3.1 Gut microbes enhance the survival of A. erythraea on the DA diet
To investigate the DA resistance of A. erythraea , we fed the copepods toxic P. cuspidata , which had a DA concentration of 4.9 ± 0.7 fg cell⁻¹, as well as non-toxic P. brasiliana . Our results indicate that the respiration rate of A. erythraea was significantly higher when fed the toxic diet compared to the non-toxic diet, starting from the second day (p < 0.05, t-test) (Fig. 2A). Ingestion rates peaked at 2000 cells mL⁻¹ (Fig. S1), which was used for subsequent daily measurements of A. erythraea’s ingestion rate on different PSN diets. In daily grazing experiments, A. erythraea maintained a consistent ingestion rate when fed the non-toxic PSN , averaging 2390 ± 359 cells copepod⁻¹ hour⁻¹. In contrast, the ingestion rate on the toxic diet showed a slight but significant decline starting from the second day (p < 0.05, t-test) (Fig. 2B). However, survival rates of A. erythraea over five days revealed no significant differences among the different diets (log-rank test, p > 0.05; Fig. 2A; Table S1). These results suggest that although the DA dietsuppressed A. erythraea growth by reducing grazing and increasing respiration, it did not affect the survival of this copepod.
To further investigate the role of the gut microbiota of A. erythraea when copepods have a DA diet, we treated copepods on different diets with antibiotics and examined their survival. Before antibiotic treatment, each field-collected copepod harbored an average of 163,174 gut microbiota. After 24 hours of antibiotic treatment, the gut microbiota count per copepod decreased to 16,428, indicating an approximate 90% reduction (Figure S2). When A. erythraea was treated with antibiotics, resulting in a substantial reduction in gut microbiota, survival on the toxic diet was significantly lower than on the non-toxic diet (log-rank test, p < 0.001; Fig. 2A; Table S1). These findings suggest that the gut microbiota of A. erythraea plays a crucial role in detoxifying DA and enhancing survival on a DA diet.