Metagenomics insights into the effect of microplastics and/or cadmium on carbon, nitrogen, and phosphorus cycles in freshwater sediments

Abstract

The accumulations of microplastics (MPs) and cadmium (Cd) are ubiquitous in sediments. However, the impact of MPs and Cd on element cycles in sediments, particularly on greenhouse gas (GHG) emissions, has not been systematically reported. Herein, a microcosm experiment was conducted to investigate the responses of sedimentary carbon, nitrogen and phosphorus transformation processes, GHG emissions and microbial communities to MPs, Cd and MPs + Cd. The results indicate 1 %w/w of polyethylene (PE) or polystyrene (PS) MPs stimulated N₂O, CO₂ and CH₄ emissions primarily through the regulation of genes associated with denitrification (nosZ and norBC), organic carbon degradation and methanogenesis (mtrA, mttB and pmoABC), respectively. Both 1 %w/w of PE MPs and PS MPs significantly inhibited the ALP activity and the expression of organic phosphate mineralizing genes (phoD, phoN and phnX), ultimately reducing the Bio-P content. Additionally, 8 mg kg⁻¹ of Cd inhibited the CO₂ and CH₄ emissions primarily through the regulation of genes associated organic carbon degradation and methanogenesis (mttB and pmoABC), but it stimulated N₂O emissions primarily through the down-regulation of gene nosZ and hence had no significant effect on global warming potential (GWP). Notably, the coexistence of MPs and Cd could impact microbial communities in MPs biofilms and promote the expression of gene nosZ, which may ultimately antagonize N₂O emissions. These findings provide a scientific basis for the ecological management of watersheds polluted by MPs and/or Cd, in particular, illustrate the need for a closer integration of microbial C-N-P interactions.