New study reveals how a common antibiotic disrupts nitrogen cycling and boosts greenhouse gas emissions in estuaries

Antibiotics that enter rivers and coastal wetlands through wastewater, agriculture, and urban runoff may have far reaching impacts on the chemistry of these ecosystems, according to a new study that identifies the bacteria responsible for breaking down the antibiotic sulfamethoxazole and links this process to increased emissions of nitrous oxide, a potent greenhouse gas.

The research team used DNA stable isotope probing combined with nitrogen tracer techniques to uncover how sulfamethoxazole affects denitrification. Denitrification is a microbially mediated process that removes excess nitrate from estuarine sediments and naturally helps control coastal eutrophication. At the same time, it can generate nitrous oxide, a greenhouse gas nearly three hundred times more powerful than carbon dioxide in warming the climate.

The results show that sulfamethoxazole disrupts this balance in complex and time dependent ways. During the first two weeks of exposure, the antibiotic strongly suppressed denitrification activity and reduced the abundance of key nitrogen cycling genes such as nirS, nirK, and nosZ. As sulfamethoxazole gradually degraded in the sediment, inhibition weakened. At high concentrations near one thousand micrograms per liter, the antibiotic even stimulated denitrification at later stages of the experiment.

Despite partial recovery of denitrification, nitrous oxide emissions increased sharply across all concentrations of sulfamethoxazole. The study found that the antibiotic hindered the final step of the denitrification pathway, where nitrous oxide is converted to harmless nitrogen gas. Because the nitrous oxide reduction gene nosZ was inhibited more than the upstream nitrite reduction genes, nitrous oxide accumulated and escaped into the environment.

To understand which bacteria were responsible for breaking down the antibiotic, the researchers employed DNA stable isotope probing with a carbon thirteen labeled form of sulfamethoxazole. This powerful molecular method enabled direct identification of microorganisms that assimilate carbon from the antibiotic into their biomass. The heavy DNA fractions revealed active sulfamethoxazole degraders belonging to Proteobacteria, Spirochaetota, Desulfobacterota, and Firmicutes.

Importantly, several of the antibiotic degraders also play central roles in denitrification. Genera such as Pseudomonas, Bacillus, Robiginitalea, Phaeodactylibacter, and Bdellovibrio were found to incorporate labeled carbon from sulfamethoxazole, demonstrating that they directly participate in biodegradation. This discovery establishes a functional link between sulfamethoxazole degradation and denitrification, revealing that some denitrifiers can use the antibiotic as a carbon source when nitrate is available as an electron acceptor.

The study also uncovered enrichment of the antibiotic resistance gene sul1 in the carbon thirteen labeled DNA fractions. This suggests that bacteria capable of degrading sulfamethoxazole may simultaneously carry resistance genes, allowing them to survive and function under antibiotic stress. Resistance gene sul2 increased in the sediment over time but did not appear in the heavy DNA fractions, indicating that sul2 carriers resisted the antibiotic without actively degrading it.

Together, these findings illustrate a dual strategy by which estuarine microbes cope with antibiotic contamination. Some bacteria degrade the antibiotic directly while others rely on genetic resistance. Both mechanisms help maintain microbial function in environments increasingly exposed to pharmaceutical pollutants.

The study provides new insight into how human derived contaminants reshape biogeochemical processes in coastal zones. By altering the activity and composition of sediment microbial communities, sulfamethoxazole can impair natural nitrogen removal while promoting climatic warming through greater nitrous oxide release. The authors note that understanding these microbial interactions is essential for predicting the ecological consequences of rising antibiotic pollution in freshwater and marine environments.

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Journal reference: Li C, Tang X, Yin G, Han P, Chen C, et al. 2025. Linking denitrifiers with sulfamethoxazole biodegradation: insights from DNA-based stable isotope probing. Biocontaminant 1: e007  

https://www.maxapress.com/article/doi/10.48130/biocontam-0025-0006  

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About Biocontaminant:
Biocontaminant is a multidisciplinary platform dedicated to advancing fundamental and applied research on biological contaminants across diverse environments and systems. The journal serves as an innovative, efficient, and professional forum for global researchers to disseminate findings in this rapidly evolving field.

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