Effects Of Several Environmental Factors On Gut Microflora Diversity Of Cladocerans

In the pelagic ecosystem, zooplankton and microorganisms are closely associated. Zooplankton (i) provide quality nutrients to microorganisms, (ii) become good habitat and refuge for microbial, (iii) promote distribution of microorganisms in planktonic systems. On the other side, microorganisms degrade biological macromolecular into more easily absorbed small molecules, then enhance digestion and absorption capacity of the host, help the host to effectively resist external pressure. However, since no direct links on nutrient cycling and trophic cascade reaction, most studies have ignored the interactions between zooplankton and microorganisms. With the emergence of global environmental problems, water physical and chemical properties change, such as ambient temperature, saltwater intrusion and frequent outbreaks of cyanobacterial blooms and so on. In the process of changes, cladocerans, the main zooplankton in fresh water, how to regulate the growth and reproduction to adapt to a series of changes in the column? In this process, whether the environment flora or intestinal flora would provide assistance?According to these scientific issues, I chose main cladocerans:Daphnia pulex and Simocephalus vetulus as the object of study, investigated the effect of individual development, food and nutrition, temperature stress, salinity stress and cyanobacterial bloom on zooplankton and their associated microflora. We tested the cladoceran growth and reproduction by life-table experiment, analyzed the surface and gut microflora combined by T-RFLP and MiSeq high-throughput sequencing, and explored their correlations with environmental stressors.First, I explored the impact of D. pulex individual development on their surface and gut microflora. For this study, three D. pulex clones were cultured separately to 3 days,10 days and 20 days. There were no significant differences between clones and developmental between composition and diversity of D. pulex gut and surface microflora. Microflora was mainly composed of β-, γ-Proteobacteria and Bacteroides.Second, I explored the impact of food and nutrition on D. pulex their surface and gut microflora. In this study, I chose a D. pulex clone, fed with 100% Chlorella pyrenoidosa,50% C. pyrenoidosa and 50% Microcystis aeruginosa while food concentration was 200μg C L-2,600 μg C L-1 and 1000 μg C-1. Food concentration and food quality have no significant effect on surface microflora. Food concentration also had no significant effect on gut microflora, while food quality influenced significantly gut microflora. In the case of cyanobacteria occurs, novel microbial population may occur in gut to assist D. pulex to enhance their resistance to cyanobacteria.Third, I explored the impact of changes in ambient temperature on D. pulex and S. vetulus. The life table experiments were conducted at four temperatures (15,20,25, and 30℃) for D. pulex and S. vetulus and their gut microflora were identified using T-RFLP and MiSeq high throughput sequencing. At low temperature, the net reproductive rate (Ro) of cladocerans decreased, the average life span (L) and generation time (T) increased. High temperature significantly inhibited the growth and reproductive ability of D. pulex, on the contrary, high temperatures promote growth and reproductive capacity of S. vetulus. When considered of common effects of temperature and M. aeruginosa, inhibition of D. pulex and S. vetulus was more significant. At high temperatures tolerance index of D. pulex to M. aeruginosa was significantly enhanced. D. pulex gut microflora was more affected by temperature and M. aeruginosa, there may be some novel microorganisms assist D. pulex to withstand common effect of temperature and M. aeruginosa.Fourth, I explored the impact of salinity changes on D. pulex and S. vetulus. The life table experiments were conducted at three salinities (0,1,2 g L-1) for D. pulex and S. vetulus, and their gut microflora were identified using T-RFLP and MiSeq high throughput sequencing. D. pulex had lower saline and M. aeruginosa. resistance tolerance than S. vetulus. Tolerance index of D. pulex was more significant increased under the joint effect of salinity and M. aeruginosa. In this progress, the role of the gut microflora in D. pulex was more significant. Gut microflora of S. vetulus may have greater function in the progress of increasing salinity tolerance.Finally, I explored the M. aeruginosa tolerance and changes of gut microflora of S. vetulus collected from different degrees of cyanobacterial bloom waters. S. vetulus collected from higher cyanobacterial bloom waters had stronger anti-cyanobacteria capacity through reduced growth and reproductive capacity as compensation. S. vetulus collected from lower cyanobacterial bloom waters had higher growth and reproductive capacity, weaker anti-cyanobacteria capacity. When faced with M. aeruginosa stress, gut microflora may enhance their capacity of anti-cyanobacteria.Based on the above studies, cladoceran individual development and food concentration had small effects on the composition and diversity of gut microflora. In both cladocerans, S. vetulus was more resistant to high temperature and high salinity than D. pulex. When faced with high temperature or high salinity and M. aeruginosa joint stress, novel microbial species may occur in gut microflora of D. pulex to improve the ability to resist of environmental stresses. When S. vetulus collected from lower cyanobacterial bloom waters increasing their ability of anti-cyanobacteria, gut microflora may take a supporting role.