Effect of Carbon Sources Addition on Water Quality and Bacterial Community Structure and Function in Micropterus salmoides Aquaculture Ponds

Abstract

Micropterus salmoides is an important freshwater species in China. Developing zero-exchange aquaculture ponds for M. salmoides is of considerable significance. Recently, carbon source technology was introduced into aquaculture as an emerging environment-friendly production method. Adding carbon sources to aquaculture water can promote the formation of bioflocs, which creates economic and environmental benefits by reducing effluent discharges and artificial feed supply and improving bio-security. In this study, bioflocs were applied to aquaculture ponds of M. salmoides, and the effects of adding carbon sources on the water quality, bacterial community structure, and function were evaluated to provide a theoretical basis for the healthy and efficient green aquaculture of M. salmoides. Specifically, two experimental groups were established by adding special and slow-release carbon sources 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 that in the control group. Bacterial abundance (BA) and bioflocs volume (BFV) in the experiment groups were approximately 5 and 2 times higher than those in the control group, respectively. This result indicated that adding special and slow-release carbon sources to the water of M. 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 that in the slow-release carbon source group. In contrast, TN, NH4,+ and NO2– in the special carbon source group were significantly lower than that in the slow-release carbon source group. This indicated that the addition of the special carbon source had a more positive effect on the formation of bioflocs, and its impact on improving the water quality of M. 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 and proteins, and micromolecular matter such as amino acids and monosaccharides, which could be rapidly utilized for bacterial production and bioflocs formation. Regarding bacterial community structure, Actinobacteria, Proteobacteria, and Bacteroidetes were the dominant phyla of M. salmoides ponds, accounting for 47.8%, 31.6%, and 16.6%, respectively, whereas 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. Adding carbon sources expanded the ecological niche of Proteobacteria, promoting the proliferation of several bacteria groups that could efficiently use organic carbon, such as α-Proteobacteria. Moreover, Proteobacteria comprise most of the bacteria with denitrification functions, contributing to nitrogen removal processes and playing an important role in the degradation of organic matter. This might have resulted in significantly lower TN, NH4+, NO2–, and NO3– concentrations in the experiment groups. Additionally, the addition of carbon sources resulted in an increased relative abundance of Limnohabitans, Sediminibacterium, Flavobacterium, Rhodobacter, and Novosphingobium. The relative abundance of these bacteria was significantly and negatively correlated with NO2– concentration, indicating that the formation of bioflocs in the experiment groups decreased 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, particularly in 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 was converted into bacterial biomass by bacterial production. This likely explains 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 by degrading carbon and nitrogen compounds. Our study provides theoretical reference and practical guidance for the low-carbon healthy aquaculture of M. salmoides. It establishes a basis for the further application of bioflocs technology in outdoor aquaculture production.

Keywords

• carbon sources addition
• micropterus salmoides
• pond aquaculture
• water quality
• bacterial community