Zooplankton impacts on the persistence of the anthropogenic pollution marker intI1 in lake water

Wastewater treatment plants (WWTP) effluents can release microbiological pollutants, including the intI1 gene (integrases of class 1 integrons), which has been proposed as a target for monitoring anthropogenic pollution in surface waters. This gene has also a strong correlation with antibiotic resistance, making of it an important proxy to evaluate the level of genetic contamination in aquatic environments. he ecological factors that influence the abundance and dynamics of intI1 within natural water bodies are largely unknown. To better understand the fate of class 1 integrons in aquatic systems, we resorted to classical limnological monitoring of intI1 over multiple years. We also conducted experiments to elucidate the impact of Daphnia grazing on its abundance. The monitoring of different size fractions of the Lake Maggiore microbial community has shown a particle-bound life-style for intI1-harbouring bacteria. Most of the bacteria hosting intI1, originating from both a wastewater effluent that discharges intro Lake Maggiore and lake water itself, grow on particulate substrates in open waters, making them particularly vulnerable to grazing by large filter feeders such as Daphnia. Daphnia grazing is independent from the origin (lake water or wastewater) of the bacterial genera; it selectively removes bacteria that are present in aggregates or even filamentous forms from both origins. To understand if intI1 is related to viable bacteria or just DNA residues, it is important to study the persistence of class 1 integrons with their gene cassettes, which often contain antibiotic resistance genes in freshwater ecosystems. Significance StatementWhile faecal pollution of freshwaters is commonly monitored, genetic pollution through wastewater treatment plant outflows, such as antibiotic resistance genes, is difficult to monitor due to the diverse nature of genes present. The intI1 gene is proposed as a proxy for anthropogenic pollution; however, there is a major lack of understanding regarding the persistence of this gene in freshwaters. In this study, we demonstrate that intI1 in freshwaters is associated with both the natural microbial community and allochthonous microbes arriving from wastewater. Furthermore, we show that intI1 harbouring bacteria preferentially reside in the aggregated microbial fraction and are easily removed by zooplankton grazing. This study is the first limnological investigation of this gene and highlights a significant gap in our knowledge regarding the ecology of class 1 integrons. Genetic pollution of surface waters is however a global problem and of very broad interest on the one hand, on the other hand, the question of the establishment of an allochthonous gene into a natural microbial community is also an interesting fundamental question in ecology, thus this study has both more applied and more fundamental aspects. Therefore, we consider it perfect for the readership of L&O.