Nitrate as a preferred electron sink for the acetogen Clostridium thermoaceticum

Nitrate enhanced the vanillin- and vanillate-dependent growth of Clostridium thermoaceticum. Under nitrate-enriched conditions, these aromatic substrates were subject to O demethylation. However, acetate, the normal product obtained from O demethylation, was not detected. Acetate was also not detected when methanol and CO cultures were supplemented with nitrate; glucose cultures likewise produced approximately one-third less acetate when enriched with nitrate. Reductant derived from the oxidation of these substrates was recovered in nitrite and ammonia. With an ammonia-limited medium employed to evaluate N turnover, the following stoichiometry was observed concomitantly with the consumption of 2.0 mM O-methyl groups (the recovery of nitrate-derived N approximated 89%): 3.9 mM NO3(-)-->2.8 mM NO2- +0.7 mM NH3. The results demonstrated that (i) nitrate was preferentially used as an electron sink under conditions that were otherwise acetogenic, (ii) nitrate dissimilation was energy conserving and growth supportive, and (iii) nitrate-coupled utilization of O-methyl groups conserved more energy than acetogenic O demethylation.