Abstract:The deterioration of the aquaculture environment and disease outbreaks occur often in high-density pond cultures of fish. Probiotics are the main and efficient method to regulate the microbiota of the fish intestinal tract and pond water environments, which will improve aquaculture conditions and reduce the incidence of diseases. To investigate the effects of probiotics on the composition and diversity of microbiota in the intestinal tract and pond culture environment of the Japanese flounder (Paralichthys olivaceus), samples from the Japanese flounder intestinal tract and environmental media (culture water, feed, and pond sediment) were collected. High-throughput sequencing of the 16S rDNA genes was performed to analyze the regulation of the microbiota of the intestinal tract and environmental samples by probiotics. Our results showed that the diversity of microbiota in the pond sediment and intestinal tract of Japanese flounder increased after the addition of probiotics, and was still in the pond sediment. Inversely, the diversity of the microbiota in the culture water declined, which was lower than that in the intestinal tract. Firmicutes, Proteobacteria, and Fusobacteria were dominant in the Japanese flounder intestinal tract, and the relative abundances of Firmicutes and Proteobacteria increased, whereas Fusobacteria decreased after the addition of probiotics. At the genus level, the relative abundances of Enterobacter and Bacillus increased, whereas Acinetobacter and Photobacterium first increased and then decreased in the intestinal tract. The relative abundance of genus represented by NS3a_marine_group firstly decreased and then increased in the Japanese flounder culture water. In the pond sediment, the change in the relative abundance of Bacillus was obvious, increasing from 3.78% to 33.64%. The relative abundances of Vibrio in the fish intestinal tract, culture water, and pond sediment decreased after the addition of probiotics. Some strains of Acinetobacter and Vibrio are important pathogens in aquaculture. The microbiota structure in the intestinal tract of Japanese flounder was similar to that in the pond sediment. Changes in the relative abundances of these main microbiota indicated that the addition of probiotics could optimize the microbiota structure of the Japanese flounder intestinal tract and environment. However, it is impossible that Lactobacillus, as the main microbiota of the probiotics, was able to colonize and persist in the fish intestinal tract, water, and pond sediment, which indicated that probiotics did not deliver bacteria but their probiotic function. Hence, the addition of probiotics could affect the microbiota structure of the Japanese flounder intestinal tract, water, and pond sediment.
