How does the cladoceran Daphnia pulex affectt he fate of Escherichia coli in water?

The faecal indicator Escherichia coli plays a central role in water quality assessment and
monitoring. It is therefore essential to understand its fate under various environmental constraints
such as predation by bacterivorous zooplankton. Whereas most studies have examined
how protozooplankton communities (heterotrophic nanoflagellates and ciliates) affect
the fate of E. coli in water, the capacity of metazooplankton to control the faecal indicator
remains poorly understood. In this study, we investigated how the common filter-feeding cladoceran,
Daphnia pulex, affects the fate of E. coli under different experimental conditions.
Daphnia ingested E. coli and increased its loss rates in water, but the latter rates decreased
from 1.65 d-1 to 0.62 d-1 after a 1,000-fold reduction in E. coli initial concentrations, due to
lower probability of encounter between Daphnia and E. coli. The combined use of culture
and PMA qPCR (viability-qPCR) demonstrated that exposure to Daphnia did not result into
the formation of viable but non-culturable E. coli cells. In lake water, a significant part of E.
coli population loss was associated with matrix-related factors, most likely due to predation
by other bacterivorous biota and/or bacterial competition. However, when exposing E. coli
to a D. pulex gradient (from 0 to 65 ind.L-1), we observed an increasing impact of Daphnia
on E. coli loss rates, which reached 0.47 d-1 in presence of 65 ind.L-1. Our results suggest
that the filter-feeder can exert a non-negligible predation pressure on E. coli, especially during
seasonal Daphnia population peaks. Similar trials using other Daphnia species as well
as stressed E. coli cells will increase our knowledge on the capacity of this widespread zooplankter
to control E. coli in freshwater resources. Based on our results, we strongly advocate
the use of natural matrices to study these biotic interactions in order to avoid
overestimation of Daphnia impact.