Distribution of bacteria in a continuous-flow nitrification column

Abstract

Nutrient solution containing (NH 4) 2SO 4 was supplied at a constant rate to Nitrosomonas europaea and Nitrobacter agilis growing in a column packed with glass beads. Conversion of NH 4 + to NO 2 − and NO 3 − was incomplete indicating that growth of the bacteria was not nutrient limited. After 7 months the column was dismantled and the arrangement of the bacteria on the beads examined using a scanning electron microscope. Nitrifying bacteria were found only in the upper regions of the column. They occurred most commonly in monolayers, less commonly in layers of about 20 cells and rarely in piles of about 100 cells in depth. Further down the column the glass beads were covered in a layer of slime and no bacteria were seen. This suggests that growth of the nitrifiers was neither controlled by diffusion of metabolites through a microbial film nor limited by competition between the bacteria for space on the surface of the glass beads.