Ecological risks of combination of multiple pollutants at environmentally relevant concentrations: Insights from the changes in life history traits, gut microbiota, and transcriptomic responses in Daphnia magna

Aquatic organisms are normally exposed to waters where multiple pollutants coexist. Although the concentration of each single pollutant in natural waters is extremely low and may not have harmful effects, the combined effects of multiple low concentration pollutants may cause substantial harm to Daphnia . Therefore, we selected 11 kinds of pollutants including microplastics, antibiotics, heavy metals, agricultural and industrial pollutants, and then exposed Daphnia magna to the combination of these pollutants at the environmental concentrations (ng L −1 -μg L −1 range) to evaluate the possible negative effects. Results showed the combination of multiple pollutants significantly decreased heart rate, body size, survival, and fecundity of D. magna and delayed maturation. In the filial generation constantly exposed to the pollutant combination, the growth, survival, and reproduction further decreased. The diversity of the gut microbiota decreased, but the abundance of bacteria with functions related to xenobiotics degradation increased under the pollutant combination. The expressions of genes related to antioxidant, xenobiotics catabolism, and energy absorption were upregulated by the pollutant combination, with downregulating expressions of the genes related to cell division and nitrogen metabolism, which reveals the underlying mechanism of the harmful effects of multiple pollutants on life history traits of D . magna . This study demonstrated the ecological risks of multiple pollutants at environmentally relevant concentrations to D . magna , providing a new perspective for evaluating the consequences of low environmental pollution in natural waters. • Combination of pollutants at ambient concentrations negatively affects Daphnia. • Offspring suffer further harm after continuous exposure to the pollutant mixtures. • The pollutant mixtures raise abundance of gut-microbiota for degrading substance. • The pollutant mixtures upregulate expression of genes related to detoxification. • Gene expression related to cell division and nitrogen metabolism is downregulated.