Nitrate affects carbon fixation by formate dehydrogenase-lacking Wood-Ljungdahl pathway in Clostridium bovifaecis

Abstract:

[Background] Unlike general acetogenic bacteria which usually reduce two molecules of CO₂ into one molecule of acetyl-CoA via Wood-Ljungdahl pathway, the formate dehydrogenase-lacking acetogen Clostridium bovifaecis reduces one molecule of formate and one molecule of CO₂ into acetyl-CoA and conducts acetogenic utilization of glucose only in the presence of formate. Nitrate is the preferred electron acceptor for acetogens which vary in their ability to utilize this alternative electron acceptor. However, the effect of nitrate on the carbon fixation by formate dehydrogenase-lacking Wood-Ljungdahl pathway remains unclear. [Objective] To investigate the effect of nitrate on the carbon fixation by formate-dependent acetogenesis in C. bovifaecis. [Methods] We measured the growth, substrate consumption, and product yield of C. bovifaecis cultured in the medium containing 10 mmol/L or 30 mmol/L nitrate and using glucose + formate + CO₂ as substrates. The medium without nitrate was taken as the control. [Results] The main product was ethanol with the concentrations of 5.80 mmol/L and 1.66 mmol/L, respectively, in the media with 10 mmol/L and 30 mmol/L nitrate, which were significantly lower than that (7.13 mmol/L) of the control. In addition, the glucose consumption significantly decreased in the presence of nitrate. Formate consumption decreased with the increase in nitrate concentration and became zero at the nitrate concentration of 30 mmol/L. [Conclusion] Nitrate of 30 mmol/L inhibited carbon fixation by formate-dependent acetogenesis, and that of 10 mmol/L decreased the glycolysis in C. bovifaecis.