Date of Award

12-2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

First Advisor

Eric S. Gilbert

Second Advisor

George E. Pierce

Third Advisor

Sidney Crow

Fourth Advisor

Christopher Cornelison

Abstract

Plastic waste can serve as a feedstock for microbial bioconversion using a chemical/biological hybrid strategy. In this work, a bioconversion process for polypropylene (PP) to produce value-added oleochemicals was reported. This bioconversion process coupled thermal depolymerization with fermentation by the oleaginous yeast Yarrowia lipolytica. PP pellets were depolymerized by pyrolysis, generating oil that consisted of mainly branched chain fatty alcohols and alkenes. The oil was mixed with biodegradable surfactants and nutrients and mechanically homogenized. The resulting medium, termed OP4, was used for fermentation by Y. lipolytica strain 78-003. Y. lipolytica assimilated > 80% of the substrate over 312 h, including 86% of the fatty alcohols. Y. lipolytica produced up to 540 mg L-1 lipids, compared with 216 mg L-1 during growth in surfactant-based control medium. C 18 compounds, including oleic acid, linoleic acid, and stearic acid, were the predominant products, followed by C 16 compounds palmitic and palmitoleic acid. Subsequently, an improved process was developed resulting in a PP-derived growth medium called OP5 that did not contain the emulsifier oleic acid and derived 81 percent of its carbon from PP. To improve on product yields during growth in OP5 medium, pH, inoculum density, C/N ratio, and osmolarity were optimized. This increased the product yield four-fold to 2.1 g L-1, and led to 46 percent cellular lipid content, the highest content reported to date for plastic-to-lipid microbial bioconversion. The maximum lipid yield occurred under conditions that balanced cell growth versus lipogenesis. Using postconsumer PP, the lipid yield was significantly lower (0.74 g L-1), likely due to toxic additives necessary for product stabilization. Overall, the work demonstrates the potential and the challenges associated with microbial bioconversion of plastics.

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