Effects of different solid carbon sources on water quality, biofloc quality and gut microbiota of Nile tilapia (Oreochromis niloticus) larvae

External carbon source is needed for biofloc system to maintain an optimal C/N ratio for the growth of bacteria biomass. In this study, three solid-phase biodegradable compounds, including Longan powder (LP), poly-hydroxybutyrate- hydroxyvalerate/LP (PHBVL) and Poly(butylene succinate)/LP (PBSL), were utilized to feed biofloc-based aquaculture systems in triplicates for nine Nile tilapia (Oreochromis niloticus) larvae culture tanks. LP was applied in the in-situ biofloc system as a "control group" (3 tanks), while PHBVL and PBSL were used in the ex-situ biofloc systems (6 tanks). During the 120-days experiment, the C/N ratio was maintained at 24.87 +/- 5.66, 22.93 +/- 3.20 and 23.12 +/- 3.54 for the LP, PHBVL and PBSL groups, respectively. There were no significant differences (P >.05) of the averaged total ammonia nitrogen (TAN) concentration among the LP, PHBVL and PBSL groups (1.10 +/- 1.18, 0.67 +/- 0.38 and 1.18 +/- 1.40 mg L-1). Significant differences of the averaged NO2--N concentrations (0.26 +/- 0.38, 0.01 +/- 0.01 and 0.08 +/- 0.12 mg L-1) were detected among the LP, PHBVL and PBSL groups (P < .05). The accumulation of NO3--N in LP group (> 40 mg L-1 on day 120) was significantly higher than that of PHBVL and PBSL groups (about 2-3mg L-1 on day 120) (P < .05). To characterize the quality of biofloc, the median diameters (D50) and essential amino acids index (EAAI) were measured for three treatments. The D50 (124.7 +/- 4.24, 131.6 +/- 2.83 and 175.5 +/- 9.19 mu m) and EAAI (0.969 +/- 0.011, 1.007 +/- 0.014 and 0.995 +/- 0.012) showed that the high quality bioflocs in the LP, PHBVL and PBSL groups could meet the requirement for feeding the aquatic animals. In addition, high throughput sequencing test showed that solid carbon source not only had a significant effect on the microbial community in bioflocs, but also on the composition of fish gut microbiota. Bacillus was the dominating genus discovered in all treatments (48.34% in LP, 49.24% in PHBVL and 50.47% in PBSL) by 16S rRNA sequencing. Overall, blending LP with biodegradable polymers as carbon source showed significantly higher removal efficiency of nitrate and nitrite nitrogen, and higher biofloc quality than using LP as the only carbon source. How exactly various solid carbon sources influence fish growth performance and health need further study.