Our previous studies discovered that the chlorophyll a content decreased dramatically when the concentration of allelochemical extracted from P. stratiotes increased (Wu et al. 2013), showing that the allelochemical affected algae cell photosynthesis. In the present study, the effect of P. stratiotes allelochemicals of different concentrations on the PC content and PC/APC ratio of M. aeruginosa were analyzed, and the results were presented in Fig. 2a, b. The PC content of M. aeruginosa was affected by the allelochemicals released from the root exudates of P. stratiotes. Starting from the second day, the PC contents in all of the treatment groups (except the low-concentration group of 0.2 g mL−1) were lower than that in the control group (without allelochemical). The PC content was decreased gradually as the allelochemicals concentration increased; it decreased to 30.16 and 39.68 % compared with the control group in the 0.8 and 1.2 g mL−1 treatment groups, respectively. At the same time, the PC/APC ratio was also affected by the allelopathy of root exudates, showing the same trend as that of PC. There was no significant difference between the low-concentration and control groups (P > 0.05). The PC/APC level decreased as the allelochemical concentration increased. The PC/APC level of M. aeruginosa was always lower in the highest-concentration treatment group (1.2 g mL−1) than in the other groups, indicating that the higher the allelochemical concentration was, the greater the effects on the PC/APC level of M. aeruginosa would be.