Analysis of Water Quality and Bacterial Community Characterization in an Industrialized Recirculating Aquaculture System of Litopenaeus vannamei

Abstract

Litopenaeus vannamei is one of the most important species of cultured shrimp in China. With the continuous development of intensive culture and increase in stocking density and feed feeding, the yield of cultured shrimp and utilization rate of the cultured environment have improved rapidly. However, a large number of residual baits, feces, fertilizers, and drugs have worsened the aquaculture water environment and led to prominent adverse environmental effects, widespread aquatic animal diseases, and severe environmental pollution. Therefore, the development of pollution-free ecological aquaculture, the promotion of industrialized circulating water aquaculture, and the research of efficient and sustainable biological purification systems have become the focus of aquaculture model research. Circulating water aquaculture is a new aquaculture mode in which water is treated and reused using modern techniques.To explore aquaculture water quality and the composition structure of microbial flora in an industrialized circulating water aquaculture system of L. vannamei, the water quality indexes of biological purification aquaculture systems such as primary moving bed biological purification, secondary fixed-bed biological purification, and aquaculture water, and the microbial flora of water and shrimp intestines were measured using high-throughput sequencing technology and bioinformatics analysis. The experimental results showed that the mass concentrations of ammonia nitrogen (NH4+-N) and nitrite nitrogen (NO2–-N) in the culture water met the standards for the healthy culture of L. vannamei after treatment with the biological purification culture system. Concentrations are 0.85 and 0.21 mg/L. Results showed that all water and carrier samples contained a certain abundance of nitrifying Spirillum bacteria, especially on the biological purification carrier, which was significantly higher than the proportion of the bacterial community in water samples. The Chao1 and Shannon indices of chemical fiber filament biological carriers were significantly higher than those in shrimp intestines. Results also showed that the number of microbial species and microbial community diversity on the biological carrier of chemical fiber filaments were higher than those in the intestine of shrimp. The bacterial community composition was characterized at the phylum and genus levels. A total of 46 bacterial phyla were identified in the samples. Proteobacteria, Bacteroidetes, and Actinobacteria were the dominant phyla in the water, and Proteobacteria, Bacteroidetes, Actinobacteria, and Planctomycetes were the dominant phyla in the biological carrier; Proteobacteria, Bacteroidetes, and Firmicutes were the dominant phyla in the shrimp intestine. The abundance of Planctomycetes in the biological carriers increased. The abundance of Proteobacteria and Firmicutes in shrimp intestines was higher than that in water, conducive to the degradation of nitrogen and phosphorus. Bacteroidetes and Firmicutes were the main metabolizing bacteria in the intestine, promoting carbohydrate metabolism and energy acquisition in shrimp. Actinobacteria can degrade organics, including macromolecules such as starch and proteins, and produce antibiotics and other antibacterial substances. The abundance of Actinobacteria in water was very high but decreased in shrimp, which played an important role in the decomposition of organic matter and nitrogen in water. The bacterial species content in the biological purification carrier of the shrimp culture circulating system was lower than that in the water sample, but the microbial diversity was higher than that of aquaculture water. The microorganisms in the biological purification carrier have the characteristics of low abundance and high diversity, possibly due to the increase in the abundance of environment-related functional bacteria on the biological purification carrier and the decrease in some flora species with little impact on the environment, resulting in the aggregation of biofilm bacteria species. In addition, the water and biological purification carrier also contained a certain amount of pathogenic bacteria, such as Vibrio and Mycobacterium. Therefore, it was necessary to regulate the content of pathogenic bacteria in the water environment and shrimp intestine to ensure a healthy shrimp culture. The results showed that the multi-stage biological purification system can effectively increase the microbial flora, promote the metabolism of nitrogen and phosphorus in the water, and regulate the quality of the aquaculture water. The dominant bacterial groups in water, biological purification carriers, and shrimp were different. Proteobacteria and Bacteroidetes were dominant in water biological carriers and intestines, whereas Planctomycetes mainly exist as biological carriers, Firmicutes mainly colonize the intestine, and Actinomycetes mainly exist in water. In this study, high-throughput sequencing technology was used to study the composition of the microbial community structure in the circulating water culture system of L. vannamei. Results can reveal the diversity of its internal microbial community, provide a basis for further revealing the "black box" of biofilters, and have important guiding significance for the study of the construction and denitrification efficiency of biofilters in seawater circulating water culture.

Keywords

• litopenaeus vannamei
• industrialized circulation aquaculture
• biological purification
• water quality
• bacterial community