Protozoa graze on the 2,6-dichlorobenzamide (BAM)-degrading bacterium Aminobacter sp. MSH1 introduced into waterworks sand filters.

Groundwater contamination by pesticide residues often leads to the closure of drinking water wells, making the development of new techniques to remediate drinking water resources of considerable interest. Pesticide-degrading bacteria were recently added to a waterworks sand filter in an attempt to remediate pesticide-polluted drinking water. The density of the introduced bacteria, however, decreased rapidly, which was partly attributed to predation by protozoa in the sand filter. This study investigated the effects of indigenous sand filter protozoa on the population density and degradation efficiency of degrader bacteria introduced into sand from a waterworks sand filter. The 2,6-dichlorobenzamide (BAM)-degrading bacterium Aminobacter sp. MSH1 was used as a model organism. The introduction of MSH1 at high cell densities was followed by a >1000-fold increase in the protozoan population size and at the same time a 29 % reduction in Aminobacter cell numbers. The protozoan population in the systems that had MSH1 added at a lower density only increased 50-fold, and a decrease in Aminobacter numbers was not detectable. Furthermore, a reduction in the number of Aminobacter and in BAM degradation efficiency was seen in flow-through sand filter columns inoculated with MSH1 and fed BAM-contaminated water, when comparing sand columns containing the indigenous microbial filter community, i.e. containing protozoa, to columns with sterilised sand. These results suggest that degrader bacteria introduced into waterworks sand filters are adversely affected by grazing from the indigenous protozoa, reducing the size of the degrader population and the sand filter degradation efficiency.