Denitrification and Assimilatory Nitrate Reduction in Aquaspirillum magnetotacticum

Aquaspirillum magnetotacticum MS-1 grew microaerobically but not anaerobically with NO(3) or NH(4) as the sole nitrogen source. Nevertheless, cell yields varied directly with NO(3) concentration under microaerobic conditions. Products of NO(3) reduction included NH(4), N(2)O, NO, and N(2). NO(2) and NH(2)OH, each toxic to cells at 0.2 mM, were not detected as products of cells growing on NO(3). NO(3) reduction to NH(4) was completely repressed by the addition of 2 mM NH(4) to the growth medium, whereas NO(3) reduction to N(2)O or to N(2) was not. C(2)H(2) completely inhibited N(2)O reduction to N(2) by growing cells. These results indicate that A. magnetotacticum is a microaerophilic denitrifier that is versatile in its nitrogen metabolism, concomitantly reducing NO(3) by assimilatory and dissimilatory means. This bacterium appears to be the first described denitrifier with an absolute requirement for O(2). The process of NO(3) reduction appears well adapted for avoiding accumulation of several nitrogenous intermediates that are toxic to cells.