Biotic and abiotic factors affecting microplastic degradation and deposition

Although microplastics are ubiquitous in marine systems, our current knowledge on how biofilms that form on them affect their degradation and removal from the surface ocean is limited. In the ocean, it is expected that plastics denser than seawater will sink (i.e., polyvinyl chloride or polyethylene terephthalate), but in the water column their fate can be unpredictable for polymers like polystyrene (PS), whose density range (960-1040 kg/m³) overlaps with that of surface seawater (1020-1029 kg/m³). While plastic biodegradation by bacteria growing on microplastics is well known, biofilms can also slow plastic degradation by shielding them from UV radiation, or by enhancing microplastic sedimentation rates, but this has rarely been studied. We incubated microplastic particles (<5 mm) of the common household consumer products polyethylene terephthalate (PETE, #1), high-density polyethylene (HDPE, #2), polyvinyl chloride (PVC, #3), low-density polyethylene (LDPE, #4), polypropylene (PP, #5), and polystyrene (PS, #6) in coastal waters of the Pacific (San Diego, CA) and the Caribbean (Bocas del Toro, Panama) to determine how biofilm formation affects their degradation and sinking rates. Due to higher water temperatures and lower salinity, ambient water in the Caribbean was less dense than that in the coastal Pacific. We found that the formation of biofilms slowed degradation processes, as indicated by scanning electron microscopy (SEM) observations and a reduced loss of phthalates, an indicator of plastic degradation. Additionally, biofilm formation enhanced sinking velocities of PETE, PVC, and PS at both sites, and PS, in fact, required a biofilm to sink at the Pacific site. Our results show that biofilm formation impacts the degradation and transport behavior of microplastics in the ocean.