3.1 The distribution of antibiotics in samples
Figure 2. showed the total amount of 8 antibiotics in the soil and water samples and their distribution in the different sampling sites using dot-line plots. Among all analyzed antibiotics in the soil samples, TC was the most abundant antibiotic with a total value of 561.62 µg/kg, and the mean value was 43.20 µg/kg, similar to Lyu's finding that tetracycline was the dominant antibiotic observed in soil (Lyu et al., 2020). The second highest abundant antibiotic was OTC with a total value of 148.60 µg/kg, and a mean value of 11.43 µg/kg. This result may be related to the excessive use of tetracycline. TC have been widely used in agricultural planting and animal husbandry because of low cost and good effect, and there are farmlands around the sampling point area. Our inspection data was consistent with the conclusion that tetracycline is one of the most commonly used antibiotics (Wang et al., 2017). As shown in Fig. 2. (cd), except for AMOX and CFX, all antibiotics were detected in soil samples. The highest content of TC in soil-5, the lowest in soil-0, and the content was 68.61 µg/kg and 21.65 µg/kg, respectively. Although OTC was the second highest content of antibiotics in soil, the concentration varied widely between individual samples. The contents of SD, SMX, SMZ tended to be close to zero.
As Fig. 2. (bef) shown, all 8 antibiotics were detected in water samples. OTC was the highest antibiotic, with a total of 0.26 µg/L and an average of 0.0221 µg/L, followed by AMOX with a total of 0.18 µg/L and a mean value of 0.014 µg/L. It may be that due to the fluidity of water, most antibiotics cannot accumulate in large quantities, but quinolone and tetracycline antibiotics can strongly adsorb suspended particles and deposits (Qiao et al., 2018), thus the abundance of TC, OTC was higher than other antibiotics in water samples. This result was found at the same level as rivers in Hong Kong (Deng et al., 2016), and other major rivers in China (Li et al., 2018).
3.2 The distribution of heavy metals in samples
As shown in Fig. 3., the contents of different heavy metals varied greatly. Mn was the most abundant heavy metal in the soil, with a total content of 18,307.7 µg/kg, and the highest content in soil7 with a value of 1,854.03 µg/kg. Zn was the second-highest heavy metal in the soil samples, with a total content of 2,675.79 µg/kg, and the highest content in soil0 at 325.79 µg/kg. because Mn is mainly present in soil solution, mines and smelters are the main sources of Mn pollution, so we speculate that the high Mn content in the soil samples may be related to the abundance of manganese tantalite in Shiyan. In our water samples, Zn content was the highest with a total of 137.95 µg/kg, followed by Mo content with a total of 121.69 µg/kg. wat3 had the highest Mn content, surpassing the content of all types of heavy metals in other samples with 37.42 µg/L. Our test results were lower than those for heavy metals in soils of other areas in China (Liang et al., 2023; Wu et al., 2015).
3.3 The distribution and concentration of microorganisms
After the blastp homology comparison of the gene set protein sequence with the Nr database, the number of microorganisms at the phylum level was 104, with the highest content was Proteobacteria, followed by Actinobacteria and Bacteroidetes. The number of microorganisms at the species level was 11127, with the highest level was Actinomycetia Bacterium, followed by Betaproteobacteria Bacterium and Burkholderiaceae Bacterium.
As Fig. 4. (a b) shown, the distance among samples was small, except for soil-5. Among all samples, the content of Proteobacteria, Actinobacteria, Bacteroidetes, and Verrucomicrobia accounted for more than 80% of the total content. The sum of the abundance of Proteobacteria and Actinobacteria accounted for more than half of the abundance of all samples. Similarly, this was similar to the results of the farmland microbial communities around the gold tailings, that Proteobacteria and Actinobacteria make up a large portion of the microbial community (Qiao et al., 2021). Compared to other samples, soil-5 had a higher content of Actinobacteria.
3.4 The distribution of ARGs in samples
A total of 1327 genes, 173 unique ARGs and MGEs (2 transposase genes, 1 transporter and 4 transcriptional regulatory protein) were detected from 2 soil and 7 water samples. Figure 5. (a) summarized the gene numbers of ARGs for each sampling site. We selected 10 genes as target genes for analysis, bacA, macB, sul1, sul2, tetA, rosB, arnA, acrB, mexF, and vanR, which accounted for 74.05%, 51.29%, 69.98%, 70.49%, 70.96%, 67.86%, 71.69%, 72.86% and 76.84% of the total ARG in soil-0, 5, and water-0, 2, 3, 4, 7, 8, and 10, respectively.
Figure 4. (cd) showed the absolute and relative abundance of ARGs. Except for soil-5, the distribution of ARGs in all samples did not differ significantly. Obviously, the content of vanR gene in soil-5 was much higher than other ARGs. The vanS/vanR two-component system is activated to express the van gene in response to extracellular glycopeptide antibiotics (Hong et al., 2008). So we analyzed the main distribution and concentration of van genes in soil-5 (Fig. 5.g). The number of vanS gene also high,and the reason that the number of van genes at this site was much higher than at other samples needed to be explored.
