Denitrification and Assimilatory Nitrate Reduction in Aquaspirillum magnetotacticum

Abstract

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.