Project description:Acetate is a simple carboxylic acid that is synthesized in various microorganisms. Although acetate toxicity and tolerance have been studied in many microorganisms, little is known about the effects of exogenous acetate on the cell growth of acetogenic bacteria. In this study, we report the phenotypic changes that occurred in the acetogenic bacterium Clostridium sp. AWRP as a result of an adaptive laboratory evolution under acetate challenge. When compared with the wild-type strain, the acetate-adapted strain displayed a tolerance to acetate up to 10 g L-1 and higher biomass yields in batch cultures, although the metabolite profiles greatly varied depending on culture conditions. Interestingly, genome sequencing revealed that the adapted strain harbored three point mutations in the genes encoding an electron-bifurcating hydrogenase, which is crucial to its autotrophic growth on CO2 + H2, in addition to one in the dnaK gene. Transcriptome analysis revealed the global change in the gene expression profile of the acetate-adapted strain. Strikingly, most genes involved in CO2-fixing Wood-Ljungdahl pathway and auxiliary pathways for energy conservation (e.g., Rnf complex, Nfn, etc.) were significantly down-regulated. In addition, we observed that a couple of metabolic pathways associated with dissimilation of nucleosides and carbohydrates were significantly up-regulated in the acetate-adapted strain as well as several amino acid biosynthetic pathways, indicating that the strain might increase its fitness by utilizing organic substrates in response to the down-regulation of carbon fixation. Further investigation into the carbon fixation degeneration of the acetate-adapted strain will provide practical implications in CO2 + H2 fermentation using acetogenic bacteria for long-term continuous fermentation. The transcriptome profiles of the wild-type Clostridium sp. AWRP and its acetate-tolerant derivative 46T-a were compared.
