Background

The gut microbiota in fish has been proven to have significant effects on the health [Full size image

OPLS-DA was used to analyse the degree of variability in intergroup samples. High predictability (Q2) and strong goodness of fit (R2X, R2Y) were reflected between every two groups (Additional file 9: Table S5), which demonstrated that the models were stable and could be used to identify significantly differential metabolites further. In the OPLS-DA score plots (Fig. 3B–D), every two groups were clearly separated, indicating distinct differences in metabolic profiling between groups. Permutation tests indicated that the models were not overfitted (Additional file 9: Table S5).

Significantly differential metabolites were screened, revealing 710 metabolites in NC compared with PY, 372 metabolites in PY compared with XHK, and 766 metabolites in XHK compared with NC. A total of 93 common significantly differential metabolites were screened in all intergroup comparisons, which included 32 lipids and lipid-like molecules, 19 organic acids and derivatives, and 7 organic oxygen compounds (Fig. 3E).

MetPA was conducted on all these metabolites, which enriched 27 pathways, of which 11 were significant (hypergeometric test, p < 0.05) (Additional file 10: Table S6). These pathways included 4 related to amino acid metabolism, 3 to carbohydrate metabolism, 1 pathway to translation, 1 to nucleotide metabolism, 1 to lipid metabolism, and 1 to metabolism of cofactors and vitamins. The significantly altered pathways included “aminoacyl-tRNA biosynthesis", "arginine biosynthesis", "glyoxylate and dicarboxylate metabolism", "glycine, serine, and threonine metabolism", "pyrimidine metabolism" (Fig. 3F).

The relationship between gut microbiota and metabolites

To explore potential associations between significant gut microbes and metabolites under environmental influence, we utilised Spearman rank correlation. At the phylum level, 26 associations between 17 metabolites and 5 microbes were found (Spearman rank correlation, r > 0.8, p < 0.05) (Fig. 4A). At the genus level, 30 associations were obtained for 6 microbes and 20 metabolites (Spearman rank correlation, r > 0.8, p < 0.05) (Fig. 4B). The details of the correlation can be seen in Additional file 11: Tables S7 and Additional file 12: Table S8.

Fig. 4
figure 4

The correlation of microbes and metabolites. A The correlation at the phylum level. B The correlation at the genus level. The lines in the circles represent the correlation between metabolites and microbes, with greener colours representing more positive correlations and yellower colours representing more negative correlations

The correlation between environmental factors and gut microbiota

Table 1 shows the environmental factors, including water temperature (WT), dissolved oxygen (DO), nitrate nitrogen (NO3-N), ammonium nitrogen (NH4-N), pH, and transparency, for the three groups in four seasons. PY had the highest water temperature on average, significantly higher than the other two groups. NC had the highest levels of DO, NO3-N, NH4-N, pH, and transparency, with NH4-N content significantly higher than that in XHK, while other factors were significantly higher than the other groups (Kruskal-Wallis test, FDR < 0.05). 

Table 1 Environmental factor parameters for the three groups in four seasons

Table 2 presents the contribution of environmental factors to the variation in gut microbiota obtained by VPA. All the variation partitioning fractions were significant in the permutation test (p < 0.05). Among the environmental factors, pH explained the most variation in gut microbiota (43.41%), followed by NH4-N (43.14%), DO (38.34%), transparency (34.41%), NO3-N (28.28%), and WT (19.40%).

Table 2 The relative contributions of environmental factors to variation in gut microbiota

To further explore the correlation between environmental factors and significant microbes at the phylum and genus levels, redundancy analysis (RDA) was conducted, and the results are shown in Fig. 5. All six canonical axes explain 23.31% of the total variability. The first two axes, which contributed 71.20% and 11.45% of the explained variance (p < 0.01), respectively, were used for further analysis.

Fig. 5
figure 5

The correlation among samples, environmental factors, and significant microbes. Blue and yellow arrows indicate vectors of significant microbes at the phylum and genus levels, respectively, while red arrows represent vectors of environmental factors. A positive correlation is indicated when the angle between vectors is less than 90°, while a negative correlation is indicated when it is greater than 90°. The vectors are perpendicular to each other to indicate irrelevance

The results showed that Fusobacteria and Macellibacteroides were positively correlated with all environmental factors except transparency. Conversely, Spirochaetae, Acidobacteria, Gemmatimonadetes, Brevinema, Gemmatimonas, and Nocardioides exhibited an opposite trend. Firmicutes, Cyanobacteria, Clostridium sensu stricto 1, Blvii28 wastewater sludge group, and Mycobacterium positively correlated with NH4-N, pH, DO, and NO3-N, while exhibiting negative correlations with WT and transparency. On the contrary, Proteobacteria and Aeromonas showed the opposite trend.