Composition and metabolic potential of microbial communities from marine plastic biofilms

Abstract

The improper disposal of plastic, a material historically synonymous with development, has led to the widespread accumulation of plastic in the environment. Most of this plastic debris ends up in the ocean where it interacts with marine organisms, including microorganisms which colonize plastic surfaces forming biofilms. These diverse and densely populated microbial communities represent an ecosystem unique to the Anthropocene and known colloquially as the “plastisphere”. Despite the great abundance of floating plastic in the oceans, which may impact marine ecology and biogeochemistry, the metabolic functioning of the plastisphere remains largely unexplored. Most attempts to predict microbial function in the plastisphere have been based on inferences made based on the taxonomic composition of plastic biofilms and their relationship to cultivated species with known metabolisms. Since microorganisms with similar taxonomy can have dramatically different metabolic potential, these inferences are likely inaccurate In addition, the great diversity in the chemical composition of plastic, such as polymer type and additive composition combined with the large-range of natural carbohydrate biomolecules and anthropogenic hydrocarbon compounds in the plastisphere may select for widely different organisms. This thesis thus tested the hypotheses that marine plastic recruits microorganisms with metabolic potential that supports biofilm formation and carry out plastic degradation in the plastisphere, and that nature of plastic, location, and season all play a role in determining community compositions. These hypotheses were test by deploying plastic coupons in Hong Kong coastal waters to recruit seawater microorganisms and promote biofilm formation. Resulting biofilms were analyzed using high-throughput DNA sequencing methods including 16S rRNA gene amplicon-based community profiling and shotgun metagenomics. Overall, I found that many factors contribute to the changing microbial composition of plastisphere and polymer type is only one of many substrate-related factors involved. Broadly, I discovered the enriched biosynthesis of non-ribosomal peptide such as microcystin and iturin in plastic associated microorganisms, and comprehensively revealed the shared metabolic pathways and the peripheral role of plastic degradation in Oleiphilaceae. My analyses expand our knowledge of the marine plastisphere and improve estimates of the potential impact of plastic pollution on marine microbial ecology and biogeochemical cycles.