Characterizing the Microbial Response to Plastic and Bioplastic Debris in the Marine Environment

Plastic is the most abundant type of debris found in the marine environment, commonly representing between 60-95% of all marine debris. Plastic debris accumulation is greatest in urbanized coastal zones and closed bays or lagoons with limited flushing, and benthic environments are the final resting place for the majority of this debris, as biofouling leads to the sedimentation of floating plastics and many plastics have higher densities than seawater. However, the impact of plastic debris on benthic microbial communities is largely unknown. Similarly, the impact of bioplastics, which are promising alternatives for mitigating plastic pollution, is virtually unknown. The goal of this study was to characterize how benthic microbial communities respond to the deposition of both plastic (polyethylene terephthalate; PET) and bioplastic (polyhydroxyalkanoate; PHA) in coastal marine sediments. The microbial community colonizing ceramic served as a biofilm control while the free-living microbial community in the overlying water served as a non-biofilm control. Findings showed that biofilm communities (i.e. PET, PHA, and ceramic) were taxonomically distinct in comparison to free-living communities. Further, the PET and ceramic communities were indistinguishable. By contrast, bioplastic selected for a distinct microbial community that was enriched for depolymerases and dominated by sulfate-reducing microorganisms (SRM). Successional patterns demonstrated that the PHA- associated communities remained atypical and dominated by SRM throughout a 424-day microcosm. The isolation and whole-genome sequencing of individuals from PHA-associated biofilms led to the discovery of Bacillus strains capable of degrading PHA and reducing sulfate. The results presented here clearly demonstrate that plastic was not colonized by a unique microbial community whereas bioplastic was. Additionally, culture-independent and culture dependent experiments showed that the PHA-associated microbial community was capable of PHA degradation and sulfate reduction. Given that SRM mediate carbon mineralization in coastal sediments, bioplastic loading and the subsequent enrichment of SRM could unintendedly alter sediment biogeochemistry. Future scientific investigation and government legislation should consider the microbial response to plastic as well as bioplastic loading when developing legislation and best-management practices related to plastic pollution.