Wastewater Chemical Stress Pushes River Microbes to Emit More Greenhouse Gases

Rivers that receive treated wastewater have long been known as hotspots for greenhouse gases such as carbon dioxide, methane and nitrous oxide. What has remained murky is exactly why. A new study published in Nature Water offers a striking answer: the very chemicals that slip through treatment plants appear to rewire the metabolism of river microbes in a way that favours gas production. The research, led by Rui-Feng Yan and Ai-Jie Wang of the Harbin Institute of Technology together with colleagues at the Chinese Academy of Sciences, tracked a wastewater treatment plant and its receiving river from end to end, combining an unusually broad chemical survey with toxicity testing, gas measurements, multi-omics analysis and controlled laboratory experiments. The result is one of the most complete mechanistic pictures to date of how pollutant mixtures translate into climate-relevant emissions in urban waterways.

The team sampled along a continuum running from the treatment plant itself into the river that receives its effluent. They measured 159 emerging contaminants, a category that includes pharmaceuticals, personal care products, endocrine disruptors, per- and polyfluoroalkyl substances, ultraviolet filters, organophosphate esters and phthalate esters. These compounds are called emerging not because they are new to the environment but because their ecological consequences are only now coming into focus. Even though the treatment plant removed 82.21 percent of the contaminants it received, the effluent still raised downstream concentrations by 53.21 percent compared with upstream water. Cytotoxicity, measured with a luminescent bacterial bioassay, climbed by 44.15 percent, and the dissolved greenhouse gas burden, expressed as carbon dioxide equivalents, increased by 11.31 percent downstream of the discharge point.

Those field numbers alone would be noteworthy, but the real surprise lay in the microbial data. Using metagenomic and metatranscriptomic profiling across the sampling sites, the researchers found that genes governing microbial defence functions, including efflux pumps that expel toxic compounds, biofilm formation that shields cells from stress, and cytochrome P450 detoxification enzymes that chemically neutralize xenobiotics, had declined by 52.34 to 57.98 percent downstream. At the same time, genes linked to greenhouse gas production through carbon and nitrogen transformations were between 1.94 and 33.67 times more abundant than at upstream sites. The pattern exceeded what simple dilution or mixing of effluent with river water could explain, pointing to an active biological reorganization rather than a passive change in community composition.

To test whether the contaminants themselves were driving this shift, the team built semi-continuous microcosms in the laboratory, comparing microbial communities exposed to EC-rich effluent, upstream river water, and effluent from which the contaminants had been depleted. The comparison proved decisive. Only the microcosms receiving contaminant-rich effluent showed the characteristic signature observed in the river: suppression of defence functions alongside activation of respiration, fermentation, and carbon and nitrogen transformation pathways. When the contaminants were stripped out, the metabolic shift largely disappeared. This controlled evidence strengthens the causal chain from chemical exposure to metabolic rewiring to greenhouse gas production, a chain that field observations alone could only suggest.

The biochemical details reveal a story of stress and adaptation that will feel familiar to anyone who has studied cellular stress responses. Enzyme and metabolite analyses showed that contaminant exposure induced oxidative stress and a transient depletion of cellular energy. In the early phase of exposure, the microbes’ energy currency, adenosine triphosphate, dropped as cells diverted resources away from growth and maintenance. But the communities then recovered: acetyl-CoA content rebounded, citrate synthase activity, a key gatekeeper of the tricarboxylic acid cycle, climbed back, and ATP availability was restored. The researchers interpret this recovery as evidence of a fundamental shift in microbial strategy, from investing in defence against chemical attack to prioritizing energy maintenance and core metabolism.

This defence–energy trade-off is the conceptual heart of the paper. Microbes facing toxic stress face a budgeting problem: the ATP and enzymatic machinery spent on efflux pumps, biofilm matrices and detoxification enzymes cannot simultaneously power other functions. When long-term exposure makes sustained defence too costly, communities appear to abandon that investment and fall back on energy-generating metabolism, including respiration and fermentation, which happen to release carbon dioxide, and nitrogen transformation pathways, which can release nitrous oxide. The genes for these gas-producing pathways became markedly more abundant downstream, and the microcosm experiments showed the same activation under controlled conditions. In effect, chemical stress pushes microbial communities into a metabolic mode that is intrinsically gassier.

