Impact of Metazooplankton Filter Feeding on Escherichia coli under Variable Environmental Conditions

The impact of beaver dams on distribution of waterborne Escherichia coli and turbidity in an agricultural landscape

Globally, freshwater environments are threatened by point source and diffuse pollution, habitat loss, and climate change. Enhancing water quality and reducing microbial pollution are priorities to realise their ecosystem services potential but challenging to achieve and require creative solutions. Beavers are receiving increasing attention as ecosystem engineers, their dams benefitting aquatic ecosystems via improved biodiversity, water quality, and flow regulation. However, effects on microbial water quality remain uncertain. Here, we investigated the influence of engineering by Eurasian beaver (Castor fiber L.) on variation in Escherichia coli concentrations and turbidity in an agricultural stream. Water samples were collected over a period of two years (2017-2019, encompassing 11 sampling dates), from a sequence of 14 beaver dams and associated ponds to quantify fluxes of turbidity and E. coli. On average, dam structures were a source whereas ponds acted as a sink for both turbidity and E. coli. The sink effect of ponds strengthened with upstream load, increasingly outweighing the source effect of dams while being moderated by season and antecedent flow and rainfall. To complement these findings, in 2023, an in-situ pollution event was simulated by adding a slurry of livestock manure (25 l) to two nearby closely comparable streams, one beaver-engineered, the other not (control), and tracking the downstream distribution of waterborne E. coli. Consistent with our field sampling campaign, E. coli was strongly attenuated in beaver ponds, which reduced peak concentrations by >95 % and slowed the flushing of E. coli compared to the control stream. Our study demonstrates that beaver dams exert a range of effects on microbial and associated pollution but, importantly, under peak loading can significantly decrease pollution reaching downstream receptors. Beaver dams, and potentially their analogues, could therefore support environmental management strategies in agricultural systems as part of a suite of nature-based approaches.

Impact of grazing by multiple Daphnia species on wastewater bacterial communities

Understanding the dynamics of fecal bacterial communities is crucial for managing public health risks and protecting drinking water resources. While extensive research exists on how abiotic factors influence the survival of fecal microbial communities in water, less attention has been paid to the impact of predation by higher organisms, such as the widely distributed grazer Daphnia. Nevertheless, Daphnia plays a significant role in regulating bacterial communities in natural aquatic ecosystems, and recent studies highlighted its potential as a biofilter in alternative tertiary wastewater treatment systems. In this study, we investigated the influence of three different Daphnia species on a wastewater bacterial community, including fecal indicator bacterium E. coli. Using a microcosm setup to simulate the discharge of untreated sewage into surface water, we conducted in-depth analysis of bacterial community dynamics through sequencing the 16S rRNA gene. Our results revealed significant changes in microbial diversity and composition following exposure to Daphnia grazing, with variations observed among the three Daphnia species. D. pulicaria exerted the most pronounced impact on microbial diversity, followed by D. middendorffiana and D. mendotae. A total of 90 taxa exhibited significantly reduced relative abundance in the presence of Daphnia, with Firmicutes phylum being the most affected. At genus level, bacteria typically associated with wastewater (e.g., Zoogloea and Arcobacter) and gut microbiome constituents (e.g., Prevotella and Akkermansia) were notably affected by Daphnia exposure. The influence of Daphnia on bacterial community composition was most pronounced for D. pulicaria, while D. middendorffiana and D. mendotae primarily impacted community structure. Furthermore, we demonstrated that the microbial response to Daphnia exposure is phylogenetically conserved, potentially reflecting a grazing resistance or grazer feeding trait. Our findings shed new light on the role of Daphnia in controlling bacterial communities in polluted water bodies and underscore its potential as biofilter in wastewater treatment and reuse contexts.

Antibiotic resistance genes are transferred from manure-contaminated water bodies to the gut microbiota of animals through the food chain

