Nitrification as a source of soluble organic substrate in biological treatment

In complex, multispecies populations, exchange of substrates can be an important beneficial interaction. Prior experimental and theoretical work has led to the hypothesis that the formation of soluble microbial products (SMP) by nitrifying bacteria can provide a supplementary organic substrate for heterotrophic bacteria, thereby augmenting their accumulation and stability, especially when inputs of organic substrates are low. In this study, chemostat experiments carried out with a NO₂-^-oxidizing strain (Nitrobacter sp.) and an NH₄⁺-oxidizing strain (Nitrosomonas europaea) demonstrated that both nitrifiers produce SMP that can support heterotrophic bacteria. The first evidence was the presence of significant concentrations of soluble COD in the chemostat effluent, even though the influent was free of organic compounds. Second, a small heterotrophic population was maintained, apparently through utilization of the nitrifier-produced SMP. A preliminary kinetic analysis suggested that SMP kinetic parameters can be adapted from parameters measured for heterotrophs.