A Preliminary Study On Bacterial Productivity In Different Grass Carp Polyculture Eco-systems And Analysis Of Bacterial Community

Grass carp (Ctenopharyngodon idellus) is one of the most popular fish infreshwater aquaculture of China. The aquaculture industry of gass carp has a broadmarket prospect, because of the rapid growth and the wide range of feed sources.Polyculture of grass carp and other kind of fishes can enhance the economic benefitsand improve the structure of biomes. It is a new model of freshwater aquaculture,which is more environmentally friendly and more healthy. Microorganism is animportant part of aquatic ecosystems. Bacteria plays the role of decomposer andproducer. It plays an important role in the material recycling, energy flow andbiogeochemical cycles. With the extensive and thorough research of bacterial biomassand productivity, the study of the bacteria-core microbial loop shows that themicrobial loop has an irreplaceable role in the dynamics process of aquatic ecosystem.In this study, we used the in situ culture method to study the bacterialproductivity(BP) in different grass carp polyculture ecosystems, to complete thetheory of trophic structure in the grass carp polyculture and to correctly evaluate therole of bacterial productivity in this ecosystem, in order to evaluate the structure andfunction of the aquaculture system, and to provide the basis for the management ofgrass carp polyculture pond. We used the PCR-DGGE analysis to determine thediversity of bacterial community in different grass carp polyculture eco-systems, andanalysis the correlation between bacterial community and the factors of water qualityby canonical correlation analysis (CCA). The main results were as follows:1. A preliminary study on bacterial productivity in different grass carp polycultureeco-systemsThe in situ culture method was used to study the bacterial productivity(BP) indifferent grass carp (Ctenopharyngodon idellus) polyculture ecosystems. The polyculture modles were grass carp-silver carp(Hypophthalmichthys molitrix)polyculture(GS), grass carp-common carp(Cyprinus carpio) polyculture(GC), grasscarp-silver carp-common carp polyculture(GSC1and GSC2). The results showedthat, the bacterial productivity of all treatments fluctuated from85.2±9.68to899.2±29.67μg C/L·d, with an average of442.3±210.51μg C/L·d. Overall, thebacterial productivity of all treatments showed the same trend, first increased, andthen decreased,. The highest value reached in August. Grass carp-silver carp-carppolyculture mode was significantly higher than other modes(p<0.05). The ratio ofbacterial productivity and phytoplankton was0.20. On the whole, the bacterialproductivity was correlated significantly with the primary productivity ofphytoplankton(PP), the concentration of DOC and POC, water temperature, phosphate.The results of PCA showed that different factors of water quality had different effecton the bacterium. In the prophase of cultivation, DIN, DOC and POC in the water hada contribution rate of44.058%; In the medium-term of cultivation, DOC and POConly had a contribution rate of32.693%; In the late stage of cultivation, DIN, DOCand POC had a contribution rate of45.921%. The study showed that, in the prophaseof cultivation, the bacterial productivity was driven by DIN, DOC, POC; DOC andPOC made an important role in the high level, but the concentration of PO4-P was thelimiting factor; In the late stage of cultivation, the concentration of nutrition was in ahigh level, but the bacterial productivity was low because of the low watertemperature. Through the whole cultivation, the load of DOC was in a high level,suggesting that DOC is the most important factor affecting bacterial productivity.2. The influence of zooplankton feeding on the bacterial productivityThe in situ culture method was used to study the influence of zooplanktonfeeding on the bacterial productivity in different grass carp (Ctenopharyngodonidellus) polyculture ecosystems. The polyculture modles were grass carp-silvercarp(Hypophthalmichthys molitrix) polyculture(GS), grass carp-commoncarp(Cyprinus carpio) polyculture(GC), grass carp-silver carp-common carppolyculture(GSC1and GSC2). The results showed that, zooplankton feeding onbacteria of all treatments fluctuated from75.19±4.97to886.45±44.67μg C/L·d, with an average of404.93±40.52μg C/L·d, accounting for103%±19.4%of the bacterialproductivity. Overall, zooplankton feeding on bacteria of all treatments showed thesame trend, first increased, and then decreased. The highest value reached in August.Grass carp-silver carp-carp polyculture mode and grass carp-silver carp wassignificantly higher than other modes(p<0.05). The results of this experiment showedthat zooplankton under3um had little influence on the bacteria. The number ofbacteria was reduced because of the predation pressure, but predation pressure alsostimulated the propagation of the bacteria. Zooplankton feeding impacted theabsorption of nutrients and promoted the growth of the bacteria through releasing theDOM and inorganic salts in predation process. Zooplankton feeding on the bacteria inenclosures were higher than that in ponds. This was because the organic matter andother nutrients was richer in enclosures than that in ponds. The high value of standingcrop of bacteria and the bacterial procudtivity and strongly zooplankton feeding wasanother reason.3. PCR-DGGE analysis of bacterial community in different grass carp polycultureeco-systemsBacterial community in different breeding period of grass carp polycultrue modewas studied by PCR-DGGE method. We used Quantity One to analyse theelectrophoresis pattern, and recycled the stripes for sequencing, in order to constructthe phylogenetic tree. We measured the main factors of the water quality and analysedthe correlation between the main factors of the water quality and bacterial communityby CCA method. The results showed that, bacterial diversity was largest in themedium-term of the cultivation, while in the prophase and late stage was smaller. Theresults of the identification and classification of the bacteria showed that bacterialcommunity differed higher between different grass carp polyculture mode.Cyanophyceae was the dominant species during the whole cultivation. In the prophase,α-proteobacteria was the other dominant species except the Cyanophyceae. In themedium-term, actinomycetes and bacteroides were other dominant species. In the latestage, the dominant species also included α-proteobacteria and Bacteroides. In thisexperiment, DOC、POC、DIN、phosphate and transparency had significant correlation with bacterial community, but water temperature had little correlation with bacterialcommunity. In the enclosure experiment, the correlation coefficient reached0.6between POC、transparency and bacterial community in the prophase of cultivation,then were DOC、phosphate and DIN; In the medium-term, the correlation coefficientalso reached0.6between DOC、phosphate and bacterial community, then weretransparency and DIN; In the late stage, the correlation coefficient reached0.7between DIN and bacterial community, then were phosphate and transparency.