Partitioning of Hydrophobic Organic Contaminants and Microbial Communities on Microplastics

Microplastic contamination of aquatic environments has only recently caught the attention of scientists, regulators and the public. Microplastics are typically more recalcitrant than naturally occurring polymers and so have the potential to cause a range of issues, including increased exposure of marine life to chemical contaminants sorbed to or leached from microplastics, negative impacts due to ingestion of microplastics by biota, and the potential to carry and transport pathogenic and invasive species long distances. Bio-based, bio-degradable polymers have begun to gain market share as an alternative to traditional petrochemical-based plastics, but not much is known about their impacts in marine environments. The overall objective of this thesis was to improve our understanding of how bio-based microplastics compare to petrochemical-based plastics in the marine environment. This information could be used to evaluate the overall sustainability of bio-based polymers as replacements for petrochemical-based polymers. The first chapter of this work investigated the potential of four types of microplastics, polyethylene (PE), polyvinyl chloride (PVC) and two bio-based polymers, poly-3-hydroxybutyrate (PHB) and polylactic acid (PLA), to sorb hydrophobic organic contaminants (pyrene, PCB-153, and BDE-47) from the surrounding water column. It also examined how co-exposure to several of these contaminants influenced their sorption. The bio-based polymers used in this work, exhibited lower affinity for the organic contaminants investigated compared to the more widely used, petrochemical-derived microplastics. This may be due to several factors including hydrophobicity of the plastic surfaces and the chemical structure of each plastic. Further, competition between several co-exposed contaminants led to an overall decrease in chemical partitioning on polyethylene microplastics. The second chapter reported on the microbial composition of biofilm communities that form on bio-based (PHB and PLA) and petrochemical-based (PE and PVC) microplastics in comparison to a naturally occurring polymer, chitin. Microbial compositions of biofilms that formed on the different plastics were similar during the first and second week of growth, but chitin exhibited a distinct community from the microplastics. By the fourth week of growth, all substrates had a similar community composition. Diversity was generally higher on bio-based plastics. Genera harboring marine pathogens and hydrocarbon-degrading bacteria were identified on all substrates. This work has implications to policy surrounding marine debris issues, exploring the more nuanced differences between bio-based polymers and petrochemical polymers, introducing concerns over additive use in bio-based polymers, and reinforcing the need for “eco-cyclable” materials in single-use items