| Micropterus Salmoides is an important freshwater species in China. It is of great significance to develop a zero exchange aquaculture ponds for Micropterus Salmoides. Recently, adding carbon sources technology was introduced into aquaculture as an emerging environmentally friendly aquaculture production method. Adding carbon sources to aquaculture water could promote the formation of bioflocs, which created economic and environmental benefits via reducing effluent discharges and artificial feed supply, and improving bio-security. In this study, bioflocs were applied to the aquaculture ponds of Micropterus Salmoides, and the effects of adding carbon sources on the water quality, bacterial community structure and function were explored to provide a theoretical basis for the healthy and efficient green aquaculture of Micropterus Salmoides. Specifically, two experimental groups were established by adding special carbon source and slow-release carbon source in outdoor ponds, respectively, and a control group without carbon source addition was also set up. A 6-week cultivation experiment was conducted. The bacterial community structure and functional prediction were explored using 16S rRNA high-throughput sequencing technology, and water quality parameters were also measured. Our results showed that the water quality parameters pH, chlorophyll a (Chl a), total nitrogen (TN), ammonia (NH4), nitrite (NO2) and nitrate (NO3) concentrations in the experiment groups were significantly lower than those in the control group, and bacterial abundance (BA) and bioflocs volume (BFV) in the experiment groups were about 5 times and 2 times higher than those in the control group, respectively. This result indicated that adding special carbon sources and slow-release carbon sources to the water of Micropterus Salmoides ponds promoted the formation of bioflocs and significantly reduced the concentration of nutrients, improving water quality. In addition, Chl a, BFV, and NO3 in the special carbon source group were significantly higher than those in the slow-release carbon source group. In contrast, TN, NH4 and NO2 in the special carbon source group were significantly lower than those in the slow-release carbon source group. This indicated that the addition of special carbon source had a more positive effect on the formation of bioflocs, and its impact on improving the water quality of Micropterus Salmoides aquaculture ponds was more significant than that of slow-release carbon source adding. This phenomenon probably resulted from the fermented organic compounds in special carbon sources including macromolecular matter such as polysaccharides, proteins, and micromolecular matter such as amino acids and monosaccharides, which could be rapidly utilized for bacterial production and bioflocs formation. In terms of bacterial community structure, Actinobacteria, Proteobacteria, and Bacteroidetes were the dominant phyla of Micropterus Salmoides ponds, accounting for 47.8%, 31.6%, and 16.6%, respectively, while hgcI_clade, CL500-29_marine_group and MWH-UniP1_aquatic_group were dominant genera, accounting for 43.8%, 10.3%, and 6.6%, respectively. RDA analysis showed that dissolved oxygen, nitrate, total nitrogen, total phosphorus, and water temperature were the key environmental factors driving bacterial community structure succession. The relative abundance of Proteobacteria in the experiment groups increased more significantly than that in the control group, which might be due to the incremental organic carbon stimulating the growth of several species in Proteobacteria such as Polynucleobacter and Limnohabitans. The ecological niche of Proteobacteria had been expanded by adding carbon sources, which promoted the proliferation of several bacteria groups that could efficiently use organic carbon, such as α-Proteobacteria. Moreover, Proteobacteria contained the majority of the bacteria with denitrification function, participating in the process of nitrogen removal, and also playing an important role in the degradation of organic matter. This might lead to significantly lower concentrations of TN, NH4, NO2, and NO3 in the experiment groups. In addition, the addition of carbon sources resulted in the increasing relative abundance of Limnohabitans, Sediminibacterium, Flavobacterium, Rhodobacter and Novosphingobium. The relative abundance of these bacteria was significantly negatively correlated with NO2 concentration, indicating that the formation of bioflocs in the experiment groups resulted in the decrement of NO2, and promoted the growth of these bacteria. The addition of carbon sources increased the relative abundance of functional genes related to Carbohydrate metabolism, Lipid metabolism, Cell motility, and Membrane transport, suggesting bioflocs enhanced the metabolic activity of the bacterial communities, especially in the aspect of the utilization of carbohydrates and lipids. Moreover, the relative abundance of functional genes related to Energy metabolism and Replication and repair in the experiment groups was significantly lower than that in the control group, suggesting that adding carbon sources reduced the energy consumption required by the bacterial community to maintain its basic growth and metabolic activity. Bacterial growth efficiency (BGE) increased correspondingly, implying that a larger amount of organic carbon absorbed by bacteria had been converted into bacterial biomass by bacterial production. This might be one of the important reasons why the bacterial abundance in the experiment groups was significantly higher than that in the control group. Our results suggested that adding carbon sources could significantly change the aquatic bacterial community structure, and enhance bacterial metabolic activity of degrading carbon and nitrogen compounds. Our study could provide theoretical reference and practical guidance for the low-carbon healthy aquaculture of Micropterus Salmoides, and lay a foundation for the further application of bioflocs technology in outdoor aquaculture production. |
