Denitrification and Assimilatory Nitrate Reduction in Aquaspirillum magnetotacticum

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Abstract
Aquaspirillum magnetotacticum MS-1 grew microaerobically but not anaerobically with NO3− or NH4+ as the sole nitrogen source. Nevertheless, cell yields varied directly with NO3− concentration under microaerobic conditions. Products of NO3− reduction included NH4+, N2O, NO, and N2. NO2− and NH2OH, each toxic to cells at 0.2 mM, were not detected as products of cells growing on NO3−. NO3− reduction to NH4+ was completely repressed by the addition of 2 mM NH4+ to the growth medium, whereas NO3− reduction to N2O or to N2 was not. C2H2 completely inhibited N2O reduction to N2 by growing cells. These results indicate that A. magnetotacticum is a microaerophilic denitrifier that is versatile in its nitrogen metabolism, concomitantly reducing NO3− by assimilatory and dissimilatory means. This bacterium appears to be the first described denitrifier with an absolute requirement for O2. The process of NO3− reduction appears well adapted for avoiding accumulation of several nitrogenous intermediates that are toxic to cells.