The interactions between ammonifying and nitrifying bacteria
The effects of adhesion to surfaces on the specific growth rates of Nitrosomonas europaea and Nitrobacter sp. were determined in batch culture systems both in monoculture and co-culture. It was found that the presence of a glass slide in co-cultures of these bacteria significantly reduced the specific growth rates of both species of bacteria. In monoculture, the specific growth rate of N. europaea was significantly lower in a mature biofilm system. The specific rates of production of ammonia by these four species of Pseudomonas were investigated in minimal medium. All four species converted approximately 60 - 80% of the provided L-alanine to ammonia, and this production of ammonia raised the pH of the medium. All four strains were capable of this when the initial pH of the medium was 5.5 or 7.5, however, lowering the initial pH of the medium reduced the specific rate of production of ammonia for P. cepacia, P. fluorescens and P. syringae, and reduced the final concentration of ammonia produced by P. cepacia. Ammonia produced by P. fluorescens could support the growth of N. europaea in liquid batch culture. The growth of the pseudomonad increased the pH of the medium from 5.5 to 6.8 and this increase in pH allowed growth of the nitrifier in the medium with the lower initial pH. The growth of N. europaea and P. fluorescens in continuous flow biofilm reactors was examined, and the addition of P. fluorescens to a nitrifying biofilm raised the pH of the bulk medium, thus removing the effects of pH inhibition on the ammonia oxidiser. Further investigations were carried out in continuous flow sand column systems and it was found that the rapid growth of the pseudomonad caused obstruction of the column.