Photooxidation promotes sinking of polyethylene microplastics during early-stage biofouling

Microplastics (MPs) are ubiquitous in the ocean and even low density polymers such as polyethylene (PE) are found below the surface and in seafloor sediments. Predicting when and how buoyant MPs lose buoyancy remains challenging, particularly because photooxidation and biofouling can co-occur at the ocean surface. Here, we tested a two-step mechanism: photooxidation followed by early stage biofouling and quantified its effect on PE MP buoyancy and sinking under controlled laboratory conditions. MPs were prepared from post-consumer PE grocery bags and photo-oxidized for 82 days in a solar simulator. Pristine and photooxidized MPs were incubated for 15 days in coastal seawater containing natural microbial assemblages, either unamended or amended with the diatom Chaetoceros tenuissimus or the coccolithophore Emiliania huxleyi . Biofilms formed within five days on both PE MPs across all live treatments, showing minor differences in crystal violet based biofilm intensity through day 15. Most notably, all pristine PE MPs ( n = 27) remained positively buoyant, while 92% of the photo-oxidized PE MPs sank to the bottom of a 31 cm water column within 5 days. Prior to biofouling, photo-oxidized PE MPs exhibited negative buoyancy in the top sinking column without reaching the bottom. Our results suggest photooxidation can precondition PE MPs such that early stage biofouling triggers buoyancy loss on timescales of days rather than the weeks to months often reported for pristine MPs. Our results highlight the need to consider photooxidation in biofouling studies to better predict the fate of MPs in the ocean. • A two step mechanism, photooxidation followed by early biofouling, rapidly induced polyethylene microplastics sinking. • Photooxidized polyethylene microplastics showed transient partial descent even before biofouling. • 92% of photooxidized polyethylene microplastics sank within 5 days across live seawater treatments. • Pristine polyethylene microplastics remained buoyant through day 15 despite biofilm formation.