Influence of Environmental Factors During Ice-Melting Period

The sea cucumber (Apostichopus japonicus) has high economic value and is one of the important mariculture species in northern China. With the continuous development of aquaculture industry, closed or semiclosed pond aquaculture has become the main sea cucumber culture method. The ice-melting period is a special period when the ice on the pond surface melts due to the temperature rise in early spring in northern China. During this period, the ice-melting layer gradually changes the water environment of pond aquaculture from a closed state to external exchange state, resulting in the formation of thermocline and halocline areas and dissolved oxygen stratification in pond aquaculture waters. Some investigations have shown that the thermocline of pond water caused by ice-melting will cause the deterioration of water quality at the bottom of the pond and the outbreak of pathogenic bacteria, which will endanger the health of cultured organisms. Microorganisms are an important part of the pond aquaculture ecosystem. They play an important role in the ecosystem´s material circulation and energy flow and are significant to maintaining its balance. As an important part of the pond aquaculture ecosystem, sediment bacteria not only play a great role in the material circulation and energy flow processes but correlate with the growth, digestion, immunity, and diseases of aquaculture organisms. However, external physical and chemical factors easily affect the pond aquaculture ecosystem´s bacterial structure. Seasonal changes and nutrient input can modulate the species and abundance of bacteria in the pond aquaculture environment, and indirectly affect the growth and health of aquaculture organisms. In recent years, investigations have been carried out on the bacterial community structure in ponds water and sediment. Nonetheless, only few reports comprise the structure and function of the bacterial community during the ice-melting period. Therefore, it is of great theoretical and practical significance to study the structural and functional characteristics of the sea cucumber sediment bacterial community in the pond aquaculture during the ice-melting period. Therefore, this study used the typical shore-based semi-open sea cucumber pond culture in northern China as its research object. A 16S rRNA sequencing library was constructed based on the sediment bacterial community during the icebound, melting, and ablation periods and analyzed using high-throughput sequencing technology. These sequences were used to evaluate the structural characteristics of these bacterial communities and identify the dominant environmental factors affecting them. The results showed that the abundance and diversity of sediment bacterial communities showed an overall downward trend in the ice melting period, fluctuating significantly in the early stage of ice-melting (P < 0.05). Compared with the icebound period, the appearance of thermocline and halocline caused drastic changes in the environmental factors of the bottom water layer. The temperature increase provides a suitable habitat for more microorganisms, accelerating enzymatic reactions and promoting microbial metabolism, which results in the up-regulation of bacteria abundance and diversity in the early melting period. As an important environmental factor in mariculture, salinity can interfere with the metabolism of microorganisms in the water and affect its diversity in sediments. This investigation showed that the bacterial community abundance and diversity in high-salinity conditions were significantly higher than those in low salinity. The substantial changes in the environmental factors destroyed the original sediment bacterial structure during the icebound period, resulting in microbial abundance and diversity fluctuation during the ice-melting period. With the gradual disappearance of the thermocline and salt layers, the pond aquaculture´s environmental factors and bacterial structure tended to be stable. In the ice-melting period, there were significant differences in the structure of sediment bacteria. The differential bacterial communities in the icebound, melting, and ablation periods were Firmicutes, Acidobacteria, and Tenericutes, respectively. Although the relative abundance ratio of microorganisms in different stages is distinct, the first dominant bacteria belong to Proteobacteria, and the relative abundance is higher than 49.04%. The secondary dominant bacteria included Bacteroides in the icebound period, Chloroflexi and Actinobacteria in the melting period, and Planctomycetes in the ablation period. The environmental factors and bacterial community structure showed significant correlation in the sea cucumber culture pond during the ice-melting period (P < 0.05). Temperature, salinity, total nitrogen, and total organic carbon were the dominant environmental factors affecting sediment bacteria. This study will provide a theoretical basis for effectively managing sea cucumber pond aquaculture.