Nitrogen dynamics, bacterial community composition and biofloc quality in biofloc-based systems cultured Oreochromis niloticus with poly-β-hydroxybutyric and polycaprolactone as external carbohydrates

Manipulating the C:N ratio is a practical way to reduce the accumulation of ammonium nitrogen (NH4+-N) in an aquaculture system. In this study, we investigated the use of poly-β-hydroxybutyric (PHB) and polycaprolactone (PCL) as carbon sources to support a limited discharge tilapia aquaculture system conveniently. The water was not changed for 120 days. No significant differences in the final biomass were observed between the PHB group (37.93 ± 8.87 kg m− 3) and the PCL group (34.29 ± 9.29 kg m− 3) or between the PHB group and the group with glucose as an external carbohydrate (44.14 ± 10.51 kg m− 3). NH4+-N and nitrite nitrogen (NO2−-N) were < 3.0 mg L− 1 and 0.8 mg L− 1 at steady state, respectively. The assimilation of NH4+-N and nitrate nitrogen (NO3−-N), nitrification and denitrification were observed in the batch tests of the bioflocs taken from the fish tank. No significant differences were detected in the bioflocs' crude protein content (31-39%) between the PCL group and the PHB group or between the PHB group and the glucose group. Microbial analysis, assessed by high-throughput sequencing, revealed that the bacterial community was similar between the three groups. Although not every parameter was better than glucose, PHB or PCL was still considered preferable to glucose because of greater convenience. Collectively, our results showed that both PHB and PCL are favorable and convenient external carbohydrates for biofloc-based tilapia aquaculture systems.