Abstract: Millions of tons of plastics entering the sea each year are a substantial environmental problem. It is expected that ocean plastic pollution will increase when considering the rapidly rising rates in global plastic production, in contrast to the relatively slow growth in plastic recycling rates, and future projections of increasing population densities in coastal areas. However, a significant discrepancy exists between the vast quantities of plastic entering the ocean and the orders of magnitude lower amounts afloat at the sea surface, indicating a substantial sink for ocean plastics. Plastics are probably degraded in a multi-step process facilitated by abiotic and biotic factors. Abiotic factors, such as shear stress induced by wave action, solar ultraviolet radiation, and heat embrittle and fragment plastics. Fragmentation of macroplastics results in micro and nanoscale particles. Photooxidation causes the release of chain scission products from the polymer matrix, e.g., nanoplastics, low-molecular-weight polymer fragments, and hydrocarbon gases. Biodegradation of plastics is mediated by microbes that have enzymes capable of inducing (1) chain scission and depolymerization, and (2) assimilate and terminally oxidize the intermediate products of initial degradation. Plastic degradation products from UV radiation could be a useful carbon source for microbes, while the role of marine microbes as initial degraders is not well understood. Several terrestrial microorganisms (bacteria, fungi) are known to degrade specific plastic polymers. For example, the bacterium Ideonella sakaiensis hydrolyses polyethylene terephthalate (PET) with a novel cutinase (termed PETase) and utilizes the degradation products as energy and carbon source. In the marine environment, complex hydrocarbon-degrading bacteria have repetitively been found in association with plastics. These bacteria have genes encoding for monooxygenases, peroxidases, and dehydrogenases, enzymes which can, in principle, facilitate the initial breakdown of plastics. Most commonly applied methods to investigate plastic biodegradation are based on monitoring weight loss of plastic over time, determining chemical changes of the polymer, investigating colonization of plastics by microbes, and measuring CO2 production rates. However, these evaluation methods often lack rigor in confirming initial depolymerization, assimilation, and mineralization. This chapter provides an overview of plastic biodegradation in the marine realm. Identified and potential microbial plastic degraders will be covered. Their metabolic and enzymatic capabilities will be highlighted with respect to valorization their potential in the future.
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