NITRATE AS A PREFERRED ELECTRON SINK FOR THE ACETOGEN CLOSTRIDIUM-THERMOACETICUM

Nitrate enhanced the vanillin- and vanillate-dependent growth of Clost ridium thermoaceticum. Under nitrate-enriched conditions, these aromat ic substrates were subject to 0 demethylation. However, acetate, the n ormal product obtained from 0 demethylation, was not detected. Acetate was also not detected when methanol and CO cultures were supplemented with nitrate; glucose cultures likewise produced approximately one-th ird less acetate when enriched with nitrate. Reductant derived from th e oxidation of these substrates was recovered in nitrite and ammonia. With an ammonia-limited medium employed to evaluate N turnover, the fo llowing stoichiometry was observed concomitantly with the consumption of 2.0 mM 0-methyl groups (the recovery of nitrate-derived N approxima ted 89%): 3.9 mM NO3- --> 2.8 mM NO2- +0.7 mM NH3. The results demonst rated that (i) nitrate was preferentially used as an electron sink und er conditions that were otherwise acetogenic, (ii) nitrate dissimilati on was energy conserving and growth supportive, and (iii) nitrate-coup led utilization of O-methyl groups conserved more energy than acetogen ic O demethylation.