Overview
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Key numbers
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- aquatic food web, as this potent neurotoxin is widely distributed in than 10% of the total MeHg load in receiving rivers4. Furthermore, the
- aquatic systems worldwide1. More than three billion people are MeHg/THg ratio (%MeHg) in rivers downstream of urban centers
- exposed to high levels of MeHg via fish consumption, with an esti- typically exceeds 10%4,9,10, which is significantly higher than the ratios
- Fig. 1 | Geographic distribution of collection sites for publicly available from untreated and treated sewage (n = 560), and samples from pristine
- ples from 19 urban rivers and the entire Han River basin (n = 490), samples the study.
- sewage is a key factor driving the enrichment of Hg-methylators in sequences in urban rivers had an average sequence identity of 90%
- urban downstream rivers. To test this hypothesis, we classified 1349 with those in sewage, significantly higher than the 56% identity with
- we identified nearly 500 unique hgcA sequences spanning 25 phyla sediment) metagenomes share up to 82% and 34% of the same hgcA
- hgcA sequences captured were sufficient to represent the entire hgcA to the river (Fig. 4c). Specifically, 91 ± 10% of the hgcA sequences in 19
- lysis is robust (Supplementary Fig. 3). These shared sequences accounted for 93 ± 11% of the total hgcA
- We were surprised to find that hgcA genes were present in 100% of abundance (Fig. 4e). Similarly, the abundance of hgcA sequences
- hgcA gene was 83% in the urban river dataset and 25% in the pristine ments—including groundwater, oceans, boreal lakes, and high Arctic
- aquatic dataset (Fig. 4a). In pristine samples, almost no crAssphage was lakes—shared on average less than 3% of the same hgcA sequences with
- environments (n = 300), urban rivers (n = 426), STP effluent (n = 129), and sewage each urban river (n = 19). f Total hgcA abundance in sewage (n = 430), STP effluent
- (n = 430). Bars indicate minimum to maximum values; the horizontal line repre- (n = 129), and urban rivers (n = 426). The black center line represents the median.
- sents the median, whiskers denote 1.5× interquartile range, and points indicate g Heatmap showing the normalized abundance of hgcA microorganisms at the
- phyla dominating, including Desulfobacterota (Deltaproteobacteria) the role of Hg-methylators in sewage, 10% sterilized sewage—filtered to
- (57%), Synergistota (14%), Firmicutes (12%) (Fig. 4g and Supplementary retain its chemical composition while removing microorganisms—was
- present in sewage samples26. Notably, metatranscriptomic analysis When sediment was mixed with river water, spiking 10% sterilized
- reducing bacterium with potent Hg methylation capability (Supple- observations17. However, spiking 10% raw sewage into these sediment-
- abundance accounting for an average of 11.2% of the total hgcA While raw sewage contains abundant hgcA microorganisms, sewage
- We collected 12 sewage samples in China and used enriched Hg stable efficiency of hgcA gene abundance reached 93.1 ± 4.3% across the six
- 10.5 ± 4.1%, which is 3-8 times higher than the rates observed in pristine urban rivers in areas with well-established sewage infrastructure
- lake sediments or the Everglades30,31. This finding suggests that raw (Fig. 2b). Additionally, previous studies have shown that over 90% Hg
- samples without raw sewage being spiked, no MeHg production was increasing from 1.3% in 1980 to 90% in 2014 (Fig. 7b). This development
- Me202Hg production rate (%)
- Me202Hg production rate (%)
- Fig. 5 | Mercury methylation potential of sewage and its impact on rivers. water samples with corresponding local sewage spiked at various ratios (0%, 5%,
- a Sewage and river sampling sites. b Correlation between the Me202Hg production 10% raw sewage, and 10% sterilized sewage). RW represents River Water, RS
- samples. R and p values are from Pearson correlation analyses. c MeHg production means ± standard in deviations from triplicate bacterial cultures (n = 3).
- Fig. 6 | Removal efficiency of hgcA in sewage treatment plants (STPs). Changes the range from minimum to maximum values, the horizontal line indicates the
- in hgcA and crAssphage abundances in the influent and effluent of six STPs in China median, the whiskers denote 1.5 times the interquartile range, and the black points
- (Hong Kong: n = 8, 7), Germany (Göttingen: n = 8, 8), USA (Virginia: n = 9, 9) and represent outliers.
- Sweden (Henriksdal: n = 4, 2; Uppsala: n = 8, 2; Kappala: n = 4, 2). The bars represent
- in Vidy Bay, where sewage overflow is a concern, high MeHg produc- Currently, about 44% of global sewage is discharged untreated
- or on sediment surfaces is more likely to enter the aquatic food web polluting about 65% of river stretches worldwide, including those
- indicator/6.3.1). The sample sizes (n values) for each urban water are detailed in samples from 1980 to 2014. Dots and error bars represent means ± standard
- Supplementary Data 11. The bars represent the range from minimum to maximum deviations of total Hg in fish as observed in various literature. This dataset was
- values, the horizontal line indicates the median, the whiskers denote 1.5 times the primarily curated by Zhang et al.71. More detailed information and references are
- interquartile range, and the black points represent outliers. The red circles indicate provided in Supplementary Data 11. Temporal changes in sewage treatment ratios
- subject metadata. In total, 490 metagenomes were collected from 19 of hgcA across metagenomes of different sizes. The count tables were
- 80% of the ambient THg in the sewage samples) and allowed to pre-
Methods (brief)
- ducting experiments with water samples across China. We find that sewage
- contamination dance was observed in the Han River samples (Fig. 3c). To further
- nificantly impacts their distribution in urban rivers worldwide. In par- of crAssphage and hgcA in the Han River samples. The results revealed
- environments18,19. We collected publicly available metagenomes from lysis clearly demonstrates that sewage contamination is a crucial driver
- Fig. 1 | Geographic distribution of collection sites for publicly available from untreated and treated sewage (n = 560), and samples from pristine
- metagenomes. The collected metagenomes are divided into three groups: sam- aquatic environments (n = 300). Each number indicates a river included in
- ples from 19 urban rivers and the entire Han River basin (n = 490), samples the study.
- urban rivers. Both hgcA and crAssphage abundances were normalized per gigabase despite varying sample sizes among rivers. The red inverted triangle indicates rivers
- namely raw sewage (430 samples), STP effluents (129 samples), urban to the similarity of hgcA sequences, the contigs encoding hgcA in urban
- rivers (490 samples, including 426 water and 64 sediment samples), rivers and sewage also showed high consistency (Supplemen-
- 300 samples) (Supplementary Data 1, 3, 4, 5). Across all metagenomes, Further quantitative analysis revealed that urban river (water and
- the sewage and 90% of the STP effluent metagenomic samples, a shared between sediments and sewage accounted for 60 ± 27% of the
- aquatic dataset (Fig. 4a). In pristine samples, almost no crAssphage was lakes—shared on average less than 3% of the same hgcA sequences with
- detected, confirming that these samples were not contaminated by global sewage (Supplementary Data 8). Overall, these results suggest
- from sewage into river systems. samples was 1–2 orders of magnitude higher than that in the STP
- To better understand the sources of hgcA sequences in urban effluent and urban river samples (Fig. 4f). Given that bacterial biomass
- United States, Switzerland, and Saudi Arabia (Supplementary Fig. 8). 430 sewage samples were assigned to 18 phyla, with a few bacterial
- groundwater, ocean, high Arctic and boreal lake), urban rivers, STP effluent and samples and hgcA microorganisms.
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