The bacterivorous soil flagellate Heteromita globosa reduces bacterial clogging under denitrifying conditions in sand-filled aquifer columns.

An exopolymer (slime)-producing soil bacterium Pseudomonas sp. (strain PS+) rapidly clogged sand-filled columns supplied with air-saturated artificial groundwater containing glucose (500 mg liter(-1)) as a sole carbon source and nitrate (300 mg liter(-1)) as an alternative electron acceptor. After 80 days of operation under denitrifying conditions, the effective porosity and saturated hydraulic conductivity (permeability) of sand in these columns had fallen by 2.5- and 26-fold, respectively. Bacterial biofilms appeared to induce clogging by occluding pore spaces with secreted exopolymer, although there may also have been a contribution from biogas generated during denitrification. The bacterivorous soil flagellate Heteromita globosa minimized reductions in effective porosity (1.6-fold) and permeability (13-fold), presumably due to grazing control of biofilms. Grazing may have limited growth of bacterial biomass and hence the rate of exopolymer and biogas secretion into pore spaces. Evidence for reduction in biogas production is suggested by increased nitrite efflux from columns containing flagellates, without a concomitant increase in nitrate consumption. There was no evidence that flagellates could improve flow conditions if added once clogging had occurred (60 days). Presumably, bacterial biofilms and their secretions were well established at that time. Nevertheless, this study provides evidence that bacterivorous flagellates may play a positive role in maintaining permeability in aquifers undergoing remediation treatments.