Fecal-contaminated water may enter the food chain and become an important route for the transmission of antibiotic resistance genes (ARGs) to the human microbiome. However, little is known about the spread of ARGs from fecal contamination in water bodies along the aquatic food chain. In this study, laboratory-raised Daphnia magna and Aristichthys nobilis were used to investigate the effects of the addition of manure on target ARGs in water and their intestinal contents to determine the potential transmission route of ARGs in the aquatic food chain system. The abundance of target ARGs in water as well as D. magna and A. nobilis intestinal contents significantly increased when fecal contamination was present. ARGs bioaccumulated along the food chain, with four ARGs (tetM-01, tetX, qnrS, and sul2) detected regularly. Mn and Cr were key environmental factors that promoted the transfer of ARGs along the food chain. Fecal addition significantly changed the structure of microbial communities in water, D. magna gut, and A. nobilis gut. The ARG spectrum was significantly correlated with the composition and structure of the bacterial community. Proteobacteria, Bacteroidetes, and Firmicutes were identified as the main host bacteria and were likely to act as carriers of ARGs to promote the spread of antibiotic resistance in the food chain. The composition and structure of bacterial communities, along with mobile genetic elements, were two key drivers of ARG transfer. These findings provide new insights into the distribution and spread of ARGs along the freshwater food chain.

Significant Factors for Modelling Survival of Escherichia coli in Lake Sediments

Currently available numerical models that describe the fecal contamination of aquatic environments using Escherichia coli as an indicator bacterium did not consider its survival in sediments. We conducted a series of comparative experiments to reveal the independent and interactive effects of sediment factors, including temperature, pH, water-extractable total dissolved solids (TDSs), coexisting microbes, and sampling sites, in lake environments on E. coli survival. In experiments, E. coli survival was observed by controlling any two factors at a time. Consequently, the decrease in pH and presence of coexisting microbes enhanced E. coli die-off, whereas the addition of water-extractable TDSs promoted its growth. To select factors to be considered for modelling E. coli survival in sediments, the independent effects of each factor and the interaction effect of the two factors were statistically compared based on their effect sizes (η2). As a result, pH (η2 = 59.5-89.0%) affected E. coli survival most significantly, followed by coexisting microbes (1.7-48.4%). Among the interactions affecting E. coli survival, including pH or coexisting microbes-which had larger independent effects-relatively larger statistically significant interactions were observed between pH and coexisting microbes (31.1%), coexisting microbes and water-extractable TDSs (85.4%), and coexisting microbes and temperature (26.4%).

Trophic interactions in microbiomes influence plant host population size and ecosystem function

Plant microbiomes that comprise diverse microorganisms, including prokaryotes, eukaryotes and viruses, are the key determinants of plant population dynamics and ecosystem function. Despite their importance, little is known about how species interactions (especially trophic interactions) between microbes from different domains modify the importance of microbiomes for plant hosts and ecosystems. Using the common duckweed Lemna minor, we experimentally examined the effects of predation (by bacterivorous protists) and parasitism (by bacteriophages) within microbiomes on plant population size and ecosystem phosphorus removal. Our results revealed that the addition of predators increased plant population size and phosphorus removal, whereas the addition of parasites showed the opposite pattern. The structural equation modelling further pointed out that predation and parasitism affected plant population size and ecosystem function via distinct mechanisms that were both mediated by microbiomes. Our results highlight the importance of understanding microbial trophic interactions for predicting the outcomes and ecosystem impacts of plant-microbiome symbiosis.

Escherichia coli removal in a treatment wetland - pond system: A mathematical modelling experience

A full-scale treatment wetland (TW) (100 inhabitants, 14 m³·d^-1), composed of two horizontal subsurface flow wetlands (TW1-400 m² and TW2-200 m²) and a small pond (13 m²), has been evaluated for Escherichia coli (E. coli) removal. The results indicate a global removal from 1.74·10⁶ to 685 MPN·100 mL^-1 (3.41 log units), reducing E. coli sufficiently to reach values suitable for reuse purposes such as agricultural reuse, without energy and reagent consumption. The small pond at the end of the treatment train plays an important role in E. coli removal and biodiversity enhancement. Data from TW1 and TW2 have been fitted to the P-k-C* model, giving values of 134 and 100 m·yr^-1 for the first-order kinetic reaction coefficient. For the pond, a process-based model using continuous stirred-tank reactor (CSTR) and a 3d-CFD model have been implemented and compared. The models indicate that solar disinfection and predation by daphnids are the most important mechanisms in the studied pond, representing 65% and 25% of the removal respectively. It can be concluded that CSTR can provide good results for small ponds and 3d-CFD model provides extra information, useful to enhance their design.

