Factors affecting protozoan predation of bacteria clogging laboratory aquifer microcosms

Laboratory aquifer microcosms were used to test the ability of a protozoan predator (Acanthamoeba castellanii) to mitigate reductions in saturated hydraulic conductivity of a porous medium caused by its bacterial prey (Bacillus sp.). Predation was able to prevent clogging and reduced bacterial numbers in the short term in continuous-flow sand columns inoculated with bacteria and protozoa used to simulate aquifer conditions. However, over the longer term, increased pressure in columns with bacteria and protozoa was similar to control columns inoculated with bacteria alone. Several factors allowed the bacteria to persist and clog the columns in spite of predation. Bacteria were able to migrate from the point of inoculation up a nutrient gradient against flow toward the column inlet, whereas amoebae were not able to migrate. The availability of the bacterial prey was limited by growth of the bacteria in crevices of the sand particles that were not accessible to the amoebae. The availability of bacterial prey was also limited by aggregation of the cells into masses that were too large for the amoebae to phagocytize. The growth of cells in crevices and the aggregation of cells were confirmed with scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) on thin sections prepared from column samples. Bacteria were found growing among the sand particles predominantly as aggregates. Growth of bacteria as aggregates coupled with the large barren surfaces and lack of biofilms added evidence to arguments that a key mechanism of microbial clogging is blockage of pores by these aggregates.