Contrasting effects of phytoplankton aging on microplastic antibiotic adsorption depending on species tolerance, and biofouling level

•More crystalline MP showed higher toxicity to M. aeruginosa and C. vulgaris.•Greater tolerance of C. vulgaris to MPs results in more MP biofouling compared to M. aeruginosa.•Functional groups mediating the adsorption of antibiotics on MPs were undermined after phytoplankton aging.•Microalgal biofouling highly restored antibiotic adsorption potential of MPs through hydrophobic interactions. Recent studies have reported conflicting results on the effects of biofouling on the adsorption behavior of microplastics (MPs). However, the underlying mechanisms driving the adsorption of MPs undergoing biofouling in aquatic environments remain unclear. This study examined the interactions between polyamide (PA), polyvinyl chloride (PVC) and polyethylene (PE) with two phytoplankton, namely cyanobacteria Microcystis aeruginosa and microalgae Chlorella vulgaris. Results indicated that MP effects on phytoplankton were dose- and crystalline-type dependent, with M. aeruginosa being more sensitive to MP exposure than C. vulgaris in the inhibitory order PA > PE > PVC. Analysis of antibiotic adsorption of the MPs showed significant contributions from CH/π interactions on PE and PVC and hydrogen bonding on PA, which decreased with phytoplankton biofouling and aging. Meanwhile, higher levels of extracellular polymeric substances on microalgae-aged MPs compared to cyanobacteria-aged MPs were conducive to adsorption of antibiotics, mainly through hydrophobic interactions. Overall, promotional and anti-promotional adsorption of antibiotics on MPs was induced by biofouling and aging of microalgae and cyanobacteria, respectively. This study provides deep insights into the specific mechanisms by which biofouling affects MP adsorption in aquatic environments, thus advancing our understanding of this critical environmental issue. [Display omitted]