Effect and track of Mylopharyngodon piceus and Silurus asotus intestinal excreta on the recruitment of dormant Microcystis aeruginosa in aquaculture water

Microcystis aeruginosa is a kind of cyanobacteria that can bloom in freshwater lakes and slow-flowing water, especially in freshwater aquaculture waters. A large amount of feces will be excreted by benthic fish in the growth process, which may provide conditions for the recovery and increment of M. aeruginosa. To explore the effects and track of the fish intestinal excreta (Mylopharyngodon piceus and Silurus asotus)on the recruitment of dormant M. aeruginosa from the sediment, and to find potential factors and mechanism, simulation experiments on the recruitment of M. aeruginosa cells from sediment were performed in the presence or absence of the fish intestinal excreta. The intestinal excreta of M. piceus or Silurus asotus were fully mixed with cells of dormant M. aeruginosa, and then they were all buried in the lake sediments. The co-culture and recruitment experiments were carried out at the gradient temperatures of 10 °C, 15 °C and 20 °C. The results showed that the recruitment rate of dormant M. aeruginosa cells in M. piceus excreta group (MP), S. asotus excreta group (SA) and M. piceus or S. asotus excreta mixture group (MP-SA) were all significantly higher than that in the control group (CK) (P < 0.05), and the recruitment rate of dormant M. aeruginosa cells in the MP group was significantly higher than those in the SA group and the MP-SA group. At 20 °C, the recruitment rate of dormant cells of M. aeruginosa in MP group was significantly higher than that in SA group, MP-SA mixed group and CK group, however, there was no significant difference in the recruitment rate between SA group and CK group (P > 0.05), and there was no significant difference between MP-SA group and CK group. From the point of view of gradient experiment temperature, the recruitment rate of dormant M. aeruginosa cells both in the control group and in the experimental group (excreta group) increased with the increase of temperature. At 10 °C, recruitment rate of dormant M. aeruginosa cells in the control group, the Mylopharyngodon piceus excreta group (MP), the S. asotus excreta group (SA) and the mixed group (MP-SA) was 14%, 45%, 36% and 28%, respectively, and at 15 °C, recruitment rate of dormant M. aeruginosa was 43%, 60%, 51%, 50%, respectively, at the experimental temperature of 20 °C, recruitment rate of dormant M. aeruginosa was 49%, 61%, 52%, 51%, respectively. For the control group and the three experimental groups (excreta group), t recruitment rate of dormant M. aeruginosa at 15 °C and 20 °C was significantly higher than that at 10 °C, but there was no significant difference in the recruitment rate of dormant M. aeruginosa cells at 15 °C and 20 °C (P > 0.05). At the same time, the main recruitment period of dormant M. aeruginosa was from day 3 to day 15. During the experiment at 10 °C, 15 °C and 20 °C, the dominant bacteria of MP group were mainly Pseudomonas, and the dominant bacteria of SA group and MP-SA group were mainly Bacillus and Firmicute, respectively. In the MP group, at 10 °C, the dominant bacteria in the sediment were Pseudomonas, Fusobacteria, Bacillus, Vibrio, Proteobacteria and Firmicute, which accounted for 33%, 13%, 9%, 7%, 4% and 4%, respectively, the proportion of dominant bacteria was 70% in total bacteria. They accounted for 61% and 54% at 15 and 20 °C, respectively. For the S. asotus excreta group (SA), at 10 °C, 15 °C and 20 °C, the dominant bacteria in the sediment were Bacillus, Bifidobacterium, lactic acid bacteria, Firmicute, Vibrio and Pseudomonas, which accounted for 64%, 50% and 42% of the total bacteria respectively. For MP-SA group, at 10, 15 and 20 °C, the dominant bacteria in the sediment were bacillus, Bifidobacterium, lactic acid bacteria, Firmicute, Proteobacteria and Pseudomonas, and their proportions in the total bacteria were 50%, 45% and 43%, respectively. The concentration of total bacteria in the sediment of each experimental group increased with the increase of temperature, and the concentration of total bacteria at 15 °C and 20 °C was significantly higher than that at 10 °C. 0-12 d was the period of bacterial proliferation. The physical and chemical indexes of the sediment-water interface (SWI) were measured and it was found that the the N/P(DIN∶SRP) ratio of MP group, SA group and MP-SA group were significantly lower than those of the control group during the period of day 3 to day 12. At the three gradient temperatures, in the same period of 0-9 d, the nitrogen-phosphorus ratio decreased the fastest in M. piceus excreta group, followed by S. asotus excreta group, the mixed group and the control group (CK). There was no significant change of nitrogen-phosphorus ratio in the whole process for the control group. In the same period, there were significant differences between the three experimental groups and the control group (P < 0.05), and there were also significant differences among the three experimental groups. During all experiments, the change trend of dissolved oxygen (DO) concentration was basically the same as that of nitrogen-phosphorus ratio, and dissolved oxygen. The results showed that the intestinal excreta of M. piceus or S. asotus could promote the recruitment of dormant M. aeruginosa, which may be the result of the decline of N/P ratio and dissolved oxygen concentration caused by flora in the excreta during the growth and proliferation period, and the effect of promoting-recruitment was more significant in the lower temperature range (10-15 °C). The results are of theoretical significance for the recruitment of dormant M. aeruginosa cells, clearing the pond of silt, and the mechanism of bloom outbreak of M. aeruginosa in spring. It also provides a new way to prevent and control the bloom of M. aeruginosa in aquaculture waters.