Chemical and microbiological changes on the surface of microplastic after long term exposition to different concentrations of ammonium in the environment

The increasing production of plastic in the world has resulted in the widespread pollution of the environment with microplastics (MP). MP enter facilities such as wastewater treatment plants or landfills characterized by various ammonium concentrations. The aim of this study was to determine the structure of the microbial community on MP surfaces at various concentrations of ammonium nitrogen, and in particular, to identify microorganisms capable of polyethylene terephthalate (PET) degradation. Moreover, changes in the chemical characteristics of the MP surface resulting from microbial activity were also investigated, and the potential of MP to serve as a vector for pollutants was determined. The tests were carried out in a reactor filled with PET for a period of 260 days. The experiment was carried out in 3 phases: in I and III phase, the concentration of N-NH4 was about 70 mg/L, while in II phase, it was about 430 mg/L. On the MP surface, biofilm-forming microorganisms from the genera Rhodococcus, Pseudomonas and Xantomonas were identified at the lower ammonium concentration. At this concentration, MP-degraders belonging to genera Acidovorax, Gordonia, Pseudomonas, Sphingomonas, and Sphingopyxis were identified in the biofilm. At the higher N-NH4 concentration, the biomass was enriched with bacteria from genera Nitrosospira, Nitrosomonas and Terrimonas, and the number of microorganisms with the potential to degrade MP decreased. Analysis of the MP surface during the experiment has showed the loss of carbonyl groups and formation of carboxyl and hydroxyl groups, which indicated the degradation of MP. Independent of the ammonium concentration in the environment, MP was a carrier of pathogenic microorganisms from the genera Mycobacterium, Enterobacter and Brevundimonas.