Nitrate Reduction in Seawater of the Deep Nitrite Maximum Off Peru1

Nitrate‐reducing bacteria were isolated from samples collected inside and outside the secondary nitrite maximum in oxygen‐poor deep waters off the coast of Peru. Under anaerobic conditions at 20C in the laboratory, these bacteria reduced nitrate to nitrite in seawater enriched with 0.5 to 1.0 mg glucose/liter. Nitrate reduction also occurred in seawater supplemented only with 20 µg‐atom NO3−‐N/liter and trace metals. Nitrite production generally was greatest during the first 24 hr while cell numbers were increasing. On continued incubation cells died, nitrate increased, and nitrite decreased. Dissolved organic carbon increased concurrently with the death of bacteria, indicating release of cell contents into the medium by lysis. Microbiological evidence is presented to show that the high nitrite concentrations in oxygen‐poor deep waters of the secondary nitrite maximum arise from nitrate reduction.