Respiratory nitrate ammonification by Shewanella oneidensis MR-1

Abstract

Anaerobic cultures of Shewanella oneidensis MR-1 grown with nitrate as the sole electron acceptor exhibited sequential reduction of nitrate to nitrite and then to ammonium. Little dinitrogen and nitrous oxide were detected, and no growth occurred on nitrous oxide. A mutant with the napA gene encoding periplasmic nitrate reductase deleted could not respire or assimilate nitrate and did not express nitrate reductase activity, confirming that the NapA enzyme is the sole nitrate reductase. Hence, S. oneidensis MR-1 conducts respiratory nitrate ammonification, also termed dissimilatory nitrate reduction to ammonium, but not respiratory denitrification.

FIG. 1.

Nitrate and nitrite respiration by S. oneidensis MR-1. Cultures in LML medium contained nitrate (A) or nitrite (B) at the following concentrations: 0.5 mM (○), 1.0 mM (•), 2.0 mM (□), 3.0 mM (▵), 4.0 mM (▪), and 40 mM (▴). The data are averages from three independent cultures. Error bars were omitted for clarity. Cultures with no added nitrate or nitrite did not grow (not shown).

FIG. 2.

Production and consumption of nitrite and ammonium during nitrate respiration. Cultures in HEPES medium contained 2.0 mM nitrate, and the culture density (▪) and the concentrations of nitrate (□), nitrite (▵), and ammonium (○) were monitored. The data are averages from three independent cultures. Error bars were omitted for clarity.

FIG. 3.

Nitrate respiration requires the napA⁺ gene. Cultures in HEPES medium were aerated (circles) or cultured anaerobically with nitrate (squares) or nitrate plus fumarate (triangles). Closed symbols, napA⁺ parent MR-1; open symbols, ΔnapA::loxP mutant CCG01. The data are averages from three independent cultures. Error bars were omitted for clarity.