3.5 The distribution of MRGs in samples
Based on Bactmet database, a total of 1085 genes, 79 types of MRGs were detected. Figure 4. (b) showed the gene counts of all MRGs, with the highest number of MRGs in water-0 and the lowest in water-3. We selected czcC, arsT, actP, arsC, pstB, copR, modB, sodB, chrA, and acn as target genes, which accounted for 62.37%, 33.45%, 46.67%, 50.88%, 55.98%, 61.12%, 65.05%, 63.22% and 57.30% of the total MRGs in soil-0, 5, and water-0, 2, 3, 4, 7, 8, and 10, respectively. As shown by the absolute abundance of MRGs (Fig. 5. ef), the MRG distribution of soil-5 differed from the other sampling sites, with only five target genes, and the highest concentration was actP (encoding a resistance mechanism to Cu). Salam et al. detected resistance genes expressing resistance to Cu, Zn, As, Cd and other metals in chronically contaminated soils using macrogenome sequencing (Salam, 2020), that was similar to our sequencing results.
3.6 The correlations between antibiotics, heavy metals, ARGs and MRGs
A correlation heat map was conducted to explore the relationship among the relative abundance of antibiotics, heavy metals, ARGs, and MRGs based on significant correlation analysis by Spearman correlation. The correlation heatmap consisted of 47 elements (8 antibiotics, 19 heavy metals, 10 ARGs, and 10 MRGs) (Fig. 6.). The correlations between antibiotics were all positive, such as SD and SMZ (p < 0.05), while the correlations between antibiotics and heavy metals were mostly positive, except for Mo and Sb which were negatively correlated with antibiotics, such as Mo and SMZ (p < 0.05) and Sb and SD (p < 0.05).
The correlation between metals was different from the correlation between antibiotics, which could be positive or negative, for example, Cu was positively correlated with Sc (p < 0.05), Cd was positively correlated with Mn (p < 0.05), Sb was negatively correlated with Cr and Zn (p < 0.05), Bi was negatively correlated with Mo (p < 0.05).
Figure 6. also showed that the correlations between antibiotics and ARGs were not significant, and the only significant correlations were between bacA and OTC, sul2 and CFX, with the significant negative correlation (p < 0.05). Our results differed from the conclusion that ARGs abundance was positively correlated with some antibiotic residues (Zarei-Baygi et al., 2019). This result also reinforced the idea that the presence of antibiotic selection pressures is not necessarily necessary for the selection of antibiotic resistance genes (Davison, 1999), and antibiotics are not the only influential factor in the production and spread of ARGs. We found that antibiotics were significantly negatively correlated with MRGs.
Interestingly, Fig. 6. showed that the correlation between heavy metals and ARGs was higher than that between antibiotics and ARGs, which was similar to the findings of others who found that the presence of heavy metals in the environment enhanced the widespread presence of ARGs (McKinney et al., 2010; Yang et al., 2018). In our data analysis, MRGs were negatively correlated with heavy metals, except for acrB was positively correlated with As (p < 0.05). It maybe due to the low concentration of heavy metals. Berg J et al. found that extensive enrichment of Cu in agricultural soils could enhance not only the tolerance of indigenous bacteria to Cu but also their antibiotic resistance (Berg et al., 2005). Alonso et al. also found that the exposure specificity of Cu in soil could not only select for strains resistant to Cu but also synergistically select for antibiotic resistant strains (Alonso et al., 2001). All of these evidences pointed to the importance of heavy metals in ARGs production and transmission, so it may be possible to reduce ARGs pollution by controlling the content of heavy metals. The analysis of our data suggested that heavy metals may be more selective for ARGs. This result agreed with Mazhar et al that may be due to the long-term bioavailability of heavy metals, whereas antibiotics were not stable in the environment due to transformation and degradation (Mazhar et al., 2021; Wang et al., 2021), so antibiotics had a weaker selective effect on microorganisms than heavy metals (Sun et al., 2021).
The correlation between ARGs was positive, between MRGs also be positive, and between ARGs and MRGs was almostly all positive, except for arsC with macB (p < 0.05). This result showed that there was an antagonism between ARGs and MRGs. And this finding was consistent with Yuan et al.'s finding that the recent ARG profiles of different species of MRGs had a bias, indicating a significant association between ARGs and MRGs (Yuan et al., 2019). Some studies had found that microorganisms exposed to environmental factors such as antibiotics and heavy metals for a long time can obtain more resistance genes during evolutionary adaptation, and because ARGs and MRGs are chained on movable elements (such as plasmids, transposons and integrated binding elements, etc.) (Baker-Austin et al., 2006; Seiler and Berendonk, 2012)resistance genes could be transmitted through HGT.
3.7 The correlation of microbes with ARGs and MRGs
As Fig. 7. (ab) shown, the effects of antibiotics, heavy metals and microbes were not very significant, but the effects between resistance genes and microbes were obvious. The correlation of microbes with ARGs and MRGs could be positive or negative. Proteobacteria was positively correlated with ARGs and MRGs (p < 0.05). Proteobacteria is the dominant bacterium in heavy metal contamination due to its good resistance to heavy metals (Qiao et al., 2021), the "take-back mechanism" for ARG transfer could lead to the transfer and spread of resistance genes (Jiang et al., 2017), and the synergistic effect of proteobacteria as part of this process with MRGs could put selection pressure on resistance genes, increase the co-existence of ARGs and MRGs in the same bacteria, and accelerate the spread and dissemination of resistance genes, which could allow the long-term presence of these genes in the environment (Pal et al., 2015). Actinobacteria was negatively correlated with ARGs and MRGs, such as copR (p < 0.05). The vast majority of antibiotics used for treatment come from soil microbial communities, such as Streptomyces, which can produce many clinically important antibiotics, and other ARGs will obtain ARGs from actinomycete resistance genes to protect themselves, and as microorganisms continue to evolve, new antibiotic resistance mechanisms may also arise (Ji et al., 2012; Jiang et al., 2017). Therefore, we should be vigilant and strengthen research on the production and spread of resistance genes.