The use of compound continuous flow diffusion chemostats to study the interaction between nitrifying and nitrate-reducing bacteria

The interactions occuring between populations of a nitrate-respiring Vibrio sp. and autotrophic nitrifying bacteria belonging to the genera Nitrosomonas and Nitrobacter have been investigated in a compound bi-directional flow diffusion chemostat at a dilution rate of 0.025 h−1 and a temperature of 25°C. When grown under NO−3 limitation, the Vibrio sp. produced NH+4 as the principal end-product of nitrate respiration, and there was a corresponding significant increase in cell numbers of the Nitrosomonas sp. population, which derived energy by the oxidation of NH+4 to NO−2. Nitrite in turn was used by the Nitrobacter sp. population as an energy source with the concomitant regeneration of NO−3. Under NO−3 excess growth conditions the Vibrio sp. produced NO−2 rather than NH+4 as the major product of NO−3 dissimilation, and growth of the Nitrobacter population was stimulated as increased quantities of NO−2 became available. In contrast, the Nitrosomonas sp. population declined sharply as the energy source NH+4 became limiting. These data demonstrate that defined mixed populations of obligately aerobic nitrifying bacteria and facultatively anaerobic nitrate respiring bacteria can co-exist for extended time periods and operate an internal nitrogen cycle which is energetically beneficial to both populations.