Enhanced Nitrification in Yellow Clay Improves In Situ Water Purification for Eel Aquaculture: A Preliminary Assessment.

To address the issues of high water exchange rates and significant negative environmental impacts associated with eel aquaculture, this study explored the use of yellow clay as a carrier for nitrifying bacterial communities. By pre-enhancing the nitrification capacity of the yellow clay, we aimed to improve the control of inorganic nitrogen in the aquaculture water. Three experimental groups were established: NF-YCA (nitrifying-functional yellow clay-added eel aquaculture system); NN-YCA (non-nitrifying yellow clay-added eel aquaculture system); and YC-F (yellow clay-free eel aquaculture system, blank control). The NF-YCA group had zero water exchange, while the YC-F and NN-YCA groups underwent water exchange equivalent to 28.36 times the system volume. Nitrification was most pronounced in the NF-YCA group, where both mean and peak concentrations of total ammonia nitrogen and nitrite nitrogen were lower than in the YC-F and NN-YCA groups, whereas nitrate nitrogen concentrations in the NF-YCA group were significantly higher than in the other two groups. No significant differences were observed in the survival rate and specific growth rate of elvers among the three systems during the experiment. High-throughput sequencing results revealed that Pseudomonadota and Bacteroidota were the most dominant phyla across all systems. However, the bacterial community structure in NF-YCA was more abundant and stable, and nitrification-related genera, such as