Abstract
Most river systems are impacted by sewage effluent. It remains unclear if there is a lower threshold to the concentration of sewage effluent that can significantly change the structure of the microbial community and its mobile genetic elements in a natural river biofilm. We used novel in situ mesocosms to conduct replicated experiments to study how the addition of low-level concentrations of sewage effluent (nominally 2.5 ppm) affects river biofilms in two contrasting Chalk river systems, the Rivers Kennet and Lambourn (high/low sewage impact, respectively). 16S sequencing and qPCR showed that community composition was not significantly changed by the sewage effluent addition, but class 1 integron prevalence (Lambourn control 0.07% (SE ± 0.01), Lambourn sewage effluent 0.11% (SE ± 0.006), Kennet control 0.56% (SE ± 0.01), Kennet sewage effluent 1.28% (SE ± 0.16)) was significantly greater in the communities exposed to sewage effluent than in the control flumes (ANOVA, F = 5.11, p = 0.045) in both rivers. Furthermore, the difference in integron prevalence between the Kennet control (no sewage effluent addition) and Kennet sewage-treated samples was proportionally greater than the difference in prevalence between the Lambourn control and sewage-treated samples (ANOVA (interaction between treatment and river), F = 6.42, p = 0.028). Mechanisms that lead to such differences could include macronutrient/biofilm or phage/bacteria interactions. Our findings highlight the role that low-level exposure to complex polluting mixtures such as sewage effluent can play in the spread of antibiotic resistance genes. The results also highlight that certain conditions, such as macronutrient load, might accelerate spread of antibiotic resistance genes.
Highlights
The majority of EU rivers are impacted by Sewage Treatment Works (STW) effluent and it is not uncommon for effluent to provide between 20% and 70% of total river flow (Graham et al, 2010a,b; EC, 2012; Halliday et al, 2015)
Nutrient and boron measurements taken in the first half of 2009 in the build-up to our study showed that the main differences in the river chemistry were soluble reactive phosphorus (SRP) and boron levels
The biofilm communities in each river were differently diverse (Fig. 3, 2-way ANOVA, F 1⁄4 7.6, p 1⁄4 0.02 for bacteria, F 1⁄4 25.4, p 1⁄4 0.0004 for algae), but there were no differences between sewage effluent and control flumes
Summary
The majority of EU rivers are impacted by Sewage Treatment Works (STW) effluent and it is not uncommon for effluent to provide between 20% and 70% of total river flow (Graham et al, 2010a,b; EC, 2012; Halliday et al, 2015). Lehmann et al / Water Research 106 (2016) 163e170 growth rates due to enhanced nutrient supply (Hill and Fanta, 2008), leading to harmful blooms, lower photosynthetic and metabolic efficiency (Wakelin et al, 2008; Bonnineau et al, 2010), potentially reducing biogeochemical cycling rates, decreased biofilm adhesion (Schreiber and Szewzyk, 2008), decreased bacterial diversity (Ricciardi et al, 2009), changes to community composition (Lawrence et al, 2005; Wakelin et al, 2008), decreased colonization rates (Verma et al, 2007) and spread of antibiotic resistance genes (Amos et al, 2014) This range of responses reflects the diversity of chemicals and microorganisms emitted by STWs. To confound the issue, contamination of river microbiota by STWeffluent components such as metals or antibiotic resistance genes (ARG), gets passed through the food web to higher organisms (Brooks et al, 2006). For example, have been found to harbour ARGs after exposure to microbial reservoirs in the environment (Ghosh and Mandal, 2010; Bonnedahl and J€arhult, 2014) which poses a potential threat to humans
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