Effect of commercially available nitrifying and denitrifying bacteria and different carbohydrate sources on the rearing of kuruma shrimp Marsupenaeus japonicus in biofloc systems

To investigate the effects of adding commercially available nitrifying and denitrifying bacteria, along with different carbohydrate sources, on rearing water quality, bacterial community, bottom sand ORP values, and growth performance of the kuruma shrimp Marsupenaeus japonicus, we conducted a 70-day rearing experiment in biofloc systems with a carbon/nitrogen ratio of 12. Four treatments were tested: sugar (treatment S) or molasses (treatment M) as a carbohydrate source, and the addition of commercial bacteria to them (treatments SC and MC, respectively). Significant differences (p < 0.05) were observed in the mean final body weight between the treatments with (SC: 3.1 ± 0.4 g, MC: 3.5 ± 0.2 g) and without (S: 2.8 ± 0.1 g, M: 3.0 ± 0.1 g) commercial bacteria. There were also significant differences in mean concentrations of total ammonia, nitrite nitrogen, and nitrate nitrogen. In the bacterial community analysis, Nitrospiraceae were consistently identified with relatively high abundance in the SC and MC, but not in the S and M. The low number of Nitrospiraceae in the S and M may have resulted in high nitrite-nitrogen concentrations, which in turn led to poor feeding and growth stagnation. These findings suggest that nitrifying bacteria play an important role in the biofloc systems with a carbon/nitrogen ratio of 12. Compared to the group using sugar as a carbohydrate source (treatments S and SC), the group using molasses (treatments M and MC) had significantly higher mean survival rates and biomass, and significantly lower food conversion ratio. The difference in carbohydrate source is thought to have affected the bacterial community in the rearing water, with the molasses treatment showing better growth performance, likely due to the predominance of bacteria beneficial to M. japonicus.