Adaptive evolution of Candida maltosa improves the bioconversion of depolymerized plastic feedstock by targeting biosurfactant production

Thermal oxo-degradation (TOD) of plastic can transform plastic waste into fermentable feedstocks. Bioconversion of the TOD products could be used as feedstocks in a biorefinery concept and lead to new avenues for plastic waste upcycling. Previous work demonstrated this concept with high density polyethylene (HDPE), the most abundant type of plastic, and identified the nonconventional yeast Candida maltosa as a promising candidate for this application. Here we describe the evolution of an improved strain of C. maltosa and characterize the uptake mechanisms of TOD products from HDPE (TOD_HDPE). Batch cultures in series passaged at the mid-exponential growth phase applied a selective pressure for faster growth and resulted in a >100% increase in specific growth rate when using TOD_HDPE as a carbon source. The evolved strain was compared to the parent strain to identify the cellular and biochemical changes associated with the improved phenotype and the uptake mechanisms involved in the bioconversion of TOD_HDPE. This comparison found that C. maltosa secretes biosurfactants capable of solubilizing hydrocarbons. The adaptive evolution resulted in changes in biosurfactant production that translated to improved emulsification of alkanes and increased solubilization of fatty alcohols and alkanes. In addition to the changes in metabolites, the study identified increases in membrane permeability associated with a reduction in ergosterol that may also play a role in the improved phenotype. These findings support the development of C. maltosa and other potential microbial cell factories for plastic biorefineries and may inform future design strategies.