Investigating whether aquatic microbes are inhibited by dissolved organic carbon formed during the photo-dissolution of microplastics

Photochemistry, driven by sunlight, is a main driver of plastic degradation in marine and aquatic surface waters. Ultraviolet light irradiates buoyant forms of plastic litter that float at the water surface, such as polyethylene (PE). Dissolved organic carbon (DOC), a complex mixture of soluble organic molecules, is a major byproduct of photodegraded microplastics. Much of the DOC produced by plastics can be used by microbes to fuel growth. However, the photochemically produced, soluble products of one PE sample in a former study inhibited microbial growth, suggesting sunlight driven photoreactions of plastics can yield products that are toxic to marine bacteria. However, it is not understood if this phenomenon is common amongst PE marine plastic pollution or if inhibition was a one-off response, unique to a single PE sample. To assess the photoreactivity of different types of PE and the potential for them to yield toxic byproducts, we irradiated ten types of PE, quantified DOC accumulation, and then assessed whether the soluble products of plastic photodegradation inhibited microbial growth. Specifically, we present the results from a three-month irradiation of 10 different PE samples, two PE standards and eight post-consumer PE microplastics, to assess how quickly sunlight removes plastics from the water surface and quantify how much DOC is produced from the photodegradation. We found that DOC accumulation increases linearly and that rates of DOC accumulation are dependent on plastic type. In addition, we took the DOC produced from each plastic samples and conducted a 45-day microbial incubation experiment during which DOC use, total cell counts, and proportional intact cell counts were measured to determine the bioavailability of plastic-derived DOC and the impact of the photochemical products of plastics on aquatic microbes. We found that short-term (2 day) microbial inhibition was a universal property of PE-derived byproducts, while three PE samples, yielded byproducts that inhibited microbes for longer. Some of the inhibition seen was short-lived, with microbes apparently processing or adapting to any toxicity and subsequently beginning to grow and use DOC as a food source. Thus, future studies focusing on a longer microbial incubation period are recommended to further determine cell's growth and inhibition potential coupled with DOC consumption data to have a complete assessment of growth and inhibition impact over time. These experiments improve our understanding of how plastics photodegrade in ocean waters and confirm that some forms of PE can release toxic byproducts as they photodegrade.--Author's abstract