Biofilm formation on microplastics in wastewater: insights into factors, diversity and inactivation strategies

Abstract

Microplastic (MP) occurrence in wastewater poses serious threats to aquatic organisms due to their ecological impact. Additionally, these microplastics may provide a unique habitat for microbial biofilms. This study explored the possible factors that facilitate biofilm formation on different MPs in a wastewater environment. Biofilm formation was determined via measurement of optical density (OD) and confirmed using scanning electron microscopy. Furthermore, microbial community profiling was performed via 16S rRNA amplicon sequencing. The highest biofilm formation occurred after 3 weeks of exposure, followed by a decline to its lowest after 5 weeks. Biofilms were predominant on MPs that were exposed to dark conditions, mesophilic temperature (25 °C) and aerobic conditions. Under these conditions, a positive correlation was observed between the OD value and ammonia (NH3) (r = 0.824) and nitrite (NO2) concentrations (r = 0.1). However, a negative correlation (r = −0.673) was found between the OD value and nitrate (NO3) levels in the medium. Furthermore, the highest biofilm formation was observed on polyethylene particles. The most dominant phyla in both the biofilms and wastewater medium were Proteobacteria, Bacteroidetes and Planctomycetes, with Methylotenera being the most abundant genera in the biofilms. In general, the polypropylene particles supported the most diverse bacterial community (H' = 2.51138 and Simpson index = 11.096), while high-density polyethylene supported the least diverse bacterial community (H' = 0.88779 and Simpson index = 1.5324). The study also demonstrated that both UV and chlorine treatments were effective in inactivating these biofilms, within 30 and 10 min, respectively.