Metabolic regulation of ethanol-type fermentation of anaerobic acidogenesis at different pH based on transcriptome analysis of Ethanoligenens harbinense

BACKGROUND: Ethanol-type fermentation, one of the fermentation types in mixed cultures of acidogenesis with obvious advantages such as low pH tolerance and high efficiency of H(2) production, has attracted widespread attentions. pH level greatly influences the establishment of the fermentation of carbohydrate acidogenesis by shaping community assembly and the metabolic activity of keystone populations. To explore the adaptation mechanisms of ethanol-type fermentation to low pH, we report the effects of initial pH on the physiological metabolism and transcriptomes of Ethanoligenens harbinense—a representative species of ethanol-type fermentation. RESULTS: Different initial pH levels significantly changed the cell growth and fermentation products of E. harbinense. Using transcriptomic analysis, we identified and functionally categorized 1753 differentially expressed genes (DEGs). By mining information on metabolic pathways, we probed the transcriptional regulation of ethanol–H(2) metabolism relating to pH responses. Multiple pathways of E. harbinense were co-regulated by changing gene expression patterns. Low initial pH down-regulated the expression of cell growth- and acidogenesis-related genes but did not affect the expression of H(2) evolution-related hydrogenase and ferredoxin genes. High pH down-regulated the expression of H(2) evolution- and acidogenesis-related genes. Multiple resistance mechanisms, including chemotaxis, the phosphotransferase system (PTS), and the antioxidant system, were regulated at the transcriptional level under pH stress. CONCLUSIONS: Ethanoligenens adapted to low pH by regulating the gene expression networks of cell growth, basic metabolism, chemotaxis and resistance but not H(2) evolution-related genes. Regulation based on pH shifts can represent an important approach to establish and enhance ethanol-type fermentation. The complete gene expression network of ethanol fermentative bacteria for pH response provides valuable insights into the acidogenic fermentation, and offers an effective regulation strategy for the sustainable energy recovery from wastewater and solid waste. [Image: see text]