Zooplankton as a Transitional Host for Escherichia coli in Freshwater

This study shows that Escherichia coli can be temporarily enriched in zooplankton under natural conditions and that these bacteria can belong to different phylogroups and sequence types (STs), including environmental, clinical, and animal isolates. We isolated 10 E. coli strains and sequenced the genomes of two of them. Phylogenetically, the two isolates were closer to strains isolated from poultry meat than to freshwater E. coli, albeit their genomes were smaller than those of the poultry isolates. After isolation and fluorescent protein tagging of strains ED1 and ED157, we show that Daphnia sp. can take up these strains and release them alive again, thus becoming a temporary host for E. coli. In a chemostat experiment, we show that this association does not prolong bacterial long-term survival, but at low abundances it also does not significantly reduce bacterial numbers. We demonstrate that E. coli does not belong to the core microbiota of Daphnia, suffers from competition by the natural Daphnia microbiota, but can profit from its carapax to survive in water. All in all, this study suggests that the association of E. coli with Daphnia is only temporary, but the cells are viable therein, and this might allow encounters with other bacteria for genetic exchange and potential genomic adaptation to the freshwater environment.

IMPORTANCE The contamination of freshwater with feces-derived bacteria is a major concern regarding drinking water acquisition and recreational activities. Ecological interactions promoting their persistence are still very scarcely studied. This study, which analyses the survival of E. coli in the presence of zooplankton, is thus of ecological and water safety relevance.

Water Chemistry, Exposure Routes, and Metal Forms Determine the Bioaccumulation Dynamics of Silver (Ionic and Nanoparticulate) in Daphnia magna

Treatment wetlands utilize various physical and biological processes to reduce levels of organic contaminants, metals, bacteria, and suspended solids. Silver nanoparticles (AgNPs) are one type of contaminant that can enter treatment wetlands and impact the overall treatment efficacy. Grazing by filter-feeding zooplankton, such as Daphnia magna, is critical to treatment wetland functioning; but the effects of AgNPs on zooplankton are not fully understood, especially at environmentally relevant concentrations. We characterized the bioaccumulation kinetics of dissolved and nanoparticulate (citrate-coated) ¹⁰⁹ Ag in D. magna exposed to environmentally relevant ¹⁰⁹ Ag concentrations (i.e., 0.2-23 nmol L^-1 Ag) using a stable isotope as a tracer of Ag. Both aqueous and nanoparticulate forms of ¹⁰⁹ Ag were bioavailable to D. magna after exposure. Water chemistry affected ¹⁰⁹ Ag influx from ¹⁰⁹ AgNP but not from ¹⁰⁹ AgNO₃ . Silver retention was greater for citrate-coated ¹⁰⁹ AgNP than dissolved ¹⁰⁹ Ag, indicating a greater potential for bioaccumulation from nanoparticulate Ag. Feeding inhibition was observed at higher dietary ¹⁰⁹ Ag concentrations, which could lead to reduced treatment wetland performance. Our results illustrate the importance of using environmentally relevant concentrations and media compositions when predicting Ag bioaccumulation and provide insight into potential effects on filter feeders critical to the function of treatment wetlands. Environ Toxicol Chem 2022;41:726-738. © 2021 SETAC.

Population dynamics and life history traits of Daphnia magna across thermal regimes of environments

The consequences of raising temperatures have been intensively studied by biologists and ecologists for the past few decades. However, current climatic changes also include many anomalous weather events, such as intra-seasonal heatwaves followed by immediate decreases in temperature. In this study, the responses of population development and life history traits to different thermal regimes were investigated. The freshwater water flea Daphnia magna (Cladocera, Crustacea) was used as a model organism. Daphnia magna populations were monitored under temperature regimes of warm (25 °C), cold (5 °C), synchronous (gradual changes between 25 °C and 5 °C) or stochastic (random changes between 25 °C and 5 °C). Population size of D. magna populations decreased with unpredictability of thermal conditions; the highest density of D. magna was found in the warm environment and the lowest density in the stochastic environment. Thermal regime had significant impact on the prevalence of asexual and sexual reproduction of D. magna. Under a synchronous regime, an accumulation of asexual reproduction was observed during cold episodes; this was followed by a phase of population disturbance, manifesting itself in high fluctuations of asexual reproduction and a pattern of sexual reproduction typical of a cold regime. Under a stochastic regime, the population disturbances were observed throughout the duration of the experiment. Daily observations of individual life history traits revealed that the development of populations under different thermal regimes resulted from the regime-specific survivability of neonates. Population development was also affected by the frequency of reproduction, which consisted of the number of broods carried per lifetime. The results indicate that not only temperature but also shifts in thermal conditions have an important influence on individual life history traits and population dynamics of D. magna. It is important to consider the effects of shifts in water temperature on demographic and individual traits simultaneously because the impact of thermal changes on population traits can be modified by individual life histories.