Carbon: Nitrogen (C:N) ratio level variation influences microbial community of the system and growth as well as immunity of shrimp (Litopenaeus vannamei) in biofloc based culture system.

ABSTRACT

Biofloc technology (BFT) is a novel modern aquaculture farming technique used to reduce toxic nitrogen concentration, act as in situ food source and eradicate pollutants using carbon and therefore to control CN ratio in an aquaculture system. In this study, effect of different CN ratios of a biofloc based system on water quality such as the level of Total ammonia nitrogen (TAN) nitrite-nitrogen (NO2--N) and nitrate nitrogen (NO3--N) were explored. Further, the growth and immunity status of shrimp L. vannamei under the influence of different CN ratios were evaluated. Two of the CN ratios (15 and 20) could significantly (P < 0.05) reduce TAN, NO2-N and NO3-N levels (0.456 ±â€¯0.01, 0.145 ±â€¯0.09, and 0.102 ±â€¯0.02 ppm) compared to control (1.45 ±â€¯0.1, 0.749 ±â€¯0.14 and 0.675 ±â€¯0.16 ppm). Large variations in the frequency distribution of operational taxonomic units (OTUs) for the bacterial community in water with different CN ration (BFT) and control were observed. Vibrios often considered as opportunistic pathogens, where the most dominant bacterial flora of water in control (79%) and CN5 (37%) group. In CN10, Thauera (62%) was most represented genus. Similarly, Attheyaceae (56%), followed by Peridiniaceae (30%) were the most dominant groups in CN15 treatment. The diversity of bacterial flora was more spread in CN20 treatments with Psychrobacter (26%), Proteobacteria (25%) and Peridiniaceae (20%) as the major groups. The trend of Vibrio dominance decreased with the increase in CN ratios and thus confirming the dominance of heterotrophic bacteria in high CN ratio groups. Upon challenge with pathogens, shrimps from CN10, CN15 and CN20 groups showed significantly higher survival (P < 0.05) compared to the CN5 and control group. Similarly, better growth rate was also observed in BFT tanks compared to control both during the culture and at harvest. Comparatively higher expression of four immune-related genes (ras-related nuclear gene (RAN), serine proteinase gene (SP), prophenoloxidase activating enzyme (PPAE), and crustin were observed in different CN ratio ponds than control and these were in increasing trend with the CN ratio. Gene expression analysis showed that the transcripts of those immune genes were significantly increased among all CN treatments than that of control. Overall, these findings demonstrated that with optimum CN ratio, BFT can be used to optimize the bacterial community composition for both optimal water quality and optimal shrimp health. This study thus indicates the possibility of obtaining better performance of L. vannamei culture with proper adjustment of CN ratio in a biofloc based system.