| Recirculating aquaculture system (RAS) is typically an indoor system that allows farmers to control environmental conditions year round. It has many advantages, such as recirculating aquaculture water use, decreasing water consumption, reducing disease risk, high efficiency untilization of resource and little pollution to environments and so on. RAS enable the aquaculture produce year round without regional or climatic restrictions, it is an important way for the sustainable development of aquaculture. Degrading organics in aquaculture water is the key link in RSA. Bacteria are efficient decomposers for organics in aquaculture environments. Bacteria have great diversity of species and metabolic pathways, they are important regulators for environmental conditions and play important roles in materials cycling and energy flowing. The aim of this work is to explore the community structure and dynamics of bacteria in a water recycling system culturing European eel. It is hoped that the information generated in this study can be served as theoretical guidance for verifying the effect of recirculating aquaculture system, improving the design of the system and facilitating the application of the system.Water samples were collected from each treatment of the recirculating system on April 15, July 15, September 15 and December 15, 2009. Attached samples were collected from biofilms on the nylon net and the oysters shells. The quantity and the composition of bacteria were determined by plate count method and 16S rRNA gene clone library analysis.1) Variations of bacterial quanity in water of the recirculating aquaculture system during four seasons.The concentration of bacteria in water decreased significantly after biofilm and sand filter treatment, indicating that the nylon net hanging and oyster shell fills have a good ability of adsorbing and filtering bacteria. The effect of water treatment is best in autumn, the concentration of bacteria in water decreased from 2.98×108 cell / L to 6.57×107cell / L after the recirculating system processing. The treatment effect is not so good in winter, the concentration of bacteria in water decreased from 1.27×107 cell/ L to 9.80×106 cell/ L after the water recirculating system treatment. The effects in the other two seasons are between autumn and winter.2) Seasonal variations of bacterial quanity on nylon net hangings and oyster shells.The concentration of attached bacteria on the nylon net hangings was hightest in autumn (9.20×109cells/g), second highest in summer (1.16×109cells/g), and third highest in winter (7.20×108cells/g). The concentration of attached bacteria in spring is lowest (1.39×108cells /g).The concentration of attached bacteria on the oyster shells was hightestalso in autumn(1.05×109cells/g), second highest in summer (3.30×109cells/g),and third highest in winter ( 1.78×108cells/g). The concentration of attached bacteria in spring is lowest (1.52×108 cells/g).3) Bacterial composition on nylon net hangings and oyster shells.In spring, the attached bacteria on nylon net hangings can be divided into four major groups: Proteobacteria (includingα-,β- andγ-Proteobacteria), Bacteroidetes, and Planctomycetes. The plasmid sequences of Bacillariophyta and Cyanobacteria were also detected. The Proteobacteria were the most dominant components, they occupied 35.3% of the analyzed clones. Plasmid sequences of Bacillariophyta were the second dominant members, occupied 33.3%. Others were Bacteroidetes (13.7%), Actinobacteria (5.9%), Bacillariophyta (2%), Cyanobacteria (2.0%) and some unknown bacteria. In addition to Proteobacteria and Bacteroidetes, there are Deinococcus– Thermus on the oyster shells. The Proteobacteria were dominant components, they occupied 70.5% of the analyzed clones. Bacteroidetes were the second dominant members, they occupied 21.6%, and then Deinococcus– Thermus occupied 2%. There were also unknown bacteria, occupied 5.9%.In summer,the attached sample on nylon net hangings can be divided into three major groups: Proteobacteria (includingα-,β- andγ-Proteobacteria) and Nitrospirae. The plasmid sequence of Bacillariophyta and Cyanobacteria were also detected. The Proteobacteria were the most dominant components, they occupied 45.5% of the analyzed clones. Plasmid sequences of Bacillariophyta were the second dominant members, occupied 20.6%. Others were Nitrospirae(9.1%) , Bacillariophyta (9.1%) and some unknown bacteria. In addition to Proteobacteria and Planctomycetes, there are Plasmid sequences of Cyanobacteria on the oyster shells. The Proteobacteria were dominant components, they occupied 47.6% of the analyzed clones. Bacteroidetes and Cyanobacteria were the second dominant members, both occupied 14.3%. There were also unknown bacteria occupied 38.1%.In autumn, the attached sample on nylon net hangings can be divided into two major groups: Proteobacteria (includingα-,β- andγ-Proteobacteria) and Planctomycetes. The plasmid sequences of Cyanobacteria were also detected. The Proteobacteria were the most dominant components, they occupied 67.4% of the analyzed clones. Plasmid sequences of Cyanobacteria were the second dominant members, occupied 21.7%, others were Planctomycetes (10.9%). The attached sample on oyster shells can be divided into five major groups: Proteobacteria (including α-,β- andγ-Proteobacteria), Planctomycetes, Nitrospirae, Bacteroidetes, and Acidobacteria. The Proteobacteria were the most dominant components, they occupied 44.2% of the analyzed clones. Planctomycetes and the Nitrospirae were the second dominant members, both occupied 14.0%, others were Bacteroidetes (11.6%), Actinobacteria (4.7%) and some unknown bacteria.In winter: In addition to Proteobacteria (includingα- andγ-Proteobacteria) and Deinococcus–Thermus, there are plasmid sequences of Cyanobacteria on the oyster shells. The plasmid sequence of Cyanobacteria were dominant components, they occupied 77.4% of the analyzed clones. Proteobacteria were the second dominant members, they occupied 13.2%, and then Deinococcus– Thermus occupied 5.7%. There were also unknown bacteria, occupied 3.7%. The attached sample on oyster shells can be divided into three major groups: Proteobacteria (includingα- andγ-Proteobacteria), Planctomycetes and Bacteroidetes. The plasmid sequence of Cyanobacteria were also detected. The plasmid sequence of Cyanobacteria were dominant components, they occupied 77.1% of the analyzed clones. Proteobacteria were the second dominant members, occupied 16.7%, others were Planctomycetes (3.7%), Bacteroidetes (3.7%) and some unknown bacteria(1.8%).4) The relationship between concentration, composition of attached bacteria and wastewater treatment.The nylon net hanging and oyster shells were in the membrane phase in spring, the ammonia nitrogen and nitrite nitrogen in the biofilm tank and oyster filter tank was not determined. In summer, autumn and winter, the concentration of attached bacteria on nylon net hangings and oyster shells increased, and the removing rate of ammonia nitrogen and nitrite nitrogen in water increased corresponding. In summer and autumn, the numbers of bacteria were higher than winter, and the removing rates of ammonia nitrogen were also higher than that in winter. However, the removing rate of nitrite nitrogen were high in three seasons and were less influence by the numbers of bacteria.On the nylon net hangings and oyster shells, Proteobacteria dominant in four seasons, and most of them, such as Serratia, Acidovorax, etc, have the ability to decompose organics. Most species of Planctomycetes and Nitrospirae can remove nitrites in environments,and the test results showed that the two bacterial has a more significant effect to the aquaculture wastewater treatment system.In the samples have been measured, Proteobacteria dominant in four seasons, and most of them, such as Serratia, Acidovorax, etc, have the ability to decompose organics. Most species of Planctomycetes and Nitrospirae can remove nitrites in environments,and the test results showed that the two bacterial has a more significant effect to the aquaculture wastewater treatment system. Some algae of Cyanobacteria existed in the biofilm tank through four seasons, and were most abundant in winter. Bacillariophyta algae were only detected on nylon hangings in spring and summer. These algae play an important role in removing nitrate and phosphate in aquaculture water.
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