The findings arrive at a moment of growing concern about inland waters as emission sources. Previous work has established that urban rivers are hotspots of carbon dioxide, methane and nitrous oxide fluxes, and that global riverine methane emissions are substantial. Earlier studies had also hinted that individual pollutants, such as the fungicide chlorothalonil or the antibiotic ciprofloxacin, can alter denitrification and nitrous oxide production in sediments and soils. What distinguishes the new study is its treatment of contaminants as mixtures rather than single compounds, its use of cytotoxicity as an integrating measure of mixture effects, and its combination of field continuum sampling with experiments that isolate cause from correlation. The risk prioritization analysis embedded in the work also identified key compounds that contribute disproportionately to both water quality health risks and greenhouse effect risks.

The implications for wastewater management are uncomfortable but clear. Conventional treatment plants are designed to remove bulk organic matter, nutrients and pathogens, and many do so reasonably well, as the 82 percent contaminant removal in this study shows. Yet the residual mixture that passes through is biologically potent enough to reshape downstream microbial ecology and measurably increase the climate footprint of the receiving water. Advanced treatment options, including ozonation and activated carbon filtration, have been evaluated at European scale for micropollutant removal, and the new results suggest that their benefits may extend beyond ecotoxicity reduction to climate mitigation. If contaminant stress is what tips microbial communities toward gas production, then removing that stress could keep the defence functions intact and the emissions lower.

There are also broader ecological questions raised by the trade-off framework. Microbial ecologists have long recognized that stress responses divert resources from growth and ecosystem functions, and trait-based frameworks in soil science have explored similar logic for carbon cycling. Extending that framework to riverine greenhouse gas production links two research communities that have largely worked in parallel: those studying pollutant effects on microbial communities and those quantifying inland water emissions. The multi-omics approach used here, spanning genes, transcripts, enzymes and metabolites, offers a template for testing whether similar trade-offs operate in other stressed environments, from agricultural soils exposed to pesticides to sediments contaminated with microplastics, which have also been reported to amplify greenhouse gas emissions from freshwater systems.

For now, the study stands as a warning that the climate cost of wastewater is not fully captured by what leaves the pipe. The effluent that meets regulatory targets can still carry a chemical load sufficient to reprogramme the microbial metabolism of an entire river reach, suppressing the communities’ defensive capabilities and channelling their energy budgets into pathways that emit greenhouse gases. As monitoring programs worldwide begin to grapple with hundreds of unregulated contaminants, the message from this river continuum is that the atmosphere may be keeping score even when the water quality ledger looks clean. Understanding and managing the defence–energy trade-off in receiving waters may therefore become an essential piece of both pollution control and climate policy in the decades ahead.

Subject of Research: How emerging contaminant mixtures in wastewater effluent drive greenhouse gas production in receiving rivers through a microbial defence–energy trade-off.

Article Title: Emerging contaminant stress promotes greenhouse gas production through microbial defence–energy trade-off in receiving rivers

Article References: Yan, R.-F., Han, J.-L., Han, Y.-N., Liang, B., Gao, S.-H., Sun, Y.-L., & Wang, A.-J. (2026). Emerging contaminant stress promotes greenhouse gas production through microbial defence–energy trade-off in receiving rivers. Nature Water. https://doi.org/10.1038/s44221-026-00704-y

Image Credits: AI Generated

DOI: 10.1038/s44221-026-00704-y

Keywords: emerging contaminants, wastewater treatment plant effluent, greenhouse gases, river microbiome, microbial metabolism, defence–energy trade-off, multi-omics, cytotoxicity, carbon and nitrogen cycling, oxidative stress, Nature Water, urban rivers