Stable-isotope probing and metagenomics reveal predation by protozoa drives E. coli removal in slow sand filters

Abstract

Stable-isotope probing and metagenomics were applied to study samples taken from laboratoryscale slow sand filters 0.5, 1, 2, 3 and 4 h after challenging with 13 C-labelled Escherichia coli to determine the mechanisms and organisms responsible for coliform removal. Before spiking, the filters had been continuously operated for 7 weeks using water from the River Kelvin, Glasgow as their influent source. Direct counts and quantitative PCR assays revealed a clear predator-prey response between protozoa and E. coli. The importance of top-down trophic-interactions was confirmed by metagenomic analysis, identifying several protozoan and viral species connected to E. coli attrition, with protozoan grazing responsible for the majority of the removal. In addition to top-down mechanisms, indirect mechanisms, such as algal reactive oxygen species-induced lysis, and mutualistic interactions between algae and fungi, were also associated with coliform removal. The findings significantly further our understanding of the processes and trophic interactions underpinning E. coli removal. This study provides an example for similar studies, and the opportunity to better understand, manage and enhance E. coli removal by allowing the creation of more complex trophic interaction models.

Applying Bio-Slow Sand Filtration for Water Treatment

“…Therefore, the formation of schmutzdecke intensifies the effective adsorptive capacity to capture and degrade pollutants. In conclusion, our work evinces that the mechanism of bio-slow sand filtration is a combination of straining in the upper filter section, and a recent study using 13C-labelled E. coli highlighted the importance of protozoan predation in the trophic network of a model slow sand filter [12].…”

Selective elimination of bacterial faecal indicators in the Schmutzdecke of slow sand filtration columns

“…In conclusion, our work evinces that the mechanism of faecal indicator removal in SSF systems is a combination of straining in the upper filter section, unsuitable habitat conditions, and enhanced predation. Indeed, a recent study using 13 C-labelled E. coli highlighted the importance of protozoan predation in the trophic network of a model slow sand filter (Haig et al 2015). Similar studies may further aid the optimization of such treatment systems for improved and stable pathogen removal efficiencies.…”

“…Despite its long track record, the underlying mechanisms for pathogen removal involved in SSF are still insufficiently understood. Elimination of retained or immobilized microbial cells may be due to abiotic stress or biotic processes such as competition for nutrients with indigenous microorganisms as well as predation by protozoa, Bdellovibrio-and-like organisms, and viruses (Bomo et al 2004;Chabaud et al 2006;Haig et al 2015;Schijven et al 2003;Stevik et al 2004;Wand et al 2007).…”

Convergence in Multispecies Interactions

“…Advances in metagenomic sequencing and stable isotope analysis (particularly stable isotope probing [83]) can provide additional information on functional genes and trophic levels for small, difficult to observe organisms such as microbes and invertebrates [84,85]. New technologies and emerging protocols should allow microbes to be incorporated into existing food webs of plants and animals, which can then be used to explore convergence across entire ecosystem networks.…”