs absorption rate. This affect was obvious when pH raised to 9.2. Molecular-state VFAs changed intracellular pH in the acidic condition. Then, the lower activity of related enzymes and protein resulted in slower substrate utilization. Energy consumption of substrate utilization and microbial metabolism reduced when pH=5.2, resulting in the decrease of intracellular PHA degradation and the increase of PHA yield. The pH value had little influence on PHA component. And HV accounted for 34.9%-38.3% of PHA weight when the initial pH between 5.2 and 10.2. This study would be a guide for PHA production using mixed VFAs as carbon source by halophilic MMCs.