Planktonic bacteria are an important part of aquatic ecosystems and interact with zooplankton. However, it is still unclear whether different planktonic bacteria differentially interfere with the responses of zooplankton to their predators. Here, we investigated the effects of different planktonic bacteria, which were isolated and purified from natural lakes, on the anti-predation (Rhodeus ocellatus as the predator) defense responses of Daphnia magna. Our results showed that the effects of planktonic bacteria on the induced life-history defenses of Daphnia were species-specific. Bacteria which increased (e.g., Escherichia coli, Citrobacter braakii) Daphnia body size also promoted the induced defense of body size, whereas bacteria which decreased (e.g., Pseudomonas spp.) Daphnia body size also inhibited the induced defense of body size. In addition, the same bacteria had different effects on induced defense traits. Some bacteria (e.g., E. coli) promoted the induced defense of body size but reduced the induced defense of offspring number, whereas other bacteria (e.g., Aeromonas hydrophila, Aeromonas veronas) weakened the induced defense of body size but had no significant effect on the induced defense of offspring number. Moreover, the differential effects of planktonic bacteria on Daphnia's induced defenses were not related to the bacterial degradation of kairomone. This study illustrated, for the first time, the species-specific effects of planktonic bacteria on predator-induced responses of Daphnia. IMPORTANCE This study is the first to reveal the differential effects of different species of planktonic bacteria on fish kairomone-induced defense traits and energy redistribution in Daphnia. Our results not only help deepen the understanding of Daphnia's inducible defenses in environments containing a variety of bacteria but also provide insights into the energy reallocation involved in anti-predator defenses.
Keywords: Daphnia magna; inducible defense; kairomone; life-history traits; planktonic bacteria.