Nitrification by marine bacteria in low concentrations of substrate and

Chemoautotrophic nitrifying bacteria, isolated from open-ocean waters, oxidized low con-centrations of substrate (5 pg-atom NHa-N or 20 pug-atom NO;.-N/liter) in liquid medium containing less than 0.1 ml of oxygen/liter and in solid medium under 0.002 atm of oxygen. Carbon assimilations for three nitrifying bacteria were higher in 0.002 than in 0.2 atm of oxygen. More carbon was assimilated per unit of substrate oxidized in low than in high concentrations of oxygen. In deeper, oxygen-poor waters of some oceans, high conccntra-tions of nitrite may arise from ammonia oxidation. INTRODUCXION Brandhorst (1959) attributed the high nitrite concentrations in the thermocline waters of the ocean to the activities of nitrifying bacteria, whereas Vaccaro and Ryther (1960) felt that excretion of nitrite by phytoplankton was mainly responsible for the observed effects. Brandhorst also suggested that denitrifying bacteria were responsible for the high concentrations of nitrite in some deeper, oxygen-poor waters where secondary nitrite maxima occurred. Subsequently, evidence was presented that some form of nitrate reduction, possibly denitrification, accounted for high nitrite concentrations in the secondary nitrit