Different effects of bio/non-degradable microplastics on sewage sludge compost performance: Focusing on antibiotic resistance genes, virulence factors and key metabolic functions.

Microplastics (MP) have aroused increasing concern due to the negative environmental impact. However, the impact of bio/non-biodegradable MPs on the sludge composting process has not been thoroughly investigated. This study examined antibiotic resistance genes (ARGs), virulence factors (VFs), and microbial community functions in sludge compost with the application of polylactic acid (PLA) and polypropylene (PP), using metagenomic sequencing. The findings indicated that both types of MPs could extend the thermophilic phase, enhance microbial activity, and inhibit the formation of humic acids. Compared to CK, the subtypes of ARGs decreased 4.22 % and 13.11 % in PLA and PP groups, respectively. But new ARGs emerged, particularly in the PLA group. The proportions of ARGs related to efflux and VFs associated with the adhesion system increased 1.62 %-2.27 % and 55.56 %-60.00 %, respectively, in MPs-added composts. The relative abundance of potential bacterial hosts (e.g., Psychrobacter) carrying multiple ARGs and VFs was much higher in PLA-added compost than in the other two. Moreover, PP facilitated denitrification process and PLA enhanced dissimilatory nitrate reduction to ammonium. Both types of MPs inhibited assimilatory nitrate reduction to ammonia but promoted inorganic nitrogen assimilation. This study broadens our understanding of the potential environmental risks posed by biodegradable and non-biodegradable microplastics on sludge compost and offers valuable insights for the management and application of compost products.