3.1 Common carp growth performance
Figure 2 shows that the body weights of common carp began to decrease significantly in group B (p < 0.05). There were no significant differences in group B, C, or D (p > 0.05), while the body weight of group E was significantly decreased (p < 0.05). Fat filling decreased sequentially from group B (p < 0.05), but there was no significant difference between groups C and D (p > 0.05).
3.2 Intestinal microbiota characteristics
3.2.1 Operational taxonomic unit (OTU) analysis
Figure 3 shows the sequences that were clustered with 97% similarity, resulting in 12147 OTUs. The maximum number of OTUs was in group D with 3493; the minimum number of OTUs was in group A with 1343. Fifty-seven OTUs were shared by groups B and C; 110 OTUs were shared by groups C and D, and 209 OTUs were shared by the five groups.
3.2.2 Intestinal microbiota diversity analysis
Figure 4A and Fig. 4B show that there was a significant difference in Chao1 and observed OTU indices between groups C and D and between groups A and E (p < 0.05), while there was no difference between groups B and C (p > 0 .05). Figure 4C shows that the Shannon index of group A was significantly lower than those of groups B, D, and E (p < 0.05), but the difference with group C was not significant (p > 0.05), while the difference between groups C and D was significant (p < 0.05). NMDS analysis (shown in Fig. 4D) indicated that all samples in group A clustered together, and the remaining four samples were not significantly separated (p < 0.05). The samples of group B were clustered together relative to the samples of group E.
3.2.3 Microbial species analysis
After the sequences were identified, there were 22 genera of bacteria with relative abundance greater than 1% at the genus level (Fig. 5A). The dominant taxa were Cetobacterium, ZOR0006, Romboutsia, Clostridium_sensu_stricto_1, and Pirellulaceae_unclassified. The dominant bacterial genus in group A was Cetobacterium; the dominant bacteria in group C was Clostridium_sensu_stricto_1, and the dominant bacteria in group D was ZOR0006. Figure 5B shows that the dominant bacteria between groups at the genus level included Cetobacterium, ZOR0006, Romboutsia, Clostridium_sensu_stricto_1, and Pirellulaceae_unclassified. The anti-stress ability of the intestinal microbiota phenotypic function in group D was significantly higher than in other groups (Fig. 5C), and the prediction of bacterial phenotypic functions showed that Proteobacteria was related to anti-stress function (Fig. 5D).
3.3 Metabolomics analysis
Figure 6A shows that group A overlapped with group B; group E intersected with the samples of group D; group B was related to group C, and group C was clustered in one group relative to group D. Figures 6B and 6C show that group B was adjusted upward by 60 and 118 differential metabolites relative to group A, primarily being enriched in glutathione metabolism, glycerophospholipid metabolism, ABC transporters, and purine metabolism, while group C was adjusted downward by 52 and 132 differential metabolites relative to group B. The main enrichment was in biosynthesis of unsaturated fatty acids, and the D group was adjusted downward by 119 and 123 differential metabolites relative to group C, being enriched in biosynthesis of unsaturated fatty acids, amino sugar, and nucleotide sugar metabolism. The E group was upregulated by 71 metabolites relative to group D, with 117 major enrichments in biosynthesis of unsaturated fatty acids being adjusted downward.
There were no significant differences in muscle amino acids or saturated fatty acids between the groups (Figs. 7A and 7C; p > 0.05). Figure 7B shows that the glutamic acid content in group A was significantly lower than in other groups (p < 0.05). The unsaturated fatty acids in group C were significantly lower than those in group A (Fig. 7D; p < 0.05), but there were no significant differences with group B, D, or E (p > 0.05).
The Hx content of group B relative to group D decreased sequentially in order (Fig. 8A); the inosine content in group B was decreased, and in group D was significantly lower than that in group A (p < 0.05) (Fig. 8B). The contents of IMP, GMP, and taurine in groups B, C, D, and E were significantly higher than those in group A (p < 0.05), and the content in group C was the highest (Figs. 8C, 8D and 8E). The contents of histamine in groups A, B, and C were unchanged, while in group D this was significantly lower than in group C (p < 0.05) (Fig. 8F). The results of purine metabolic enrichment analysis showed that AMP, IMP, GMP, and IDP were upregulated, and inosine, xanthosine, guanine, guanosine, Hx, urate, and allantoate were downregulated (Fig. 9).
3.4 Correlation analysis of 16S sequencing and metabolomics
A Spearman correlation analysis of 16S sequencing and metabolomics was performed to explore the relationship between the intestinal microbiota and host metabolism using the data of differential genus-level microbiota and differential secondary metabolites (Figs. 10A and 10B). Clostridium, Streptococcus, Pirellula, Cyanobium_PCC-6307, Methylocystis, Rhodopirellula, Deltaproteobacteria_unclassified, and Synechocystis_ BDHKU-20401 were positively correlated with IMP and GMP and negatively correlated with Hx and inosine.