Date of Award

Fall 2026

Abstract

Microplastics (MPs) are ever-present pollutants throughout marine environments, yet important questions remain about how they move vertically through the water column and interact with marine microorganisms. This dissertation demonstrates that low-density polyethylene (PE) MPs, the most prevalent form of MPs in surface ocean, are not merely passive debris, but dynamic, biologically active particles that systematically alter biogeochemical processing from the ocean surface to the seafloor. At the ocean surface, photooxidation of inherently buoyant PE MPs serves as a trigger for vertical transport. Solar simulator experiments showed that 82 days of photooxidation pre-conditioned PE MPs, causing 92% of particles to lose buoyancy and sink within five days of early-stage biofouling by microbes, a process operating on timescales of days rather than the weeks to months typical for pristine plastics. During transit through the water column, sinking MPs establish compositionally and functionally distinct plastisphere communities. A 23-day flow-through seawater experiment revealed that MP biofilms differed significantly in microbial community structure (explaining 44% of variation) and extracellular enzyme profiles compared to natural suspended matter and marine snow. While natural aggregates were dominated by leucine aminopeptidase activity, MP biofilms exhibited elevated lipase activity, demonstrating that suspended plastics act as differentiated, active sites of substrate processing within coastal waters. Upon reaching the seafloor, biofouled MPs persist as distinct hydrolytic microsites. Sediment reactor incubations demonstrated that deposited MPs retained attached microbial cells and unique enzymatic signatures (initially dominated by peptide hydrolysis). While MP exposure did not alter overall sediment microbial community composition or total hydrolytic activity, it reorganized local organic matter processing by shifting sediment enzyme profiles, elevating alkaline phosphatase activity. Together, these findings establish a continuous vertical framework showing how photochemical and microbial processes transform buoyant MPs into sinkable, enzyme-rich microsites that actively reorganize carbon and nutrient dynamics from the surface ocean to coastal benthic sediments.

Document Type

Dissertation

First Advisor

Kai Ziervogel

Second Advisor

Aron Stubbins

Third Advisor

Ashley Bulseco

Department or Program

Oceanography

Degree Name

Doctor of Philosophy

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