The molybdenum (Mo) non-point source pollution in the mining area has an irreversible impact on the surrounding water and soil ecosystems. Herein, three integrated vertical subsurface flow constructed wetlands (CWs) were constructed to assess the effects of combination substrates and plant on the removal of Mo(VI). Results showed that CW1 with combination substrates and cattail exhibited a favorable removal performance for Mo(VI) at 80.90%. Moreover, most Mo(VI) retained in the CWs was retained in the substrate (58.13–88.04%), and the largest fraction of Mo(VI) retained was the water-soluble fraction on the surface of the combination substrates. Mo(VI) removal was also influenced by the microbial community composition in substrate, especially their co-occurrence networks. The species that showed significant positive correlation with Mo(VI) removal were Planctomycetes, Latescibacteria, Armatimonadetes, and Gemmatimonadetes. Moreover, CWs added plants showed that more co-occurrences interaction between taxa occurs, which means that the wetlands efficiently select recruitment of potential microbial consortia and change the co-occurrences to remove pollution in the substrate. These results could be useful in providing an ecology-based solution for the treatment of Mo(VI) in wastewater, especially in adjusting the microbial communities for Mo(VI) removal at the genetic level.

Microcystin–LR (MC-LR) and phenanthrene (Phen), which commonly co-occur in eutrophic waters, have been extensively studied as individual contaminants, but their combined ecotoxicological effects on submerged macrophytes remain unclear. In this study, we examined the individual and combined toxicity of MC-LR (2, 10, 50, 250, and 1000 μg/L) and Phen (0.2, 1, 5, 25, and 100 μg/L) on the submerged macrophyte Vallisneria natans over a 7-day exposure. Key toxicity biomarkers, including growth, photosynthetic efficiency, and antioxidant responses (catalase, superoxide dismutase, glutathione S-transferase, and malondialdehyde), were evaluated. The results showed that high concentrations of each contaminant alone (MC-LR ≥ 1000 μg/L; Phen ≥ 100 μg/L) significantly inhibited growth and reduced photosynthetic efficiency. In contrast, synergistic toxicity was observed at much lower combined concentrations (≥50 + 5 μg/L), with effects substantially exceeding those of individual exposures. Co-exposure intensified antioxidant activity, but it was insufficient to mitigate oxidative damage. Notably, Phen at concentrations above 25 μg/L significantly enhanced the bioaccumulation of MC-LR in V. natans. These findings demonstrate that environmentally relevant mixtures of MC-LR and Phen induce remarkable toxicity even at concentrations where individual compounds show negligible effects. The results highlight that co-existing cyanotoxins and polycyclic aromatic hydrocarbons may present greater ecological risks than predicted from single-contaminant assessments, underscoring the need to update current ecological risk frameworks for the accurate evaluation of complex pollution scenarios in freshwater systems.

Brownification denotes increasing water color, partly caused by increasing dissolved organic matter of terrestrial origin in freshwater. Brownification has become a wide-spread environmental problem because water color alters the physicochemical environment and biological communities in aquatic ecosystems. However, our understanding of its ecological effects on aquatic macrophytes is limited. Here, an indoor mesocosm experiment with a common submerged macrophyte, Potamogeton crispus, along an increasing gradient of brownification was conducted over a period of 42 days. Results showed that P. crispus was able to overcome low degrees of brownification owing to the plasticity in morphological and physiological traits and P. crispus growth even benefitted from the concomitant nutrients along with brownification. However, the macrophyte growth was negatively affected by a 10-fold increase in water color beyond its current level. Additionally, collapse in antioxidant systems and potent photosynthesis inhibition implied that P. crispus could not adapt to the low-light stress generated under the high degree of brownification. Epiphytic bacteria are more sensitive to brownification than their hosts. Any degree of brownification initially caused a decrease in microbial diversity of epiphytic biofilm, whereafter the concomitant nutrients under brownification favored the growth of epiphytic microorganisms. The shading effect of a large number of epiphytic biofilms under brownification may further aggravate the low-light stress on macrophytes. Overall, the study provides new insights into the comprehensive effects and underlying mechanisms of brownification on macrophytes.