Fecal metabolic variations of Apostichopus japonicus induced by dietary sea mud components in the recirculating aquaculture environment

Sea mud is an important component of diet in Apostichopus japonicus recirculating aquaculture system (RAS), which has a crucial effect on the water environment and metabolic performance of A. japonicus. In this study, we investigated the potential toxic mechanism of sea mud on A. japonicus and interaction mechanism between fecal microorganisms and metabolites in RAS. Thus, this study set up five experimental groups to obtain the optimal amount of sea mud addition to diet. The proportion of sea mud added to diet was 50% (FB1), 67% (FB2), 75% (FB3), 80% (FB4), and 83% (FB5), respectively. The results showed that there was a significant difference in fecal metabolite expression in the comparison group of FB4_vs_FB1 and FB5_vs_FB1 (P ≤ 0.05). A total of 73 potential biomarkers, such as uridine and inosine, were identified by ROC curve analysis that could indicate the toxic effects of sea mud. Topological analysis showed that glycerophospholipid metabolism pathway was upregulated in the comparison group of FB5_vs_FB1 (P ≤ 0.05). However, five pathways including the pyrimidine metabolism pathway, arginine and proline metabolism, purine metabolism, aminoacyl tRNA biosynthesis, and carbapenem biosynthesis pathway were significantly downregulated in group FB5_vs_FB1 (P ≤ 0.05). Glycerophospholipids and pyrimidine pathway in group FB5 was disrupted, leading to immune dysfunction and stress tolerance, which might be the important cause of intestinal inflammation in A. japonicus. Furthermore, a significant correlation between fecal metabolites and microorganisms was found. In brief, our study demonstrated the optimal sea mud addition to diet should be less than 80%, which could provide a basis for the environment protection and optimization of A. japonicus RAS.