As global water resources come under increasing pressure, more efficient and sustainable ways of treating wastewater are needed. Constructed wetlands (CWs) offer a nature-based solution (NBS) that benefits biodiversity. These engineered ecosystems use vegetation, soil and microbes to remove pollutants from wastewater, while also providing habitats, absorbing carbon from the atmosphere, and boosting resilience to flooding. NBS are key to addressing a number of key EU policy priorities, and are supported by the European Green Deal, EU Biodiversity Strategy for 2030 and EU Adaptation Strategy. Several projects funded by Horizon Europe also promote research into NBS.
CWs remove pollutants including nitrogen, phosphorus, heavy metals and medications through a variety of physical, chemical, and biological mechanisms. One key biological process is microbial remediation, in which microorganisms (e.g. bacteria) break down or transform pollutants into less harmful substances. Research has explored the best ways to optimise this process, but the synergistic ways in which plants and microbes interact to improve remediation remains poorly understood, say the researchers behind a new review.
Their study aims to fill this gap by presenting a review of microbial remediation mechanisms in CWs, based on findings from 539 relevant studies published globally between January 2000 and May 2025. They look at key genes, enzymes, and pathways of transformation, in particular focusing on the role of plant-microbe interactions in removing conventional and emerging pollutants, especially antibiotics and polycyclic aromatic hydrocarbons (PAH). They also consider ‘bioaugmentation’ strategies, where specific microbial strains with favourable metabolic traits are added to a CW to enhance pollutant breakdown.
The review highlights that CWs have proven effective at removing pollutants from wastewater across a variety of wastewater types (aquaculture, rural, industrial, sewage), with average removal efficiencies ranging from 76.9 to 91.9% for pollutants such as nitrogen, phosphorus, heavy metals, antibiotics and PAH . Microbial remediation plays a crucial role in CWs, accounting for 90.5% of antibiotic removal and 60.9% of PAH removal.
The presence of plants in CWs increases average pollutant removal efficiency by up to 11.3%. However, plants’ direct uptake of pollutants is relatively low; their impact lies in how they interact with relevant microbes and enhance their activity, say the researchers. This happens because plant roots release oxygen, enhancing microbial action, but also because roots release chemicals (metabolites) that fuel microbes with energy and nutrients, promoting their growth and reproduction.
The latter process is driven by plant stress: plants emit useful metabolites as a signal or ‘cry for help’ in order to recruit beneficial microorganisms. For example, microbes can fix nitrogen from the atmosphere to feed the plant, produce antifungal compounds to ward off plant pathogens, or form protective biofilms on roots. However, while well-documented in soil ecology, research on this mechanism is limited in CWs, say the researchers.
Bioaugmentation (directly adding useful microbes into a CW), meanwhile, has been found to enhance removal of conventional pollutants by up to 16.6% and removal of emerging pollutants by up to 64.3%. The researchers note, however, that these results predominantly arise from short-term laboratory-based experiments, highlighting the need for more research to validate findings at larger scales and in the field.
The study also finds that pollutant removal rates may be enhanced by up to 14.8% when both plants and plant-associated bacteria (isolated from roots, stems, fruits, etc.) are synergistically introduced to a CW. This could be because these bacterial strains adapt better to CWs than strains isolated from soil or sludge.
Stabilising microbes and confining them to a localised area, meanwhile, can boost bioremediation rates by up to 21.2% more than just using free bacteria. ‘Immobilising’ bacteria – for instance by attaching them to a carrier material – can protect them from being washed away or damaged by environmental conditions such as extreme pH and temperature fluctuations. Immobilising bacteria is not straightforward, note the researchers, but new techniques such as 3D-printing could help.
Another challenge for the bioaugmentation approach is that microbes often fail to colonise effectively, with their success hindered by competition between species, leading to inconsistent experimental results. Finding optimal microbial ‘consortia’, where different bacteria work together to remediate pollution, should be given more research attention, the study suggests.
The researchers acknowledge that a risk of using antibiotic-degrading bacteria is that they could contribute to the spread of antibiotic resistance genes (ARG) – a major threat to global health. Synthetic biology tools such as gene editing that targets ARG could mitigate these risks, they say. This technique has been successfully applied in diverse bacterial species such as Escherichia coli, Staphylococcus aureus, and Klebsiella pneumoniae, demonstrating its potential for application in CWs. Researchers have also developed ‘intelligent’ synthetic microorganisms programmed to self-destruct after completing degradation of pollutants, preventing those microbes from proliferating. Implementing these tools in CWs or natural ecosystems requires further evaluation to ensure safety, stability, and scalability, however.
The researchers conclude that there is significant opportunity to scale up laboratory findings to pilot- and field-scale CW applications for sustainable wastewater treatment. However, to advance the microbial remediation potential of CWs for wastewater treatment, more research is needed on the molecular mechanisms driving degradation of pollutants. The risks of introducing beneficial functional bacteria into CWs should be carefully evaluated, and innovative technologies should be applied to overcome the challenges of bioaugmentation.
Reference:
Zhao, M., Jiang, S., Wu, D., Xue, H., Zhang, W., Wu, X., Yu, Q, He, Y. and Geng, J. (2026) Microbial remediation as a key pathway for pollutant removal in constructed wetlands: Mechanisms, bioaugmentation strategies, and perspectives. Bioresource Technology 441, 133570. https://doi.org/10.1016/j.biortech.2025.133570
