Polylactic acid microplastics facilitate nitrogen removal in freshwater sediments by modulating carbon-nitrogen...

Area of Science:

• Environmental Science
• Microbiology
• Biogeochemistry

Background:

• Biodegradable microplastics (BMPs) are known to enhance denitrification in freshwater sediments by releasing dissolved organic matter (DOM).
• The precise mechanisms by which BMPs influence denitrification processes remained largely unclear.
• Understanding these mechanisms is crucial for assessing the environmental impact of microplastics on nitrogen cycling.

Purpose of the Study:

• To evaluate the effects of polylactic acid microplastics (PLA-MPs) on nitrogen transformations in freshwater sediments.
• To elucidate the underlying mechanisms by which PLA-MPs enhance denitrification.
• To investigate the role of PLA-MPs in facilitating nitrogen removal and biogeochemical cycling.

Main Methods:

• A 45-day incubation experiment was conducted to assess the impact of varying PLA-MP concentrations (0.05% and 0.5% w/w) on sediment nitrogen transformations.
• A subsequent 10-day anaerobic incubation with 0.5% PLA-MPs and different nitrate concentrations was performed to identify specific microbial and genetic responses.
• Analysis included measurements of total nitrogen removal, microbial community structure (denitrifying bacteria), and functional gene abundances (nitrate/nitric oxide reductase, mcrA).

Main Results:

• Significant enhancement of denitrification and nitrogen removal was observed with 0.05% and 0.5% PLA-MPs.
• The presence of PLA-MPs led to increased abundances of denitrifying bacteria and genes encoding nitrate/nitric oxide reductase.
• An increase in the ANME-2d mcrA gene, indicative of nitrate-dependent anaerobic methane oxidation, was also noted.
• PLA-MPs degradation produced low-molecular-weight labile DOM and methane, serving as electron donors for nitrate reduction, supported by upregulated esterase and lactate dehydrogenase.

Conclusions:

• Polylactic acid microplastics (PLA-MPs) significantly enhance nitrogen removal in freshwater sediments by promoting denitrification.
• PLA-MPs degradation fuels multiple heterotrophic denitrification pathways, including nitrate-dependent anaerobic methane oxidation, by releasing labile DOM and methane.
• These findings highlight the crucial role of BMPs in nitrogen biogeochemical cycling and provide insights into microplastic-driven environmental processes.