The widespread distribution of nanoplastics and dissolved organic matter (DOM) in sewage raises concerns about the potential impact of DOM on the bioavailability of nanoplastics. In this study, the effects of different sizes (100 nm and 350 nm) of polystyrene nanoplastics (PS-NPs, 50 mg/L) and combined with 10 mg/L or 50 mg/L DOMs (fulvic acid, humic acid and sodium alginate) on the growth and denitrification ability of Thiobacillus denitrificans were investigated. Results showed that 100 nm PS-NPs (50 mg/L) cause a longer delay in the nitrate reduction (3 days) of T. denitrificans than 350 nm PS-NPs (2 days). Furthermore, the presence of DOM exacerbated the adverse effect of 100 nm PS-NPs on denitrification, resulting in a delay of 1–4 days to complete denitrification. Fulvic acid (50 mg/L) and humic acid (50 mg/L) had the most significant adverse effect on increasing 100 nm PS-NPs (50 mg/L), causing a reduction of 20 mmol/L nitrate by T. denitrificans in nearly 7 days. It is noteworthy that the presence of DOM did not modify the adverse effect of 350 nm PS-NPs on denitrification. Further analysis of toxicity mechanism of PS-NPs revealed that they could induce reactive oxygen species (ROS) and suppressed denitrification gene expression. The results suggested that DOM may assist in the cellular internalization of PS-NPs by inhibiting PS-NPs aggregation, leading to the increased ROS levels and accelerated T. denitrificans death. This study highlights the potential risk of nanoplastics to autotrophic denitrifying bacteria in the presence of DOM and provides new insights for the treatment of nitrogen-containing wastewater by T. denitrificans .
Excessive accumulation of nitrate in natural ecosystems can lead to eutrophication of water bodies, resulting in environmental impacts such as algal blooms and red tides that pose serious threats to environmental and human health. Autotrophic denitrification is a resource-efficient biological nitrogen removal technology, but the slow growth and low electron transfer efficiency of autotrophic denitrifiers limit their practical application. In this study, the heterotrophic denitrifying bacterium Pseudomonas sp. JM-7 ( P. JM-7) was used in mixed culture with the autotrophic denitrifying Thiobacillus denitrificans ATCC 25259 to form a heterotrophic-autotrophic synergistic denitrification process (mixPT) to enhance electron transfer and improve the denitrification efficiency. Our results for an initial nitrate concentration of 430 mg/L showed that P. JM-7 reduced approximately 210 mg/L nitrate (48.8 %) in 90 h, with 1 g/L yeast extract powder (YEP) as the electron donor, while Thiobacillus denitrificans barely removed nitrate. In contrast, the mixPT system reduced approximately 360 mg/L nitrate (83.3 %), nearly 1.7 times greater than P. JM-7 alone. The mixPT culture also exhibited excellent denitrification in low carbon waters (10 mg/L), and removed 40 mg/L nitrate completely within 120 h. Cytochrome c produced by Pseudomonas sp. JM-7, which is an excellent electron transfer mediator for denitrification by Thiobacillus denitrificans , played a crucial role in the heterotrophic-autotrophic syntrophic denitrification. The adsorption effect of Pseudomonas sp. JM-7 shortened the contact distance between the autotrophic denitrifying bacteria and cytochrome c, which made it easy for cytochrome c to transfer the electrons to the autotrophic bacteria directly, thereby reducing the loss of electrons in the process of transfer, and improved the electron utilization efficiency. These findings have demonstrated the potential applications of mixPT system in controlling eutrophic waters and propose cytochrome c of Pseudomonas sp. JM-7 could serve as a novel and high-quality electron transfer mediator. Furthermore, this study have overcome the restriction of low denitrification efficiency exhibited by heterotrophic denitrifying bacteria Pseudomonas sp. JM-7 in low-carbon environments and expanded the utilisation of Pseudomonas sp. JM-7 in low carbon environments.
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