Abstract
The Yellow River is the most important water resource in northern China. In the recent past, heavy metal contamination has become severe due to industrial processes and other anthropogenic activities. In this study, riparian soil samples with varying levels of chromium (Cr) pollution severity were collected along the Gansu industrial reach of the Yellow River, including samples from uncontaminated sites (XC, XGU), slightly contaminated sites (LJX, XGD), and heavily contaminated sites (CG, XG). The Cr concentrations of these samples varied from 83.83 mg⋅kg-1 (XGU) to 506.58 mg⋅kg-1 (XG). The chromate [Cr (VI)] reducing ability in the soils collected in this study followed the sequence of the heavily contaminated > slightly contaminated > the un-contaminated. Common Cr remediation genes chrA and yieF were detected in the XG and CG samples. qRT-PCR results showed that the expression of chrA was up-regulated four and threefold in XG and CG samples, respectively, whereas the expression of yieF was up-regulated 66- and 7-fold in the same samples after 30 min treatment with Cr (VI). The copy numbers of chrA and yieF didn’t change after 35 days incubation with Cr (VI). The microbial communities in the Cr contaminated sampling sites were different from those in the uncontaminated samples. Especially, the relative abundances of Firmicutes and Bacteroidetes were higher while Actinobacteria was lower in the contaminated group than uncontaminated group. Further, potential indicator species, related to Cr such as Cr-remediation genera (Geobacter, PSB-M-3, Flavobacterium, and Methanosarcina); the Cr-sensitive genera (Skermanella, Iamia, Arthrobacter, and Candidatus Nitrososphaera) were also identified. These data revealed that Cr shifted microbial composition and function. Further, Cr (VI) reducing ability could be related with the expression of Cr remediation genes.
Highlights
River contaminants, especially heavy metals, because of their high toxicity, abundance, and persistent properties, can damage aquatic ecosystems (Varol, 2011; Liao et al, 2016)
Based on the Cr contamination states, the six sites were divided into three different categories: (i) uncontaminated regions, including Xincheng district (XC) and Xigu district (XGU), where the Cr concentration was less than the natural background (90 mg kg−1); (ii) slightly contaminated regions where the Cr concentration is greater than the natural background (90 mg kg−1), but less than the limited standard of Chinese Soil Quality Criterion (250 mg kg−1)
These included regions such as Xigu district (XGD) and the Liujiaxia reservoir (LJX); and (iii) heavily contaminated regions, including Xigu district (XG) and Chengguan district (CG), where the Cr concentration exceed the limited standard of Chinese Soil Quality Criterion (250 mg kg−1) (Table 1)
Summary
Especially heavy metals, because of their high toxicity, abundance, and persistent properties, can damage aquatic ecosystems (Varol, 2011; Liao et al, 2016). Studies about heavy metals toxicity, mobility, and bioavailability along Gansu industrial reach of the Yellow River are still scarce. Microorganisms exposed to strong selective pressures from heavy metal contaminated environments can process corresponding function for the ecosystem (Epelde et al, 2015). This has resulted in the evolution of heavy metal resistance mechanisms, including the structural changes of microbial communities (Fan et al, 2016), and transfer of heavy metal resistance genes to other community members by transposons or plasmids and expression level changes of such genes (He et al, 2011; Epelde et al, 2015).
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