Minimal contribution of bioerosion in the deterioration of biodegradable plastics in the marine environment: evidence from laboratory and field experiments

Biodegradable, bio-based plastics are widely used alternatives to conventional plastics; however, understanding their degradation under natural marine conditions remains incomplete. Previous work has highlighted the potential of bioerosion by marine invertebrates in the fragmentation of plastic films. This study investigated the role of bioerosion by marine invertebrates in the deterioration of three biodegradable (one PLA/PBAT (‘Bio1’) and two Mater-bi formulations (‘Bio2’, ‘Bio3’)) and one conventional polyethylene (PE) film, using complementary field and laboratory experiments. In a temperate coastal experiment, PE and biodegradable films were deployed for 9 weeks (−1.5m depth), with and without invertebrate exclusion chambers. Biodegradable films lost ≥96% mass regardless of exclusion treatment, while there was no change in PE-film mass. While acknowledging material-specific differences in in-situ degradation profiles, statistical analysis identified exposure duration as the dominant driver (η2partial = 0.946); while exclusion treatment had minimal effect (η2partial <0.0001). Biodegradable films had micrometre-scale pitting, but no evidence of invertebrate bite marks. Laboratory experiments exposed Bio1, PE and natural food (kelp) discs to amphipods (Marinogammarus marinus; detritivore ‘shredders’) and periwinkles (Littorina littorea; omnivorous grazer) for 21-days. Plastic discs showed evidence of bioerosion by amphipods (Bio1-film 60 ± 16%; PE-film: 20 ± 6% of disc segments), but not by periwinkles. However, Bio1 and PE microplastics were detected in tissue and faeces of both invertebrates, with amphipods containing 3.5-fold, and 31-fold more microplastics, respectively, than periwinkles. Collectively, findings demonstrate that while marine invertebrates can bioerode test materials and generate microplastics, their contribution to degradation under natural conditions is minor.