Acetate production and metabolic characteristics of Clostridium aceticum under heterotrophic and autotrophic conditions

Abstract:

[Background] Acetic acid-producing bacteria can utilize one-carbon gas to produce high-value chemicals such as acetic acid by fermentation, which is an important way for carbon resource recycling and green biomanufacturing. In addition, most of the acetic acid-producing bacteria can utilize organic matter for metabolism. However, the two ways have different energy metabolism patterns. [Objective] To investigate the acid production characteristics of a typical acetic acid-producing bacterium, Clostridium aceticum, under heterotrophic/autotrophic conditions, and to reveal the mechanism of different culture conditions in affecting the metabolic patterns of this bacterium. [Methods] The acid production and metabolism of C.aceticum cultured with glucose and syngas (H₂ and CO₂ mixed in the ratio of 6:4), respectively, were investigated. The distribution of the products and the specific titer of acetic acid production were analyzed on the basis of the results of the experiments. The key enzyme activity indexes were employed to reveal the mechanism of the differences in the heterotrophic and autotrophic metabolism patterns. [Results] C.aceticum performed stepwise degradation in the presence of 10 g/L glucose. It firstly decomposed glucose to produce lactic acid, and subsequently produced 6.3 g/L ethanol, 13.2 g/L butyric acid, and 5.6 g/L acetic acid. However, only 1.96 g/L acetic acid was detected in the case of syngas as the substrate. The results of product distribution showed that the specific titer of acetic acid production and acid/alcohol ratio under autotrophic conditions reached up to 28.6 g/g and 17.57, respectively, which were much higher than those (8.23 g/g and 1.53) under heterotrophic conditions. The NADH and ATP production as well as pyruvate dehydrogenase activity were higher under heterotrophic conditions, while the acetate kinase activity showed an opposite trend. This result revealed that most of acetyl coenzyme A as an intermediate of pyruvate metabolism, flowed to cell synthesis and end products (ethanol and butyric acid) under heterotrophic conditions, while it flowed to acetate production under autotrophic conditions. [Conclusion] C.aceticum could achieve higher acetic acid production under autotrophic conditions, which provides a reference value for the utilization of syngas by microorganisms and the regulation of metabolic pathway of acetic acid-producing bacteria.

Key words:Clostridium aceticum;heterotrophic metabolism;syngas;acetic acid;acetyl-CoA