Bacterial composition and inferring function profiles in the biofloc system rearing Litopenaeus vannamei postlarvae at a low salinity

This study aimed to investigate the bacterial composition and inferring function profiles in the biofloc system rearing Litopenaeus vannamei postlarvae (PL) at a low salinity condition. PL (~ stage 15) were stocked in four parallel tanks filled in water with a salinity of 5.0‰ at a density of 4000 individuals per m 3 for a 28-days culture experiment, during which glucose was added as carbon source with a C:N of 20:1. At the end of experiment, water was sampled from each tank and pooled to extract microbial DNA for high-throughput sequencing of V3-V4 region of 16S rRNA gene. Results showed that the bacterial community at 28 d was dominated by phyla of Proteobacteria (45.8%), Bacteroidetes (21.1%), Planctomycetes (13.5%), Chlamydiae (10.3%) and Firmicutes (6.8%). A proportion of 81% inferring KEGG functions of this bacterial community associated with metabolism. Among functions relating to nitrogen metabolism, 48.5% were involved in the conversion of ammonia to glutamate, but the proportion of those involved in transformation among ammonia, nitrite and nitrate was 18.0% in total, inferring higher protein-synthesis but lower inorganic nitrogen-transformation capacities of the bacterial community. At the same time (28 d), high levels of total nitrogen (231.3±6.0 mg L -1 ) and biofloc (127.0±63.0 mL L -1 ), but low concentrations of ammonia (0.04±0.01 mg L -1 ), nitrite (0.2±0.1 mg L -1 ) and nitrate (12.9±2.5 mg L -1 ) were observed. The results supply a novel insight for understanding the function of bacterial community in the biofloc system nursing L. vannamei PL at a low salinity.