Potential benefits of Bacillus subtilis-mediated bioflocs in supplementary feeding on triangle sail mussels Hyriopsis cumingii: a pilot study of fluorescence-labeled floc-forming bacteria

As micro -particle aggregates, bioflocs may be potential baits for ingestion by triangle sail mussels and supplementary feeding raises more concerns. Besides, a plasmid carrying the green fluorescent protein (GFP) gene was transformed into floc -forming Bacillus subtilis (GFP-B. subtilis) to visualize its colonization during the bioflocs ingestion. To investigate the potential benefits of bioflocs on H. cumingii, Chlorella vulgaris diets were supplemented with nothing (control group, CG) or bioflocs accounting for the microalgae biomass at 25% (T1), 50% (T2), and 100% (T3) respectively. The community composition of bioflocs was analyzed and some species like phylum Proteobacteria, order Rhodobacterales and genus Paracoccus with potentials to improve water quality were found. Furthermore, supplementary bioflocs could reduce the concentrations of inorganic nitrogen in the aquacultural environment. Differences in growth traits across all groups were insignificant, but the digestion, metabolic and immune response of H. cumingii were promoted remarkably. Interestingly, some colonies of GFPB. subtilis were detected in the intestines of H. cumingii from T1, suggesting H. cumingii could uptake bioflocs efficiently and intestinal colonization of B. subtilis might be accomplished. Besides, supplementary bioflocs with the highest ratio (T3) could improve the physiological status of organs and suppress the dominance of potential pathogens such as Aeromonas and Pseudomonas. It was assumed that B. subtilis might stimulate the healthy intestine development via intestinal colonization. Overall, B. subtilis-mediated bioflocs is beneficial to H. cumingii and more feeding strategies of bioflocs in H. cumingii farming are expected to be developed in future.